LANES CORNELL UNIVERSITY LIBRARY GIFT OF The Estate pf Theodore P. Wright UG630 .LiriSir"'*" '"""* + ''in»f>!9[in?inM?l?!'''"^^i simplified, enlarge olln 3 1924 030 744 647 Overs . "^ Cornell University W Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924030744647 All books are subject to recall after two weeks. Oiin/Kroch Library DATE DUE ^ 'y^ iftrsi? ?!j T|fH l»L7 ^ WR 1 ^ 2D04 S^ tflAMJ'l GAYLORD PRINTED IN U.S.A. MILITARY AEROPLANES SIMPLIFIED ENLARGED AN EXPLANATORY CONSIDERATION OF THEIR CHARAC^ TERISTICS, PERFORMANCES, CONSTRUCTION, MAINTENANCE AND OPERATION SPECIALLY ARRANGED FOR THE USE OF AVIATORS AND STUDENTS BY GROVER C. LOENING, B. Sc, A. M., C. E. Author of "Monoplanes and Biplanes" Member, Society of Automotive Engineers Member of Committee on Engineering, National Advisory Committee for Aeronautics Formerly, Chief Aeronautical Engineer U, 5. Army EDITION FOR YEAR 1918 Copyright by G. C. Loenihg ALL RIGHTS RESERVED Both In U. S. and all Foreign Countries Passed and approved by U. 5. Military Censor Printed by W. S. Best Printing Company Boston, Mass. 1918 TABLE OF CONTENTS INTRODUCTION 1 CHAPTER I Aircraft in General 7 CHAPTER II Types of Aeroplanes 11 CHAPTER III Prjniarily for Reference 25 CHAPTER IV Air Resistances 41 CHAPTER V Inclined Surfaces 57 CHAPTER VI Characteristics of Aerofoils 67 CHAPTER VII The Aeroplane's Elements 85 CHAPTER VIII Performances of the Aeroplane 95 CHAPTER IX Stresses and Safety Factors , 109 CHAPTER X Assembly and Construction 124 CHAPTER XI Marine Aeroplanes 143 CHAPTER XII Flying, Stability and Airworthiness 151 CHAPTER XIII Flying and "Stunting" 1 73 CHAPTER XIV Eyes of the Army and Navy 185 CHAPTER XV Conclusion 191 PREFACE It is a noteworthy tribute to the soundness of the fundamental principles of aviation, that "Military Aeroplanes," as originally written in 1915 — three years ago — as the official textbook for the United States Army Aviation School at San Diego, has required practically no corrections in its text. It has become universally recognized as one of the most valuable textbooks for student aviators, as well as an im- portant work of reference and is in extensive use at American, British and Canadian Aviation Schools. Year by year, war has brought on new developments in detail, and has made it more and more necessary to concentrate the education of the aviator in the technique of flying into as small a compass as pos- sible, because of the many other subjects such as telegraphy, map-reading, navigation, etc., which require attention. Many writers on Aviation, hold therefore, that a student should have at his elbow a childishly elementary work, which can be read in a few hours, in which only the flimsiest explanations are given, and containing little to Which the inter- ested student can turn, during the progress of his work for constant reference and further enlightenment. As a consequence, a surprisingly large number of so-called elementary works have appeared recently, many of them distinctly praiseworthy, but most of them, unfortunately, written in the spirit that if a book has a formula in it, it must of neces- sity lose its elementary value. Many qualified instructors recently have versed the opinion that such works are entirely too "thin;" and cer- tainly the continued large popularity of "Military Aeroplanes," which is ■admittedly somewhat technical, appears to bear this out. It has been the studied aim of the author to present in this work the happy combination of explaining these valuable and essential tech- nical features in so direct and simple a manner that "Military Aero- planes" might be, in reality, a most elementary work, and yet one cap- able of being studied over to advantage. In order to meet the progress in new models of aeroplanes and the continued advance in the technique of flying them, the author has adopted the plan of making "Military Aeroplanes" essentially a year book, with a completely revised and enlarged edition each year, even though the censorship makes it impractical to add many military feat- ures, which would greatly enhance its value. In the 1918 Edition some new chapters have been added, and the very finest half tone and line cuts available have been incorporated, all of which has necessitated a complete re-arrangement. Wherever necessary to simplify the student's work, every elementary feature has been explained and laid out in such a way that the layman need have no reason for difficulty in understanding the simple text. In this way the author has endeavored to give the aviator inf on nation that will add directly to his skill, in a language that blends with the spirit of the avia- tion field. New York, February, 1918. MILITARY AEROPLANES INTRODUCTION Newcomers in aviation hardly realize what a difference there is between war flying as we know it today, with stunts at 15,000 feet done as a necessary adjunct to proper maneuvering, and the day when flyers had to be most careful to make their turns wide, because if banked too steeply, their low reserve of power would be unable to prevent the aeroplane's losing the fifty or sixty feet of altitude they had attained with such effort. It is equally true that as aeroplanes have increased in excellence, the skill demanded of flyers is less and less, and it is safe to say that many a stunt flyer of the greatest distinction today would hopelessly crash in a 1910 aeroplane. It may be admitted that the palm for much of the great flying now going on must first go to the machine and motor, which despite many shortcomings, have nevertheless attained a perfec- tion undreamt of seven years ago. A good flyer respects his machine, and it is only the poor flyer who constantly blames it for his difficulties. So the beginner should start out with the idea that if he himself is sufficiently skilled he can fly almost anything well. With a thorough enough understanding of the elementary funda- mentals of aviation, gained by persistent study, the aviator can start out absolute master of the air, of himself, and of his machine, his control of the craft almost perfect, and his responsibilities centered upon his own skill and knowledge. It is of direct and definite value to him to know how the aeroplane is built, and how it flies. Studies that add to his knowledge of flying render him far more efficient than if he is restricted merely to learning the mechanical control of a craft he does not understand. It is interesting to emphasize the development of the controlla- bility of aeroplanes from those early days at Kitty Hawk, when Wilbur and Orville Wright were risking their lives in discovering balance in the air. Their historic experiments, first solving the control of flight, have led very gradually to the present day, when men can even -WTite their nam.es in the air in a series of weird spirals and loops — due to the perfection of the aviator's comm.and of his craft. In flying, as in almost no other activity, the personal equation has a most vital importance. Some men, who drive motor cars very badly, and seem entirely unsuited to mechanical things, turn out to be excellent flyers, and others apparently born for anything like flying prove on actual test to be utterly unsuited and incompetent. About the only basis on characteristics that has held is that a good cavalryman or a good polo player almost always makes a good fljer. And a com- bination of polo and riding, with skill in driving a motor car in trafl&c, would be about the best indication of a "make up" for aviation. But in a flyer all characteristics are more or less secondary to the main one which should be possessed by every aviator, namely — ordi- nary common sense. There is nothing difficult or mysterious about aviation either in theory or practice that cannot be grasped by any one who can think a little, and use common sense, in doing so. The difficulties of flying have been much over-rated. In fact, many a "hero" has been featured in aviation for deeds which almost any ordi- nary mortal could have accomplished. It was the peculiar early exhi- Underwood and Underwood The Nieuport scout — a popular French type which is very nearly a Monoplane, because of the small size of the lower wing. bition game, which systematically wove a glamor and human interest story of great danger about flying, that in reality did not exist, and greatly delayed progress along the right lines. As a matter of fact, flying in the air oil a calm day in a good machine, is easier than driving a motor car to any one who flies conservatively, and who has learned the reflexes of the controls. It is in getting into the air, and more particularly in landing, that the real work for the skill of a flyer comes in. One of the most important things for the pupil, is not to rush over the initial stages of fljdng. More important at first than actual skill on the controls is the acquirement of a very sound "feeling at home" in the air, and a very healthy respect for the aeroplane. If a pupil rushes thru his early instruction without getting that feeling of thorough ease and conafort in the air, he will constantly be forcing himself to fly against his sub- conscious will and sooner or later will be bound to "blow." On the other hand, the flyer who finds himself at home, and really likes to fly, and then after only a short flying experience, gets over-confident — is riding to a fall, almost surely. The most dangerous time for a pupil is that pecuhar stage, common to every student, where he knows he has learned to fly practically as well, if not better, than his instructor. Then is exactly the time for him to begin to use his head and common sense, and practice most conservatively and thoroughly. The average impulsive pupil after a few hours flying wants to start "stunting." If given a free hand, the mortality lists at aviation schools would be shocking. Stunting to a skilled flyer, is easy enough. The Spad tractor aeroplane — a typical European fighting scout, with a high speed of over 130 miles an hour. The biplane wings are mounted to the body at the front, and in examining the controlling surfaces at the rear it will be seen that the horizontal elevator flaps are turned down. This has lifted the tail off the ground and the machine is just starting to fly. but it particularly requires ability to know what position the aeroplane is in, aU the time. Many fliers do "stunts," but in reality they cannot fully describe exactly what they do. Unless they can, they are indeed playing with death. The only sure way to acquire the proper skill in flying up to the point of "stunting" and knowing every moment what the machine is doing, is to build up to it by persistent steady practice. Gradually, every few turns, bank just a shade more, turn a shade sharper, extend the feel of speed range, to higher angles and slower speeds, finally at sufficient altitude working up to a stall, and so on. Throughout, the essential effort of every pupil should first be to get in sympathy with his machine, to know every "hit" of the motor, and every tone of the singing wires — to be one with his mount. Brutality on one's controls is often a characteristic of beginners and one of the hardest to eradicate. A good aeroplane can be flown with two fingers; throughout a flight on a calm day, the elevator column need not be moved more than half an inch. But to do it with the proper delicacy, the controls must be grasped with the highly sensitive touch of a musician, and not gripped with a death defying determination to "fly or bust." The set face, and the controls grasped with an iron grip with the obvious determination to defy death just once more, are the sure signs of a very ill-suited flyer. It is true that very few people really like to fly, but the ones that do, if sufficiently practiced, certainly are the ones who become the "stars." The American type of training aeroplane, in which the pilot and his pupil sit one behind the other, each with a set of control levers. We have said stunting requires a steady attainment by practice. Even more so is this true of making landings. There is the real test of an aviator ! For on making landings there is required repeatedly, the very best of quick judgment, absolute mastery of the controls, instant reflexes, and with a calm collective ability to do and watch about ten things at the same instant — the speed of the motor, the speed of the aeroplane, its angle, its side drift, its lateral level, the nature of the ground, the direction of the wind, the presence of other aeroplanes, of obstructions, trees, — and yet* ready instantaneously to alter one's plan of landing to meet a new condition, just become apparent, and to use the right judgment. Thousands of bad landings are constantly made without any damage or any realization by the pupil (and frequently even by the instructor) of how badly executed they were. On the other hand, what a delight it is to see a "master flyer" deposit his machine like a feather a few feet from an appointed spot with every regard shown for the wind, for the terrain and for other aeroplanes! The keen judgment required of aviators on landings becomes no- where more evident than in a forced landing on a cross country trip. It has been the writer's good fortune to have flown across England on a very modem machine with one of the best flyers of the day, and it is not easy to forget the skill shown when a fog arose in making a landing on this trip, in a tight little field in a cross wind. g^ ^>^ A typical European two-seater tractor of light construction and power, for fighting purposes. This is the Sopwith "one and a half strutter." While flying is much like skill in playing a game, the fact remains that the instantaneous judgment required is better, the more thoroughly the flyer understands his "metier." He need not be a theorist, he need know little about mathematics, but the fundamentals as to why and how an aeroplane flies must be part of himself. So thorough must this grasp be that in those brief moments on making a landing, or coming out of a flying difficulty, he can instinctively picture the whole mechanical relation of the aeroplane to the air around it, so as with the correct move to meet some entirely new condition, or accident, which may never have happened before. Skillful pilots in the war have brought aeroplanes down safely, with an entire wing shot away, with the tail half gone, etc., by performing some new combina- tion of maneuvers instantly devised, which it would take many theorists and designers a long time to work out. It is remarkable how instinctively a thoroughly well trained flyer will do the right thing at the right time, by the use of marvelously quick judgment, based on a thorough knowl- edge of the science and practice of aviation. It is proposed, therefore, to furnish the aviation pupil with a knowl- edge of the fundamental features of aeroplane forces, balancing, flying and construction, to aid him in acquiring that necessary instinct of appreciation of what an aeroplane is and what it is doing. iXjLTiiTacit. a\ rp.st^due jgitiiiiiUialiJ-iiESs: THE DIFFERENCE BETWEEN, "HEAVIER THAN AIR" AND "LIGHTER THAN AIR" CRAFT. A submarine immersed in water, and a dirigible balloon, immersed in air, are exactly the same type of craft. Their characteristics of fluid motion, of steering and of floating are the same — • even to the fact that both use water ballast in tanks, to vary their flotation! But in the one case, the submarine is buoyant in water because it is filled with air which is l/800th the weight of water, and the dirigible balloon is buoyant and floats in air because it is filled with hydrogen which is l/13th the weight of air. Whereas — the aeroplane must acquire speed, in order to fly! CHAPTER I. AIRCRAFT IN GENERAL The application of Aviation to the art of warfare has quickly be- come a specialty, and it must at once be appreciated, that military require- ments have a most vital and important influence on m.any features of aeroplanes — not only in the art of using them in military operations, but in their fundamental design and construction, — features, such as extreme lightness, which will disappear, \yhen the needs of war are re- placed by commerce. It is planned, however, to give particular attention here to the military aeroplane, as we find it today- — emerged from a crude state of invention and development into a more or less' finished product, which, in the greatest war of history, has gloriously demonstrated its strategical and tactical importance. A Kite balloon floating on its line. The basket- contains the observer. Central News Service A "k. b." after being hauled down by- its attendant motor car auxiliary. It is no longer necessary to speculate on the uses of aeroplanes in warfare. What has actually been accom.plished in directing artillery fire, in reconnaissance, in dispatch-carrying, and ia offensive work has opened a new phase of warfare, as significant as it is surprising. The technique of the use of aeroplanes in strategy and tactics is decidedly a subject for the military expert, but the general desiga and construction of aeroplanes to accom.pl ish certain definite purposes, and their operation and maintenance in the field, are subjects that may properly be considered here. 8 A full consideration, therefore, is given to elementary theory and practice applied in aviation, and the information used is primarily designed to be of definite service, in the field, where many unforeseen difficulties constantly arise. During the centuries of effort to conquer the air, the aeroplane is but one of many types of craft that have been devised. Before taking up the determination of its elements, therefore, it is necessary, clearly, to distinguish the aeroplane from other craft designed to navigate the air. Aircraft may be divided into the following classes: ^»^ A Zeppelin "Rigid" airship and above it an aeroplane. The rigid balloon is one type of floating airship. 1. FLOATING AIRCRAFT. The "airship," is distinctly a lighter-than-air machine, con- sisting of a balloon or gasbag, containing a gas — hydrogen for example — lighter than air, which by displacement of an equal volume of air, gives a flotation, the magnitude of which is de- termined by the kind of gas, the size of gas container, and atmos- pheric conditions. The ordinary free balloon is, in short, nothing more than a harnessed "bubble," and the dirigible, or airship, is a balloon of elongated shape, fitted with steering apparatus and propelling mechanism. The Kite Balloon — This type of balloon is used for artillery observation both on land and at sea. It is "Captive," in that it is held by a cable, which is let out from a mechanism on the ground permitting the balloon to float up to an altitude of a thousand feet or so. It has no motor and its peculiar shape, makes it very handy and stable in winds, pulling up steadily on its cable and preventing it from spinning around, hence the name "Kite Balloon." Airships are constructed mainly in three different types, the "Rigid," the "Semi-Rigid" and the "Flexible or Non-Rigid." These designations refer, entirely, to the manner of combina- tion of gas container and framework carrying the weights of en- gines, etc. A flexible gas container, held in shape only by the pressure of gas within and to which the load is hung, character- izes the "Non-Rigid" system. A gas container, held in shape by gas pressure, with an additional stiffening keel to which the weights are attached, is descriptive of the "Semi-Rigid." Whereas, in the "Rigid" system, a stiff, braced frame-work or hull, carrying di- rectly the motors and loads, is formed to contain within it numer- ous separate, drum-shaped gas containers instead of balloons. The stiff frame provides, in itself, that necessary rigidity of hull, which interior gas pressure on the envelope provides in the other types. The Zeppelin airship was the first successful development of the rigid system, and is useful in a naval fleet unit. The "Blimp" — This type of airship is a small scouting type used very largely for short range coast defense reconnaissance. It is a typical example of the Flexible dirigible system. Close view of the car of a Blimp, similar to a trac- tor aeroplane fuselage. A " Blimp " dirigible starting out. The rudders at the rear may be noted. 2. FLYING MACHINES. The Aeroplane — In distinction to the airship, supported in the air by a buoyant gas, the aeroplane is supported by an upward wind pressure, generated by its own speed through the air. This lift- ing pressure is obtained on specially formed wing surfaces, which are set at an inclined angle, and forced through the air at the re- quired speed by an air propeller. Suitable auxiliary surfaces and rudders are used to preserve the equilibrium of the craft and to enable the pilot to steer it. 10 The Helicopter — Air propellers are similar in character to marine screw propellers, and not only are they made use of to push or pull an aeroplane, but it has been proposed, in operating them on a vertical axis, to use their thrust directly^ in lifting loads. This type of flying machine is called the "Helicopter" or "direct lift" machine, and does not involve the principle of lift from the inclined arched plane, used in the aeroplane. The Ornithopter — Nature's flying machines — the birds — are neither screw propelled aeroplanes nor helicopters. They derive their support from the wind pressure on their outstretched wings pre- cisely as does the aeroplane, but for propulsion, the bird flaps its wings in a rowing, weaving motion, which gives a forward push. When an aeroplane glides, it resembles in character the soaring of a bird, with wings outstretched, but attempts to de- rive propulsion from a reciprocating movement of wings, have not been successful, as yet. Machines of this type are called "Omithopters" or "Flapping-wing" Machines. Although little has been accomplished with them, the possibil- ities of the helicopter and ornithopter have by no means been fully investigated, and whether or not a combination of "direct lift" and aeroplane, often called the "gyroplane," has any future, is still a sub- ject for study. Airships, on the other hand, are quite highly developed, and al- tliough thc'>- arc difficult to handle in bad weather and are very expensive, they arc uscfid for slow speed scouting requiring long duration — for example in naval scouting against submarines along the coast. In addition, large airships such as the Zeppelin have a radius of action of thousands of miles, and while entirely too slow and clumsy for bombing raids, they have the great ability of putting out to sea as a unit of a fleet, and thereby becoming most formidable as long range patrols. Their design and construction are full of interesting, and difficult, en- gineering problems, only remotely related to the study of aeroplanes — and requiring a very special training. Of the various kinds of aircraft, only the aeroplane type of flying machine is to be considered here, primarily, because we find the aero- plane, at present, the most successful, the most economical and the best developed means of na^'igating the air. The development of the large aeroplane, as a competitor of the load carrying airship is one of the most interesting developments of the war, and because of its greater speed, lesser vulnerability, and cheaper production, the large size heavier than air machine, has proven highly successful, and so practical to operate, that a great commercial future is in immediate prospect for it. CHAPTER II. TYPES OF AEROPLANES At the present time the early inventive stage in the development of the aeroplane is gradually but perceptibly giving way to a state of more precise engineering. And, in this step in its progress, aviation is but following the course taken by almost every other art and sci- ence. Any classification of aeroplanes, therefore, is subject to modi- fication as newer craft are developed, and old ones rendered obsolete. But the general principles of the machines do not change as rapidly as do their concrete interpretations. The principle of sustentation of an aeroplane from the upward push of air flowing past it, has been stated, and, in the following chap- ters, will be analyzed. The support being derived from the free air, an aeroplane is readily subject to loss of balance, due to air disturb- ances, gusts, convection currents, etc. It follows, therefore, that many features designed to overcome loss of balance, are used on aero- planes. Organs are also introduced to give the pilot control over the craft within definite limits. An aeroplane consists, therefore, of lift-generating surfaces at- tached to a frame carrying motor, fuel, pilot and equipment, and in combination with devices to balance and steer the craft. Flying freely, in the air, an aeroplane has three axes of rotation. 1. It may ascend or descend, by virtue of changes in its longi- tudinal path. The nosing up and nosing down of an aeroplane is termed "pitching," as in boats. 2. An aeroplane, in flight, may change its direction of travel. This is termed "yawing," as in boats. 3. In addition to these, an aeroplane can tip over to either side, on a transverse axis, and this movement is termed "banking" or "roll- ing." In making turns, it is necessary to "bank" up an aeroplane, sidewise/ sufficiently to overcome the centrifugal force, and prevent skidding. This "banking" is obtained by manipulating the lateral control. The locomotive driver, is steered by the tracks, and has to give his attention, only to the control of the speed of his engine; an auto- mobile driver, controls his motor, also, but in addition must steer his machine; whereas the aeroplane pilot both steers and operates his engine, and in addition must give his best attention, continually, to balancing the machine, fore and aft and side to side. 12 Like every science, Aviation has a language of its own, and a method is used here of expressing this language in photographs. Study of the explanatory caption and of the photographs themselves, there- fore, is equal in importance to the reading of the text. The types of aeroplanes considered here are merely typical ones of distinct features and are constantly varying in detail, as dictated by the advances in military experience. THE "TRACTOR" AND THE "PUSHER" An aeroplane that is pulled through the air by a propeller situ- ated at the front of the machine, is called a "tractor." On the other hand if the propeller is back of the main lifting planes, the machine is called a "pusher." These terms are very expressive and very widely used. The single propeller "tractor" is the most widely used type now but the "pusher" type, particularly for gun-carrying, has still a"raison- d'etre." Twin motored aeroplanes may be either "tractor" or "pusher," depending on the position of the propellers relative to the wings. The term "biplane" refers to an aeroplane with wings, super- imposed, and "monoplane" to a single deck type of plane. And in a "triplane" there arc three superimposed planes. THE CONTROLS Since there are three axes about which an aeroplane may rotate, it follows that three controlling organs are required : 1. The "elevator," for pitching; 2. The "rudder," for steering or "yawing;" 3. The "lateral" or "rolling" control. The principle of the air force deri\'ed from an inclined plane,* is used in all of these controls. The "elevator" is inclined up or down, to lift or depress the tail of the machine. The rudder is turned so as to permit the wind to blow on it, to one side or the other, whereas the lateral control consists, merely, in giving a difference in angle to the two sides of the wings, causing one side to lift more than the other. There are three general means of lateral control: 1. "Ailerons," or separate small planes, on either side independ- ent of the main lifting surface ; 2. "Wing flaps," or portions cut out of the main surface and hinged thereto ; 3. "Warping," which consists in twisting the main lifting sur- face, so as to get a greater angle of inclination to the wind on one side and less on the other. In the construction of rudders and elevators, the necessary change in angle to alter the wind pressure, is accomplished either by pivoting the entire surface, or by turning a flap hinged to a fixed siuface in front of it. * See Chapter V. 13 Trip lane Wings 5P^.,,, /Machine Gun 1 ; / 1 1 It ,-Fin 4^^RQMJB|Bf^PB|h|^^te '""rllf ~ "^' ^mSin^^^^^^^^^^^^^^^^^^'^' f/Vec/ Sfabilizei '" ^^^^'' Movable Elevafor Flaps--' Single sealer Triplane tractor scout, used for fast pursuit and fighting. The three superimposed lifting surfaces give a light strong structure and the view is good, but they are difficult to line up, and simpler forms are displacing the triplane. ; Upper Winn >Un,j Flap or Aileron Machine Gun shoofintjfhru !-, Propeller Triil Surfdces ,- Rudder Tail Skid ''■Landino, Wheels This is a typical German single seater pursuit machine — the Albatross — equipped with a very large motor, but hea\'y and, because of its greater weight, lacking in maneuvering power. ^Mofor with Propeller a^ Rear Pi 10+. Gunne Siabilizer ■ Tail Skid Two seater pusher type, popular some time ago as a gun carrier and reconnaissance type, but now being rapidly displaced by twin motored machines. 14 THE TRACTOR BIPLANE. The form of aeroplane that at present approaches the nearest to a standardized type is the Tractor Biplane. The main lifting surface, as may be seen from the photographs, consists of two super-imposed planes, with their widest dimension across the flight path. The main planes are attached to a long, fish-shaped body, termed the "fuselage," which, in effect, is the backbone of the machine, since it carries the motor and propeller at the front and the seats near the center, while at the extreme rear are mounted the rudder and elevator. The use of an enclosed fuselage in a tractor type is almost uni- versal, and greatly increases the efficiency of a machine, by reduction of head resistance in the wind. The disposition of the seats in the body gives excellent protection to the aviators, particularly designed to do so in very fast machines where the wind pressure can become very uncomfortable. In the types of tractor biplanes shown, the chassis is mounted to the body, as is also the center section of the wings. By taking the outer wings off, this type is readily made transportable by road. In the photograph of the biplane tractors in flight, several de- tails show up clearly, — particularly, angles of view of the pilot, whose vision is interfered with by the lower plane in such a way as to create "blind spots," or obstructions to observation. PUSHER BIPLANES. The older types of machines, particularly the early Wright and Curtiss, were pusher types — the Wright, however, had two propellers and the Curtiss only one. These types were open-bodied, entirely unprotected, and with the motor to the side of or behind the aviator. A few years of development, led to the adoption of either a na- celle — • short fuselage, protecting seats and motor only, — or a fuselage. In using a fuselage on a "pusher" machine, it becomes necessary either to mount a propeller at the extreme rear "torpedo" fashion, to mount a propeller on either side, or to have a propeller running on a large bear- ing around the fuselage. In "pusher" flying boats the propeller tips just clear the boat. The earliest Wright machine had the elevator in front, so that to ascend the elevator was turned up, thus lifting up the nose, and vice versa; whereas, when it was later changed to the rear, for reasons of stability, to ascend it became necessary to turn the elevator the opposite way, thereby pressing down the tail. This distinguishes "front elevator" and "rear elevator." Above: Two views of the Bristol "Fighter," a popular and very useful British two seater. Below : A close view of the nose of a German Tractor and a seaplane climbing steeply. Courtesy C. G. Grey, Editor "The Aeroplane" Above : Two views of the Sopwith "Camel," a single seater scout, armed with machine guns. Below : Views of the S. E. 5, a British single seater with very high performances. IS The Sopwith tractor bipIane-'-a very widely used type. The side view at the top shows, right to left, the motor and propeller at the front, the machine gun shooting thru the propeller, the wings, and- at the end of the body, the control surfaces; the "rudder" is the vertical one, the "elevator" the horizontal one. The top wing is set ahead of the lower wing and therefore "staggered." The bend-up across the wings shown in the lower cut is the "dihedral." 16 f^udder mam lUma am rmrpedoban^ / i^rm for likral colftrol j OLD "PUSHER" BIPLANES Above — Left — ^Wilbur Wright, the inventor, and the early type of Wright double pusher biplane, with elevator out in front. Right — Double screw pusher Wright biplane, of later pattern, elevator in rear. Center — Twin screw, pusher fuselage biplane, with engine in front. Bottom — Left — Early Curtiss open body, pusher — one screw, three wheel chassis, rudders in rear. Right — Farman pusher biplane with nacelle or enclosed body. A "fuselage" encloses motor seats, etc., but in addition serves as the main structural unit of a machine, whereas a "nacelle" serves merely for wind protection, since a separate frame carries the rudders. The term "empennages" refers to the tail surfaces of a machine, whether they be "bal- anced" or "flap and fin." The term "fin" largely replaces the term "keel." It will be noted that the early Wright machines have no fins or keels in the empennages. The side surfaces of an enclosed fuselage are virtually keels. 17 EARLY TYPES OF MILITARY AEROPLANES 1. The Bleriot Monoplane used by France early in the war. 2. The Taube Monoplane used by Germany, at the start of the war. 3. The Aviatik Tractor, a German high powered biplane. 4. The B. E. 2 British Reconnaissance Tractor. 5. The Twin-Motored Caudron, used by the French. This machine climbed very fast but was not very speedy. 6. The Vickers Pusher with gun. 7. The French Nieuport Speed Scout — a highly successful type with excellent speed and splendid climb. 8. The Martinsyde Biplane, a British speed scout. 18 Courtesy of Handley-Page, Ltd. The Handley-Page twin-motored bomber, one of the most successful aeroplanes of large size, the advent of which virtually marked a new epoch. 19 ^^^m «; <>< T3 bo . C C o +" H -a 5P U 'S .3 ^ >'^ :s s Q S I J3 ^' 4> < '^ th -C 20 MONOPLANES. It has long been the custom, distinctly to separate biplanes and monoplanes as different types. This is hardly justified, since the only distinguishing feature is the use of a siagle deck, "king post" type of truss, to carry the air pressure lifting load, in the monoplane, and a double deck, "Pratt" type truss, in the biplane. Biplane surfaces do interfere slightly with each other, but in tractors the disposition of motor, wings, body, rudders and even chassis, is identical, whether bi- plane or monoplane. A further misconception, in this connection, is that the monoplane is faster than the biplane. The more recent speed scout biplanes have proved the fallacy of this. As will be seen in later chapters, the biplane and monoplane are very similar aeroplanes, differing primarily in wing surface bracing. Attention is called to the monoplane photographs on the opposite page. Monoplanes, like biplanes, may be tractors, pushers, open-bodied, and have two propellers. Several European firms construct a body and chassis, complete with rudders, to which either monoplane or bi- plane wings may be mounted. In general, the biplane carries more load, and the monoplane is simpler in construction. But even these differences are fast disap- pearing. A distinct advantage of the tractor monoplane over the tractor biplane, is found when the wings of the monoplane are raised slightly above the body, thereby enabling the pilot to look under them and to have a free and unobstructed view. AEROBOATS OR FLYING BOATS. For the purpose of starting from and alighting on water, aero- planes of tractor, pusher, or any type are readily modified. Merely adding pontoons to a tractor, in place of wheels, gives the hydro-aeroplane; and the construction of aeroplanes, fitted to receive either wheels or pontoons, as circumstances require, has de- veloped considerably. Craft of this kind are called "convertibles." But in order to obtain greater sea-worthiness and better co-ordi- nation in design, a special type of aeroplane has been developed, suit- able only for over-water work. The keynote in its design is found in its treatment as a boat with wings, rather than an aeroplane with floats. The aeroboat, or flying boat, therefore, is primarily charac- terized by a staunch, boat-like body, around which the rest of the aeroplane is built. The photographs show several different types. For further discussion of aeroboats and hydro-aeroplanes refer- ence is made to the chapter specially devoted thereto. {Post, f . 143.) 21 -o o i a! .^'§S -t-* +-' - C m o o I- a ".hg o u- 2 c s u2 "^ O O : o n « S 9 S S- WT3 2 S «J 2 J > m S - - _o 'a H Ci b O < o •a c > o E^ o O S bo o II (L) r- J3- = CO O ■(-) > ■r (U - otJ n ■^ .— -Jj ii SE D. 1" - S o g .2 3 c^ u to--; O ^^ S2 o o < PQ 22 GOTHA AVIATIK D.F.W. ROLAND D.ll. ALBATROS O. I . . HALBERSTADT Types of German Aeroplanes developed for A. E.G. AOO t AteATROS C V RUMPLER FRtEORICHSHAFEN )ombing, fighting and reconnaissance purposes. Courtesy C. G, Grey, Editor "The Aeroplane" 23 THE "DUNNE" AEROPLANE. In the preceding types, the auxihary organs for pitching and yaw- ing are separated from the main planes and are distinct. In the Dunne aeroplane, there is only one set of controlling organs, and due to the peculiar shape and construction of the machine, the control of yaw- ing, pitching and rolling is combined and governed, only, by the double wing flaps. As m.ay be seen from the illustrations, the main planes are set in a "retreating" position. Their position in plan, and their angle setting, give inherent stability characteristics. The "Dunne" principle of a retreating plane was used, though in a modified way, in the German aeroplanes, called "Pfeilfliegers" or "Arrowplanes," but the customary fuselage and rudders were retained. The German "Taubes" were monoplanes with pigeon-like retreating wings. (See p. 170.) It may be stated here, that the "retreating" planes have much the same effect as a dihedral angle, on lateral stability, but are not so sensitive to side puffs. The effect on pitching stability, obtained on the Dunne, by the negative incidence at the tips, can be had, though in a lesser degree, on the more ordinary types of aeroplanes, by a nega- tive setting of the tail planes. While "inherent" stability is descrip- tive of that obtained by the construction of the aeroplane itself, in shape, M-vtf, Flaps Off ^hf aifjer 5/i^c THE U. S. ARMY DUNNE TYPE BIPLANE An obsolete but interesting type. The changing wing section and reducing angle of incidence are clearly seen. The bustle is used to deflect the air sideways. The wing flaps on upper and lower planes, are the only means of control. To ascend all flaps are turned up, and to de- scend they are all turned down. Inverse movement rolls the machine laterally, causing it to turn. 24 wing setting, balance and fin disposition, a clear distinction is drawn between stability of this type and that obtained by adding to any aero- plane an auxiliary mechanism, designed to be actuated by movements of the aeroplane, and automatically operating the controls, for proper corrective effect. Such a mechanism is virtually an automatic pilot, and is often termed, a stabilizer. "Automatic" stability may be ob- tained, by use of a mechanism of this nature, on an aeroplane that is inherently lacking in stability. There are many other types of aeroplanes, but their general features resemble those described, and the art moves too quickly to give them all consideration. A general idea of the various types, with specially selected illustrations, having been given, a more detailed study of the aeroplane may be taken up. A squadron of French fighting aeroplanes of high speed, single seater type, armed with machine guns. CHAPTER III. PRIMARILY FOR REFERENCE As much as possible, mathematics are avoided in the technical parts of this work. Where formulae are of real help, however, in stat- ing clearly the relation between quantities, they are used and fully explained. In a field like this one, so eminently practical in its nature, com- mon-sense is of much greater benefit than abstruse scientific knowl- edge. There is justification for decrying the vast amount of com- plicated mathematics that have been built up on fundamental assump- tions which the practical air pilot knows are wholly erroneous, but in doing so, let us not forget that scientists and the laboratories have contributed a great and valuable share, in advancing the aeroplane's efficiency. It is praiseworthy in presenting a subject, to simplify it, and to avoid a too technical impression, but where this is at the expense of a clear and full understanding, it is inadvisable. Aeroplanes, as machines, naturally involve many scientific ele- ments, and it is certainly best, at the outset, to realize this and. to ac- quire a working conception of what they are. 1. It is necessary to -know the simpler types of equations and why they are so handy. 2. The elements used in solving triangles, such as sines and cosines of angles, should be familiarized, and a logarithm table is sometimes very convenient. 3. Mechanics dealing with momentum, inertia, accelerations, centrifugal force, and gyroscopic force, should, at least, be understood, and a comprehensive review should be made of Elasticity, Stress and Strain, and Fluid Motion. 4. A clear conception of Work, Energy, Power and Power Ef- ficiency, is of fundamental importance. 5. Graphical representations, composition and resolution of forces, are constantly of use. 6. Various modes of representing variations of quantities on charts, serve as the basis of recording air pressure results, and should be fully appreciated. 7. The relative values and conversion factors of different sys- tems of units, are most useful, and areas, volumes, etc., are frequently called for. Recalling these elements is made simpler, if a brief summary of the features particularly applicable to this study be given. 26 FORMULAE. To attempt to present a study of flight without any formulae would make it necessary to express relations between quantities in long paragraphs of words, that could more readily be stated in simple equations. There is nothing mysterious about an equation. It is merely a sentence tersely expressed. Thus, if it was desired to state the rule that the quantity A mul- tiplied by twice the quantity B is equal to C, the formula represent- ing this would be, A X 2B = C Each letter or symbol in a formula represents some factor that is substituted when its value is known. If A = 16 and B = 4, then C = 128, since, the rule interpreted, reads, 16 X 8 = 128 Besides equations, other relations may be represented by formu- lae. Thus, the sign "« ," signifying "varies as," would permit the statement that "wind pressure varies as the square of the velocity of the wind," to be expressed P oc V^ Equations are of two kinds, derived and empirical. A derived equation is susceptible of proof, by use of mathematical processes based on proven assumptions. An empirical equation is neither derived nor proven. It is merely a statement of the results of experiment, regardless of mathematical proof. In many branches of engineering, empirical formulae are con- stantly used, and in Aviation, the lack of a satisfactory basic theory of air flow makes empirical formulae based on experiment, most neces- sary. Empirical formulae are really experimental averages. As an example: The theory of long columns, has not as yet permitted of the mathematical derivation of a satisfactory set of formulae for the stresses. Very extensive experiments have been conducted therefore, on the loads necessary to deflect and break such columns. Grouping these experimental results together it is found that if 1/d denotes the length ratio of a certain column, and p, the stress per square inch of cross section, the average of the experiments, may be expressed as p = 32,000 - 277 1/d This is strictly an empirical formula. The engineer is interested in its practical application, not in its derivation, and when a column of this type is to be. designed, for any value of 1/d he can find the value of p. Formulae of empirical nature are fundamental in a study of Aviation. 27 It is often found necessary, particularly in an experimental field, to introduce numerical constants, to balance the two sides of an equa- tion. It may be known, for example, that the horse-power of a pro- peller varies as the cube of the revolutions and the fifth power of the diameter, but we could not express this relation as an equation, capable of solution, until a numerical factor is found which gives a value to the h. p. (horse-power) for any r. p. m. (revolution per minute) or diameter, that agrees with the experimental results. Thus the relation could be written, merely as an example for expla- nation, H. P. = kN^D^ but unless k = 1, the equation cannot be solved until a value of k is found. Since the equation is empirical, it becomes necessary, actu- ally to try many propellers, until an average is found. As a matter of fact, k, in the above formula, has been determined by experiment to be 0.54 when certain units are used. The formula becomes, H. P. = 0.54 N^ D5, and is capable of simple arithmetical solution by substituting values for the letters. A term like k is called a "constant." The majority of formxdae for air pressures involve "constants," and the great advance in designing during the past two years may be traced directly to the determinations by the aerodynamic laboratories, of better values of these constants, for use in empirical formulae. SOLVING TRIANGLES. Every triangle has six parts, three sides and three angles, and if we know any three (including a side) the triangle may be solved — that is the other sides and angles may be determined. Triangles may be solved in two ways : 1. By trigonometry. 2. By graphical methods. In aviation work only the simplest trigonometry is used, and about the orJy functions of angles used are the sine, the cosine and the tangent. It is well to recall, here, that "sine" and "cosine" are merely numerical ratios, representing the fractions that certain sides of a triangle are to the hypothenuse. The accompanying chart shows what these functions are, and also gives formulae for solving the triangles. In later chapters it will be found that in the representation and solution of forces, in the determination of angles of incidence, glides and climbs, and in stress determinations, many occasions arise for solution of simple triangles. But in aeroplane work great accuracy of computation is not ne- cessary, so that a simpler way of solving triangles may, at times, be used, i. e., the graphical method. This consists merely in a mechani- cal process of laying off on a sheet of paper the known angles, by a 28 MATHfrnTICAL SlliVS ;> fftvlrf **ow < /«3 i^ /♦ -rocT yn - :>fnt ion - tan^fwtt cet ' cesirrt JL ^ke-^tn.Jbrtrntn^rh a ' l>aralM h a-- hif^/jm/sr C- C 3mB > ■- . eciB^^ , -tun B - -k. e * " au3 * " " '**^ ' - " ■'"' f" (tnphfcnl Sokitut tf ct friaikfJe — Fi'nf ewtfrurf fine A5^ J- it, AX ^ on^ Aav /«• ■ fhf^^oifi, A*y f// an^ of 7c' ^,'f) a ^nrtroclir. Bi>j^1 hnt BY fflt>din^9ii4h^ivir9et»r^,f',sfevn4 */^l angkCr J»/» ^ /ifn^r^ nili, atoif tJ^un^ /A« feunil^ t^at *.- /S.I and i . /4 ?r JW*!* * t^A • / rtr'. - itnf / Jrrt 1*3 4 J^a r /yi it fi Awr»^ tHrmory fe rmtff ^ *>,.t /. - /4// t^0fonm rsfmpPtl vi>Mtrd^ h^ 48, are ^irt.r ^antt f^ia toon ^ ^ffuvjtflffi Jt-wirta-Kb r^3 - i. - h%m en^ ^kp* Mlks Qf P = 2 TT Vl/g Moment of Inertia. Inertia has been defined, but "Moment of Inertia" must be con- sidered when we come to rotary motion. Moment of inertia is the quantity obtained by multiplying the mass of each particle of a body by the square of its distance from the axis. Whether a propeller, a flywheel, or a wing spar, every object has a "moment of inertia" I, about any axis. It would be a laborious 31 comptitation to find I for various shaped bodies. Fortunately it has been done for us, and values are given later in a table. I is expressed in pounds X feet squared (lbs. ft. 2). Angular Velocity The "radian" is often used as the measure of a distance along the circumference of a circle. There are 2 x or 6.28 radians, covered in one revolution of a circle. So that one revolution per second, r. p. s., equals 2 ir radians per second. If w is called the rotational or angular velocity of a particle, and n the r.p.m., then, w = 2 irn It has been indicated that acceleration of a rotating particle, due to change in direction, gives rise to centrifugal force. But the rotational velocity of a particle, may increase or decrease. This is called angular acceleration and is a rate of change of angular velocity, called s. Torque. In linear accelerations, we have Forces, while in rotational accel- eration, forces are also to be considered, but instead they are called Torques. Torque, T, also equals mass x acceleration, but in its case mass is the moment of inertia and acceleration is angular. 1 .-. T = Ixsx - 32 where T is in pounds weight x feet and s in radians per second per second. The Gyroscope Linear motion and rotating motion have been considered. The axis upon which a body is rotating can be moved in a linear motion. In addition the axis of a rotating body may change its direction continually. This brings us to the gyroscope. An unbalanced force is of course necessary to change the direc- tion of linear motion of a particle. In the same way an unbalanced force is necessary to change the direction of the axis of a rotating body. When a wheel is set rotat- ing, the direction of the axle tends to remain unaltered, as long as no unbalanced external force acts upon it. But when an unbalanced force is applied suddenly enough the axle's fixed position in space gives rise to a curious phenomenon, not only resisting movement by this force, but actually causing the axle to move in a direction at right angles to the applied force. It is unnecessary here to take up the relation of this phenomenon to the earth's rotation or the derived formu- lae, representing it. 32 An example of gyroscopic force, however, may be given. If a bicycle wheel is held out in front of one, by one end of its axle, and set rotating clockwise as viewed by the holder, when the axle is pointed down the tendency is for it to swing around and point to the left, and any effort to point the axle upward, meets a pronounced resistance, the axle at the same time turning sharply to the right. The effect of this phenomenon on the aeroplane's stability is taken up later. In steadying ships or monorail cars, or in stability devices for aeroplanes, the movement at right angles to the direction of the applied force of a sensitive "gyro" is made use of. Elasticity — Stress and Strain. The phenomena which are associated with the distortion of bodies due to stresses are excessively complicated, and one has but to think of the many familiar properties of brittle substances, like glass or chalk, elastic ones like spring steel or rubber, and plastic ones like clay or wax, to realize that this is in itself a formidable study, much too ex- tensive to be given anything but a meagre consideration here. The importance of the study of Resistance of Materials, to aviation, can- not be overestimated, since in the design of the aeroplane proper, this is the branch of engineering that solves the fundamental problem — to build light and yet strong. This necessary combination is one that truly represents a cri- terion of the excellence of an aeroplane, as a structural engineering unit, and although it often does not, nevertheless, the aeroplane should, involve the most refined, advanced and expert structural features that engineering development has made possible. It has been a great detri- ment to a^'iation that fo many of its devotees have failed to realize that the very best material obtainable, and the most ingenious and perfect construction, still scarcely suffice to carry and distribute the strains properly . Of all the great variety of solid substances, having almost every imaginable degree of elasticity, softness, hardness and brittleness, we are concerned in later chapters, only with the behavior under stress of those which are used as materials of construction, such as steel, aluminum, brass, linen, spruce, ash, glues, paints and rubber. Of the three classes of substances, solids, fluids and gases, let it be recalled, that an "elastic" solid, like spring-steel, can withstand a stress which tends to change its shape for an indefinite length of time, whereas a "plastic" solid, like wax, does not recover from strain when the stress ceases to act. One must qualify the above, however, since the best spring steel never completely recovers from distortion, and even wax is slightly elastic. A fluid is a substance which at rest has no power definitely to resist a stress, and when at rest it is always pressing, normally, on the sides of the vessel containing it. A gas is a 33 matter with no independent shape, adjusting itself to take the form of the vessel in which it is confined, and tending to diffuse and expand indefinitely. Substances are of two kinds — grained and ungrained. Glass and water are examples of ungrained substances, while wood, steel, and practically all materials of construction, have a grained struc- ture. The grain in steel is well marked, and though often lost sight of, it is most necessary in aeroplane work, that care be taken not to put too great a stress across the grain of a steel plate. Elasticity may properly be defined as the resisting property of a body to motion of its molecules. Strain is the distortion of a body measured at a given point. Stress is the force by which the molecules resist a strain at any point. Stresses are developed, and strains caused, by the application of external forces. Each stress is accompanied by its own character- istic strain. Stresses are of five kinds — Tension, Compression, Flexure, Tor- sion and those induced by Fluid pressure. They are illustrated on an accompanying cut. It is a fact of fundamental importance in the theory of elasticity, that however irregularly a body may be distorted, any small portion of the body suffers that simple kind of distortion which changes a circle into an ellipse, the change of shape consisting essentially of an increase or decrease of linear dimensions in three mutually perpendicular direc- tions, sometimes accompanied by a slight rotation of the small parts of a body. The stress on a body is usually represented as pounds per square inch, or the force in pounds acting on a one-inch square part of the body. The total force P on a body divided by area A, of its cross- section gives this unit stress which is called "intensity of stress." The strain 1 accompanying this is not represented in actual inches or units of total defiection d, but is given as a fraction of the span L of the piece, such that strain 1 equals d/L. The basic law of Resistance of Materials is that intensity of stress p is proportional to strain I. And to balance the proportion into an equation, a constant is introduced, called E, giving the simple rule, that p = P/A = d/L X E = 1 X E This constant E, is called the "Modulus of Elasticity," and is of the greatest convenience in indicating what the proportion of stress in a given material is to strain. Thus, it is readily seen that steel is stronger than aluminum, when it is learned that E for steel is 28,000,- 000 and for aluminum 1,700,000. 34 ihr/ci^ hnds of Jtmses ^^hkal iMm cf fortes fenn icakd a1 rondom The ft^nrrit tfa^fcefO^t^aiy given poitt 5- feree K hffratm^ Hje hverarmfalno onti^ deditniiit n I' •• ^ " ciock^ise. Gmposffion ef forces — Pfvm i/nekr /Tejrore qfChnguhr CffAPUS /c , /J 1 1 1 1 / l: •r 1 / 1 ) 1 ^< 1 / ''/ / 1 ' / / / / / / / ^ _ e i _. of fl^et . %ar (o -onifja^ Me cvfri on i. "*etume ' cvrtt on ilte net- fen^^ ej ate a^ of tubem feet Cu4frr /- firm ^rofih - ^ties orft) ei>tk ^Hh Mamr aeafe an OWTT /. KINDS OF STRESSES GRAPHICAL FORCE DIAGRAMS GRAPHS CHARTS AND 35 For all materials, however, there is a limit beyond which the ratio of stress to strain or coeflficient of elasticity E = p/1, does not hold. This region is called the "elastic limit" of the material, and while con- siderable stress can be added beyond this, the material begins to stretch out of all proportion and rapidly reaches the breaking away point, which is called the "ultimate resistance." When relieved of stress, before reaching its elastic limit, a ma- terial will return more or less to its former state, but when the stress has exceeded the elastic limit the material takes a permanent set. The forces necessary to bring any material to the elastic limit, and the value of the ultimate resistance, are entirely matters of experi- ment, from which are derived empirical values. Fluids and Gases. In . liquids the phenomena of surface tension, capillary action, cohesion, etc., are of but minor interest excepting in hydro-aeroplane studies. It is important to recall of liquids, however, that the pres- sure exerted on any part of an enclosed liquid, is transmitted undi- minished in all directions (air-pressure fuel tanks). When a fluid is in motion it is being acted upon by an unbalanced force, giving it velocity and by a pressure, or in other words, it has the energy of a "velocity head" and a "pressure head." Any increase in one is at the expense of the other. A device very widely used for the measurement of velocities of both water and air is the Pitot tube, which measures the velocity head V = V 2 g h. It consists, merely of a bent tube with a nozzle, point- ing into the relative flow and measuring by means of the length of a column of liquid, the head h, which substituted in the above, gives the velocity v. In considering liquids the losses in head, in long pipe lines and the effects of expansion and contraction and of nozzles, are of inter- est with reference to the gasoline and radiator connections. Buoyancy and Specific Gravity should be considered. A body immersed in a liquid or a lighter gas immersed in air, is acted upon by a lifting force which equals the weight of the liquid or air displaced. In other words, the law of Floating Bodies is to the effect that a floating body will displace a volume of liquid of gas whose weight equals its own. A body immersed in pure water has a flota- tion of 62.4 lbs. per ft.^ The density of a substance is its mass per unit volume, while Spe- cific Gravity of a substance is its weight as compared with the weight of an equal "bulk" of pure water. So that, given the specific gravity of a substance, it is necessary to multiply by 62.4 to obtain its actual weight in pounds per cubic foot, since water weighs 62.4 lbs. per ft.' Specific gravity is sometimes referred to other substances — air for 36 example. The specific gravity of gold is 19.26. Its weight per cubic foot is consequently 1,200 lbs. A table of weights and specific gravi- ties is given later. Gases are highly compressible, in distinction to water and solids, and are perfectly elastic, though in distinction to solids their elasticity is one of volume and not of form. It must be borne in mind, with reference to gases,- that the tem- perature remaining the same, the volume of a gas is increased exactly jn the same proportion as the pressure is decreased. Or, the product of volume X pressure equals a constant quantity. The study of Aerodynamics which constitutes the major part of this work, takes up the mechanics of gases, making it unnecessary to give them further consideration here. WORK, ENERGY. POWER. Work is said to be done when a resistance is overcome, so that movement takes place through a certain distance. The air propeller which puUs against a resistance of 200 pounds, causing the machine to which it is fixed to move 80 feet, is doing work, inasmuch as it is continually overcoming this resistance. The unit of work is the foot-pound, which is equivalent to the work performed in moving one pound of weight through one foot of space. Work may be done in several ways — pushing or pulling weights, or working against pressures, such as the work performed by a piston in driving a fluid of gas before it, which is equal to the intensity of pressure X area of piston X distance traversed or stroke. In the above example, the propeller is doing 16,000 foot pounds work by overcoming a resistance of 200 pounds and moving against it 80 feet. Work, in whatever units it is expressed, is always "resistance overcome" multiplied by "distance traversed." Energy is distinct from work, in that it represents capacity to do work, but not the actual jssork done. It is expressed in the same units as work. There are two kinds of energy — Potential and Kinetic — since a body when at rest may have stored up "potential energy" due to its peculiar position or condition, and when in motion, a body is capable of performing work against a retarding resistance, due to its "kinetic energy." A reservoir fioll of water, capable of turning a water wheel, if re- leased, is an example of potential energy, and another is the stored energy in storage batteries or gunpowder. The weight of the stored body X the distance through which it is capable of acting is the meas- ure of potential energy. 37 Kinetic energy or K. E. of a body, is equal to the work which must have been done upon it to have brought it to its actual velocity from a state of rest. While potential energy is due to the acquirement of "strategical position," kinetic energy is due to the acquirement of "tactical impetus" or velocity. Kineti c Ene rgy = wv^/2 g and is derived from the familiar rela- tion V = V 2 g.h since K. E. equals the weight of the body X height from which it would have had to fall to acquire its velocity. Finally, it becomes obvious that Energy exerted = Work done. In referring to the amount of work done in a unit of time, it is necessary to consider Power, which may be defined as the rate of doing work. Whether the propeller in the above example traverses the 80 feet of distance in one second, or in one hour, the actual work done in foot pounds is the same, since time is not a dimension of work. Ob- viously, it would take more "power" to overcome any resistance in one second than in one hour, and to measure power it is necessary not only to consider the resistance and the distance traversed, but also the time it takes to do it. Power, then, is the number of foot pounds per second or per minute or the number of mile-tons per year, if we choose to use such units. The customary unit of power is the Horse-Power. One horse-power equals 33,000 foot pounds per minute, or, 1 h.p. = 550 foot pounds per second. Thus, when a weight of 5.5 pounds is moved 100 feet per second, one horse-power is exerted. An aeroplane, with a resistance in the air of 200 lbs., requires 29 h.p. when travelling at 80 feet per second, since 200 X 80 H- 550 = 29 h.p. It js interesting to note here, with reference to the possibility of man- power flight, that, for a few minutes a man can exert at the limit 200 ft. lbs. per second, and for an hour about 100 ft. lbs. per second, less than l/5th of one horse-power. Although much energy is generated and expended, the fact re- mains that the sum total of all the energy in the universe remains the same. Mechanical energy and heat are converted one into the other, the heat of the boiler, taken from fuel coming from the earth, passes into the engine', and into parts which do work against various kinds of friction, until finally the sum total of the mechanical energy has returned to the earth, from whence it originally came, as heat. The law of the Conservation of Energy is the most firmly estab- lished of the laws of mechanics, and only by the creation of an addi- tional amount of energy in the universe, which is impossible by any known human agency, could perpetual motion be achieved, although some magnetic and atmospheric phenomena may be used very nearly to approach it. 38 POWER EFFICIENCY Any machine, in order to accomplish an amount of work in a given time, must have work put into it in proportion. Due to friction and other losses, it is always true that the power obtained from a machine is not as great as the power put into it. Now, call P, the power delivered by a machine, and P' the power necessary to put into it, then the ratio P/P' will be less than unity, ordi- narily; it might be equal to 1, if the machine were a perfect one with no losses ; but never can it exceed one. The ratio of the power delivered by a machine and the power it used in doing so is called the Power Efficiency of the machine. We have used above an example of an aeroplane, with a flj'ing resistance of 200 lbs., which, when it was travelling at 80 feet per second, required 29 h.p. If the h.p. of the engine were 50 h.p. then the efficiency would be 29/50 or 58%. It is most important in tliis study clearly to understand the sig- nificance of Power Efficiency. FORCES REPRESENTED GRAPHICALLY. The development of a simple notion into an extensive science is well illustrated in Graphic Statics. Based upon the elementary fact that a force can be represented by a line, — ■ long enough to measure its magnitude to some convenient scale, and placed so as to indicate the direction in which the force acts with reference to some fixed point — there has been built up a com- plete science of the action of every kind of force, and in many cases simple solutions are obtained for problems that would require com- plicated mathematics. For all ordinary engineering the numerical computation of the characteristics of forces has almost entirely given way to their determi- nation by machine-like graphical methods. In later chapters the particular application of graphical methods to determine the stresses in aeroplanes will be taken up. It wiU suffice here to give a general idea of how the combined effect of several forces can be determined, — - composition of forces : and how a single force can be split up into an equivalent set of forces — resolution of forces. The single force, that would have the same effect at a point as a set of several forces, is called the Resultant. Referring to the diagrams, illustrating the action of forces, it is indicated that two forces of 4 and 9 lbs. are acting at a point o. It is desired to know what their combined effect is, so that a single force could be placed at o that would resist their combined action. The mechanical process of finding their resultant consists merely in applying what is often called the "parallelogram of forces," p. 34. 39 Graphically, the mechanical process is as follows: Lay off AB parallel to the 4-lb. force, and from A lay off AC parallel to the 9-lb. force. Com- plete the parallelogram to E, and draw AE. Then choose some scale, such that AB when actually measured on the drawing measures 4 units, and AC 9 units. With this same scale measure AE. It scales about 103^ units. Therefore, its value is 10 J^ pounds. Its direction is given by the direction of AE so that by drawing the force through o, parallel to AE, and making it 10?/^ pounds long to scale, we completely determine it in direction, magnitude and point of application. Finding the resultant of any number of forces, whether co-planar or not, consists in finding the resultant of two, then finding the re- sultant of this resultant and one other, and so on. Moments are defined on the diagram as merely the forces times their perpendicular lever arms, from the point about which moments are taken. If the force is expressed in pounds and the lever arm in feet, the moment is in foot-pounds. The unit is the same as in Work, but obviously, moment expresses what could be termed the Potential Energy of the force. Scaling lever arms of forces, from diagrams to scale, is by far the easiest and quickest way to obtain them. Of course, if a point is in equilibrium, all the forces pulling one way are balanced by forces pulling the opposite way. In the same way the sum of moments of all the forces will be zero. This is a very important conception to keep in mind. The resolution of forces into parts or complements, along given directions or axes, is indicated in the diagram, and is, briefly, a reverse application of finding the Restdtant. The intricate-looking but simply-made stress diagrams of braced frames, like bridges, are made of an elaboration of compositions and resolutions of forces. In all this graphical work, it is best to appreciate at the outset, the necessity of learning the mode of procedure of laying off the lines like learning to run a machine and then merely keeping the scales used clear and unconfused. Successfully to determine stresses it is as un- necessary to know the theory involved, as it is for the average taxi- driver to know the theory of why certain mixtures of gasoline and air are explosive. 40 Charts and Graphs. The representation of the variation of something, as a graph on a chart, is merely a convenient way of tabulating restilts. Instead of having long, cumbersome tables, giving values, at certain intervals, it is far easier to represent them on a chart. If it is but appreciated that a graph is a table with values for all intervals between the limits indicated, its convenience becomes very evident. Diagrams are given, as an example, of two types of co-ordinates, the Rectangular and the Polar. Graphs are used very extensively in studying Aviation, and the power curves for Aeroplanes bid fair to become as universal as the power curves for electric railway cars, etc. The combination of several curves on the same chart is illustrated in the diagram, and consists merely in keeping the same cross lines, but assigning to them different scales. SEVERAL VIEWS OF SEAPLANES HYDROPLANING AND PLYING CHAPTER IV AIR RESISTANCES A general description of various Aeroplane types has been given and the conceptions of science applicable to an elementary study of aviation have been outlined. We may proceed therefore, with our de- tailed study. The first feature to which attention must be given, is the simplest — merely the resistance to motion thru it, set up by the air. But to under- stand this we have to appreciate the nature of air as a medium, its characteristics, etc. And finally, it will be realized, that the efficiency of aeroplanes is very largely determined by the shape given to their various parts so as to enable them to slip easily thru the air. An aeroplane presents to the air numerous wires, struts, fittings, bodies, wheels, the landing gear, the aviator's head, the radiator, and other items, all of which — when the machine begins to tear thru the air, at its high rate of speed — set up a resisting force, which must be overcome by the motor. But these air forces do not help to fly the machine at all; they are, on the contrary, great detriments, though essential. In fact 10 pounds of air resistance saved, by a better. shaping of a part, or greater care, in lining it up to present its best angle to the air, readily' permits of a saving in power which will of itself carry 70 or 80 pounds more load on the machine. Not only that but, a clean design by saving head resistance, has better climb, better high speed and is more controllable, because it will pick up speed quicker. Speed and climb do not always necessarily require high power — first comes the requirement of low resistance. The converse is true — that the military pilot, unfamiliar with what good and bad shapes are, may ruin the performances of his machine, perhaps causing great danger, by recklessly adding bomb gear, rockets, flags, cameras, etc., on the outside without figuring how much power it .would take to drag all this thru the air. The nature of air resistance and its increase with speed as con- sidered in this chapter, will lead to the realization that a high speed record of 150 miles per hour is not going to stand very long, as Structural Resistances are constantly being reduced. 42 These structural resistances, however, are distinct from the re- sistance to motion of a wing that generates a lift — a distinction which will be explained in detail when lifting air pressures are studied in a later chapter. However, it is well to make it clear that the air propeller in flight actually overcomes two sets of forces: 1. The air resistances of the parts of the aeroplane. 2. The resistance caused by the wings themselves. .^Column of Air The air pressure at Point X in pounds per square foot is equal to the weight of the column of air 1 foot square in section above it. The Atmosphere. The atmosphere is an ocean, consisting of a mechanical mixture of several gases with water vapor, and even on the highest mountain we are still living at the bottom of this ocean. The atmospheric en- velope has a definite extent, and at any point exerts a pressure which is given rise to by the weight of the amount of air above it. We are constantly carrying around, therefore, on our shoulders, on the roofs of buildings, everywhere, the weight of the column of air directly above. The higher up, however, the less is the weight of air, and, consequently, the less the pressure. Air being compressible this increase in pressure with decrease in altitude affects the weight of air per cubic volume. We would have quite an exact measure of height in the atmosphere, in noting the corresponding pressure, were it not that this pressure is also affected by temperature and great wave movements of the air ocean, storms and winds. As the temperature increases the density decreases, and the volume of a pound of air increases at the same pressure. The unit of atmospheric pressure is the mean pressure of the air at sea level, at 60° F. and is called one "atmosphere." Its value is 14.7 lbs. per sq. in., and it causes the mercury in the barometer to rise 30 inches. Over one sq. ft., a pressure of one atmosphere is equiva- lent to a weight of 2,116 pounds. 43 For every 1000 feet increase in altitude the pressure decreases about y2 lb. per sq. in. At a height of 18,500 feet, atmospheric pres- sure is one-half of that at sea level, and at' a height of 40 to SO miles the air must be practically weightless, if the density decreases gradually. At atmospheric pressure and 60° F., the weight or density of air is .081 lb. per cubic foot. It is convenient to recall that air is about 1 /800th as heavy as salt water, and 13 times heavier than hydrogen. Since air has weight, it follows that, as a substance, it has inertia and momentum. Air is a "continuous" medium, each particle tending to hold together with every other particle, and the tenuous manner in which any air disturbance influences adjacent air filaments is beau- tifully demonstrated in photographs of air flow. Disturbances of the air cause up and down currents, complicated air vortices, aerial fountains, waves and pulsations, with changes in the velocity and direction of air stream.s. Just as water boils, so will air boil, when heated. The action of the sun in boiling the air over a dry, open space, can be distinctly felt when flying. PHOTOGRAPHS OF THE EIFFEL LABORATORY IN PARIS, SHOWING THE TESTING ROOM AND THE TWO WIND TUNNELS The study of meteorology shows the many other important characteristics of the atmosphere, the variation of weather due to great movements and disturbances in the air ocean, etc. But what we must emphasize here is that air varies in weight and density both reducing as altitude is increased. Therefore air resistance, the force with which air will strike an object at a given speed, will vary with altitude, because the heavier the air, the greater the resistance. This means that the aeroplane's action is most decidedly affected by altitude, and a study of this will lead us to see why every aeroplane has a ceiling, i. e., a definite limit to the height it can attain in flight. There is another very important conception with regard to air resistance determinations. Disregarding the effects of inertia and acceleration of an obiect, the air pressures are the same in action 44 whether the object is moved against the wind, or the wind against the object. Motion through the air gives rise to two distinct kinds of resis- tance : 1. Pressure, generated by the impact of the air on an object, and 2. Friction, generated by the flow of the air filaments past the surface of the object. Characteristics of Air Flow. Having defined air, the manner in which it flows may be con- sidered. Air either flows smoothly past an object in stream lines — continuous filaments — or it breaks up into swirls and eddies, due to too abrupt a change in flow. The accompanying photographs of air flow illustrate this. THE FLOW OF AIR. UPPER LEFT, .\ FLAT SURFACE — UPPER RIGHT, A SPHERE — LOWER LEFT AND RIGHT, STRUTS OF DIFFERENT FINENESS R.-VTIO. It is apparent that a spindle or fusiform shape, gently dividing the air at the front, and gradually permitting the filaments to close together at the rear, will give a smooth flow, which amounts to the same thing as a very low resistance. It is also evident that a flat sur- face creates very great disturbance, and consequently high resistance. The curve of the stream lines, necessary to prevent disrupting them, may be computed for any speed, by applying fluid dynamics. But it must be kept in mind that a form of this kind gives its low re- sistance, only at one particular speed, since the path of flow is affected by the speed. It is unnecessary here to take up the determinations of these forms. If the stream lines flow smoothly past an object, and close up again without eddies, it follows that the only resistance ex- perienced is frictional. There is hardly any shape, however, which does not create some small eddy resistance. 45 There are many ways of determining the manner in which the air flows past an object, such as noting the directions in which light silk threads are blown, or introducing smoke or particles into the air and photographing it. Ammonium Chloride is a very convenient smoke. Importance of Visualizing the Air. It is of great value in aeroplane work, to become accustomed to visualize the streamline flow of air, and ability to "see the air" often solves many problems of stability and reduction in resistance, with- out any recourse to mathematics or measurements. Besides this, there is offered in. the study of air flow by photography, a field of in- vestigation of great promise and absorbing interest. THE AIR FLOW OVER A WINDSHIELD VIZUALIZED It is a common experience that in a wind, at the front of a flat surface, there is a dead region of air, where no wind is felt. Photo- graphs show this air cushion clearly. In stability discussions, effect of following planes, interference and propeller stream action, priceless secrets would be revealed if the air could be followed in its every movement. The air flow over the front wind shield throws the air into the rear man's face — a feature that could have been corrected, if the designer could have "seen the air." Determination of Air Resistance. The nature of the action of air on objects has been consideredi but we must know in addition with what force in pounds P, the air pushes 0X1 aa object when it passes it at velocity V. We cannot refer to theory for this, satisfactorily, so we must obtain actual measurements of the air resistances on various objects. 46 Methods of measuring the resistance of the air that have been widely used, are the following : 1. Dropping surfaces from a height and measuring time of drop and pressure, used by Newton, and Eiffel in his earliest experiments. 2. The whirling arm, used by Langley, and consisting of whirling the surface at the end of a large arm around a circle of large diameter and recording the resistance automatically. 3. The moving carriage, an automobile, trolley or car, as used in the experiments of the Due de Guiche, Canovetti, and the Zossen Electric Railway tests. 4. By blowing or drawing air through a tunnel in which the object or a model of the object is placed. This method is the most modern and convenient, and permits of a uniformity of the air current, which cannot be obtained as easily in the open. In wind tunnels, the best practice is to draw the air in, through screens and channels, that straighten it out, past the experimental chamber, and thence to the fan. Practically all the great Aerodynam- ical Laboratories use the wind tunnel method of experiment. The prominent ones are, the Eiffel laboratory in Paris, the National Physi- cal Laboratory in England, and the tunnel at the Washington Navy Yard. The speed of the wind in the Eiffel laboratory can be brought up to almost 90 miles per hour (40 metres per second), and its size permits of testing many objects such as struts, to full size, and complete models of aeroplanes to one-tenth full size. Such a magnitude per- mits of exceedingly valuable determinations, and the work of the laboratories is daily being applied with entire success to full-sized aero- planes, altho the higher speeds of aeroplanes require considerable cor- rection of wind tunnel results. This is particularly true in the measur- ment of pressures oa wing sections at low angles. It must be borne in mind, therefore, that the air in a tunnel is con- fined and that all tunnel results are not perfectly adaptable to machines unless suitable corrections are applied. Combining the results of all the laboratories, we may draw some general conclusions with regaid to air resistance, as follows: 1. The resistance of an object in an air stream is proportional to the square of the velocity of the air. In other words, if the velocity is doubled, it follows that the re- sistance will be increased four times, or if velocity is five times as great, the force on the same object would be twenty-five times as great. This is merely an experimental fact. There are variations from this, however, due primarily to the fact that friction resistance alone, as distinct from impact resistance, varies as V .^ increasing in less proportion than V^. On very large surfaces, and particularly on dirigible balloons of streamline shape, the frictional part of the resistance is by far the greatest, and consequently makes the total resistance increase in a proportion less than V^. For our purposes, however, the total resistances of objects including the pressures and frictions are considered aa varying with V''. 47 2. Air Resistance Increases as the Object's Size Increases. This experimental fact is also subject to modification, since, as the size of surface increases, the pressures are somewhat greater in proportion. But we can disregard this also without serious error. 3. The Air Density Influences the Air Resistance. It has already been pointed out that heavy air (low altitude), has more resistance than light air (high altitude). 4. The Shape of an Object Controls its Air Resistance. The beautiful streamline photographs have already discovered this for us, and show how easily and with what small resistance the air slides by a streamline shape. Let us combine all this into a compact sentence called a formula, where P = the resistance inlbs., S = the areain sq. ft., V = the velocity thru the air in miles per hour, d = the density of the air in lbs. per cubic foot, and finally describe the shape of the object in order to ascertain whether it is clumsy or streamline, by a numerical multiplier, which we will call k, and which we will define as a "shape coefficient." In other words, P = k. d. S Vl But to simplify matters, since all of these shape coefficients for various shapes h^ve had to be measured, and mostly. at sea level, we can call d also a numerical factor, and combine it with k, so that kd = K, which is a number, always applicable to that particular shape, and represents the coefficient at sea level, which we can use, for any size body of that shape, at any speed, in order to obtain the resistance at sea level. For many differei^t shapes, all we need, therefore, is for some- one to measure and tell us what the values of K are for different shapes. Just as a grocer will tell ypu that a piece of cheese weighs half a pound because he measures it, so has M. Eiffel told the aviation world that K for a wire "is .0026. Therefore P = KSV^ This formula applies to all air forces, whether they are resistances, as we are considering here, or lifting pressures as we will consider later — with this reservation, that the shape coefficient is either for resist- ance, or for lifting power, and one must always be careful to determine which it was meant to be measured for. 48 Air Resistance in lbs. Force / Shape \ /Prqjected\ / Coefficient \ I Area \ i I 0.001 p\ in 1^1 yas measured/ \ sq.feet / 'Air Speed Squared in Miles per hour ? EXAMPLE-- For Z sq.-ft. projeded area, ai 50 miles per hour, on a shape wHh a coefficienf of O.M2,as measured by test, aisea level. ^^ ^^^^ x 2 X 2500 = 10 lbs. Here is a picture of the fundamental and only necessaiy formula for practical aeroplane knowledge. We may now proceed with the interesting study of what values of K have been found for various shapes, and not only will those be de- scribed by diagrams, but frequent numerical examples are given, of how to a]j]jly the data to determine values of air resistances. A study of this subject will give the student a most valuable insight into this branch of the science, and an appreciation of what shapes are "good" and "bad." S^mHi itMie Aireer, 3C K' MSB K-.ooie iS'Hl K-.oon K= .oofs- imiHC HlKllfnCKl mom cniNPtf Um, ctunputi 1, .1 ^ ' ^= ^^, '(^1 ' K'Ooi ' C-^ ^^) K-- .0019 ""< K'.oooi fUmJANCt or WlKl% A,f9 CABLU rm mrtMw [ ;'" 5.*::::::::: / BS _- ^ n f.. ^ \/. ^5 «::::::::2 S5 / S* .* . - -..L.. e / n ta n ai 96 K- .0016 K- ocoM K'.oecei K' ,oo»» The shape coefficients of various surfaces and bodies K- ■ OOi 49 '^'■^ ^"° = -g^ AAhmal fltfsxne J R.AM fhnM Of A FffCTHNCLK ^^ When ne nm snr£t» is ftamm; m pt/recrtoN A AtiAIHST ^ THE. LONtien 3ipK TW ASPfTT MTIO fj = J" MUSN THE AM afUMM /i nAWH^ Iff ^HPgrTtOfl -^ /f 7ve Jim JTjiMf, flows reitPeNPKvtAnCi AqAtrar r«r Sfo^r s'p^ Fineness Hatio AtfAmST A SVftFMf, 773 Tt>7AL rvKce , R^ a CAU£P , THE NoKtlHL f^CSSUKt /tMsni{ pwisn K - » TUB AM*: ^m** . j>, p M»rmAL PlA Nt - « me AL4»£ AifSA, or the A ffUtf^ on TAtL^ Appev -To A /V A JTHeAfflfffe SMAFt ^ THE ffATlO OF LBt^TH TV MAtr. Definitions. In Aerodynamical studies it has become customary in defining objects to use unfamiliar terms. Aspect Ratio — is a term used to define the shape of a surface, and is the long span of the surface across the wind divided by the width. Fineness Ratio — is a term used to define the general shape of bodies, and is obtained by dividing the fore and aft length of the body by the greatest width across the wind. Master Diameter — is the greatest width of a body across the wind. Fairing — is used to denote the additional "tail" or filler used to make a poorly shaped body more streamline in form, thereby reducing its resistance. Diametral plane — is the plane, passed through a body, facing the wind perpendicularly, and cutting through at the master-diameter. Normal plane — is another expression for diametral plane, and merely refers to the maximum cross-sectional projection of the body. It also refers to a flat surface held normal (perpendicular) to the air current. Equivalent Normal Plane — is the si'^e of normal flat surface, ihafc would give the same resistance as does the body referreo to. Flat Surfaces, Normal to the Air Stream. Square Planes — In square planes, normal to the air, the value of K is .003 for sur- faces up to two or three feet square, and .0033 for very large surfaces like the sides of buildings. It may be stated, therefore, for aeroplane usage, that P, the air resistance in lbs., of a square surface, S sq. ft., in area, at a velocity V miles per hour, is P = .003 S V2 Thus, for a surface 2 feet square, at 70 miles an hour : P = .003 X 4 X 4900 P = 58.8 pounds 50 Rectangles — The aspect ratio of a square is one. Rectangles have aspect ratios above one, when presented normally to the air. Up to an aspect of 5 or 6, K remains about .003. An increase in the value of K is found for rectangles as the aspect ratio increases. When the aspect ratio of the rectangles increases to 15, K becomes .0035 and on further increasing the aspect ratio to 30, K = .0038. A flat rectangle, perpendicular to the air current, with its dimen- sion across the current, thirty times as large as its width, might be met with in rods, temporary struts, etc., and it is interesting to note how high the resistance would be. Discs — The shape of flat surfaces also affects their air resistance. Pass- ing from a square plane to a round disc, reduces K to .0028, so that the air resistance of a disc 2 feet in diameter, at 60 miles per hour, is p = K S V2 = .0028 X .7854 x 4 x 3600 P = 32 pounds In general rounded edges may be expected to reduce K, for flat surfaces. Discs or flat rectangles, placed one in front of the other, interfere with each other and exhibit a most important phenomenon, shown on page 52. Cylinders — Passing from the disc to the cylinder, with the circular base facing the wind, the resistance is found to be less as the length of cylinder is increased, until the length becomes greater than 5 diameters, when the resistance is found to increase again. Some values of K are given on the chart on page 48. Wires and cables are merely very long cylinders. Extensive ex- periments have been conducted on them, and values of K found. For smooth wires K = .0026, whereas cables are found to have considerably higher resistance with K = .003. Thus, a machine having 200 feet of 1/8 inch cable, giving a pro- jected area of 200/96 = 2.08 sq. ft., will have an air resistance due to the cables at 80 miles an hour of P = .003 X 2.08 X 6400 = 40 lbs. This high value immediately suggests the advisability of reduc- tion of cable resistances. In double cables, it would prove beneficial to tape them together, so as to streamline each other. A graph is given showing the reduction in resistance due to inclining the wires i. e., staggered planes, on page 48. 51 Shape DESCR. Fusiform Body Fusiform Body SHAPE COEF. K = kd (Sea-level) .00012 .OOOEO Strut Shape Max.Diam.ai Center Strut Shape - Max. Diam.'/s Back Strut Shape Max.Diam.'/sBack A='AB Aeroplane Cable Flat Surface .00170 .00046 .00038 .00290 .00310 Fusela9e .00120 VALUES OF SHAPE COEFFICIENTS FOR SEVERAL SHAPES Spheres — The resistance of the air on spheres presents a study of interest. The sphere is the simplest geometrical form, and, as a basic one, it shoiild long ago have served as the unit form for air resistance. Lack of agreement in the experimental results of different laboratories was only cleared up when Eiffel discovered that an increase of speed of the air above 20 miles per hour caused a change of flow, due to the flatten- ing out of vortices back of the sphere, which reduced the resistance considerably. And that above this speed, the nature of the air re- sistance remained constant. K = .00044, for a sphere, at speeds above 20 miles per hour, whereas at very low speeds K becomes .001. In having a smoother flow at the higher speeds, less lbs. of air are put in motion, which means that the resistance is less. This action of air, in tending to smoother flow with speed increase, is important to bear in mind. 52 iNTEKFtntHcE OF Discs f^ ioTol resistance = S7% 'f " ef one ch'sc P= //oX res. of apt doc b^i itatlf^ /Irrewi intlKcitr tl>r action ef The bodies are placed in the order of their least resist- ances. For the top one K = .00012 For the lower one K = .0002 dodifi win somr Mosler Piomeler oncf circular Jfctiaij, mJfh difftteni fndi and lenqlh^ , gs iested, iy Elffe), K l>i/iyii/S OS foOens -^ *v;M s/ieed 1^ /t g o 30 an JO to 76 SO 9e m fih- 1 - 1 \z.cm n nc ^_ > " ^ i;:;;;^ %.oc(a OXf .DOOi <;£■/. TOP LEFT - INTERFERENCE OF FOLLOWING DISCS — TOP RIGHT, THE BODIES TESTED AT GOETTINGEN - BELOW, BODIES TESTED BY EIFFEL. 53 Streamline Shapes — In this class may be included bodies of fusi-form or streamline form, shaped for least resistance. Their application to the design of tanks, fuselages, nacelles, hoods, etc., is of fundamental importance. In a most interesting set of experiments, conducted by M. Eiffel, on streamline shapes, illustrated in the diagrams and chart on p. 52, the bodies consist of a nose, a cylindrical central portion, and a tail. The results of the experiments show that : 1. The blunter the nose, the greater the resistance. 2. The shorter the central cylindrical portion is, for the same nose and tail, the lower the resistance. 3. The effect of shortening up the tail is not very great, although slightly increasing the resistance. In each case, however, measurements made at speeds up to 90 miles an hour showed that the resistance does not vary as V'', the value of K becoming constantly less with speed increase. This is a very sig- nificant determination, and may be explained on the ground that, in bodies of this kind, the major part of the resistance at high speeds is- frictional and therefore increases at much less than V^. In addi- tion the effect of velocity increase is to flatten out the flow and suppress eddies. The values of K for these bodies are given. The Goettingen Laboratory conducted extensive experiments on the best shapes for dirigible balloons which it is important to consider. The models tested measured 3.75 feet long and .62 feet in diameter, giving a fineness ratio of 6. The shapes in their order of least resist- ance and values of K for 25 m. p. h. are given. At higher speeds, still lower Ks would be expected. The form No. 1, having the least resistance, is, perhaps, the best form that has ever been tested in a laboratory, and at high speeds would give a resistance about l/25th of the normal pressure on its diametral plane. It is the form used in the Parseval non-rigid diri- gibles. It is interesting to note in studying low resistance bodies, how closely they resemble the shapes of fishes, and of birds, measurements of a fast swimming fish showing an almost exact resemblance to this Parseval shape. As a general rule, the best streamline body is the one having a fine- ness ratio of 6 and with the master diameter about 40% back of the nose, both nose and tail being fairly well pointed. Struts — The application of fineness ratios, and shapes of least resistance, to improvement in the form of struts, has in; many instances tremen- dously improved the performance of aeroplanes. 54 «/NPL OUTUNE. OP and contipoitdinj value* of K, I" tff/iere^ 3- prvjetttd r}ornyil^ SOrftxe cf Strut. 'J flirt- . K= .0 0O36 *4 NPL &ffe, ^Finenevb»^ AT eN9A ^ pOe.a ^OT THE RESISTANCE OF SEVERAL STRUTS OF DIFFERENT SHAPE 55 In addition to the form for least resistance, however, the weight of the struts and their strength are factors that must be considered in choosing the best shapes. We will confine ourselves here, how- ever, to a study of the resistance of various shapes. A group of strut sections are given and K for each one. It is to be noted that the effect of yawing is greatly to increase these resist- ances by presenting the strut sidewise to the air, and it will be neces- sary later to consider the amount of this increase. Inclining the strut to the vertical, as in staggered planes, has the effect of increasing the length of section in the air stream, and, con- sequently, the resistance does not decrease for streamline shapes, while for blunter shapes, inclination reduces the resistance considerably. In struts, as in bodies, an increase of velocity is accomplished by a reduction in the value of K, that is more noticeable the greater the fineness ratio, i. e., the longer the section of the strut. This is again due, probably, to the preponderance of friction in the total re- •sistance. The results obtained in studies of strut resistance indicate the importance of having struts well made and of a uniform section. Just as in bodies, abrupt changes in contour must be avoided and atten- tion paid to a smooth curve on either side of the central portion. It is found, in general, that a fineness ratio of 5 to 1 is best for use, where a fin effect is desired, and where not, — the best fineness ratio is 3 to 1. Wheels — The air resistance of chassis wheels is a considerable item in flight. Experiments have been conducted on various-sized wheels, and the results are as follows : 283/^" diameter by 2>^" tire, K = .0025 24 " " " 3 " " K = .00265 21 " " " 3 " " K = .0018 18 " " " 2 " " K = .0021 When the wheels are covered in, it is found in almost every case that the resistance is halved, so that for the 24" x 3" wheel, when covered in, K = .00133. An average K for wheels would be .002. As an example, it is desired to determine the resistance of two 26" X 4" wheels at 80 m. p. h. The projected surface = 1.4 sq. ft. .-. P = .002 X 1.4 X 6400 = 18 lbs. If the wheels were covered in at this high speed, about 9 lbs. would be saved in resistance; this would permit of carrying about 60 lbs. ihore load on an efficient machine, or would add 10 gallons more fuel. 56 Summary. The data given in this chaptpr enables the air resistance of vari- ous shaped bodies to be computed for any speed V and any sjze sur- face S, where S is the maximum cross-sectional projection of the body, perpendicular to the air stream. It is merely necessary to supply the numerical values of K, S (in sq. ft.), and V (in m. p. h.), in the formula P = K S V^ Where K = kd, d being the density of the air, and the values here being correct only for sea level and theretfore largely comparative. It is well, again to recall that the propeller of an aeroplane must give a pull or push great enough to overcome: I. The resistance to motion of the struts, wires, body, wheels, fittings, skids, gas tanks, etc., called Structural Resistance. II. The dynamic resistance of the wings and rudders, called the Drift and generated by the same pressure that gives the Lift. In this chapter the first has been considered. And a study of the second may now be taken up. Uncovered Wheel Covered Wheel Tube wi+h Fairing Tube witliout Fairing The little wheel uncovered and the big wheel streamlined have the same air re- sistance; similarly, the tubes drawn to scale show how much larger a streamlined tube can be than a round one for the same air resistance. Air ^ F/af Surface '-Max. Cross-Sectional Area The realization of what streamline shape will do, is strikingly shown here, where the flat surface and the large body are drawn to the sizes that will give the same resist- ance. CHAPTER V. INCLINED SURFACES. The air resistance force on the strut of an aeroplane is of the same nature as the force resisting the movement thru the air of a railway train or an automobile or a projectile. To move at the desired speed, these resistances must be overcome by a pysh or pull exactly opposite to them. But when we consider the flow of air past an inclined plane surface a new condition presents itself for our study, the fundamental element of which is that the total air force is not directly opposite in the line of direction of the air flow, but is inclined upwards. A study of the ac- companying diagram shows that the air flow, as it is forced apart by the inclined surface, causes an upward pressure on the under side as the air is accelerated and forced downward under the surface, and on the top the air tending to stay behind and being torn away, there is caused a suction, the total effect being an upward force, if the surface is inclined upward to the air flow as shown. A downward force would be obtained if the surface were set with its leading edge below the trailing edge, so that the air hit the top face, but the streamline action would be the same. Air flow on an inclined flat surface. We have conveniently represented these air forces as lines on a diagram, and quite properly so. Forces going in any given direction are divisible into components in other directions, which together would give the efEect of the original force. This resolution and composition of forces is a convenient scientific concept, which comes in very handy here because we can at once resolve this air force, P, into two components D and L, one horizontal and in the hne of motion of the surface through the air, and the other vertical to this direction. Each of these will be smaller than the original force, since they are the "backward and upward" parts thereof. If we were to let the air force on the surface, have its way, it would push the surface upwards in the direction of L and backwards in the direction of D at the same time. 58 So we put weight on the surface enough to overcame the force L, and then quite logically call this force the Lift. And as for D, we push agaiiist it, with the thrust from a propeller, and we call D the Drift, since it is always tending to cause the surface to drift backwards. This simple explanation enables us at once to state the reason why flight in heavier-than-air machines is possible. By pushing the inclined surface into the air with a horizontal force D, we create a pres- sure on the surface equal to P, the force of which D is the horizontal component. But by doing so we have also created the other component L, which is a lifting force, capable of carrying weights into the air. This is the simple fundamental, which is so difficult for many to acquire, because of the apparent incongruity of obtaining a vertical force out of an inclined surface when only a horizontal force is actually applied to it. LIFT JAM d;DRIFT Since Drift is a function of the pressure necessary to lift the weight, it now becomes apparent why Drift was classified as distinct from the head resistances of the various parts of a machine. The latter are due soleh' to their form and the speed of travel, and they exert no effect on the lifting power itself. Consideration of this resolution into Lift and Drift, at once in- dicates that the characteristics to be sought tor in a surface are great lift with a very small drift, so that tor a minimum expenditure of power a maximum load carrying capacity is obtained. The ratio of lifting power L to drift D is a function widely used in considering the efficiency of surfaces, and the higher the value of L/D the greater is the weight that can be carried per pound of resist- ance. muUNC, CBti£ -^ LfADtNC EIKS ^l£aptMi fptie t t f It Mil t ^ t -5FAH- Definitions Span is the dimension of a surface across the air stream. Leading edge is the first edge of the surface upon which the air impin^s, whereas, trailing edge is the rear edge of the surface. Cliord is the dimension between the leading edge and the trailing edge of a surface. It is the depth of surface along the air stream. Surfaces are of two kinds — flat in section and curved in section. Camber is the rise of the curved contour of an arched surface, above the chord line. It follows from the above that for any inclined surface, that the aspect ratio is equal to the span divided by the chord. 59 Flat Planes. It is necessary to draw a distinction between planes that have a flat cross-section, and surfaces that have a curved cross-section, be- cause the variations of the air pressures in magnitude, position, and direction are quite distinct. Let P90 represent the normal pressure on a surface set at 90° to the air stream and determined as explained in Chapter IV, pp. 49-50. And let -Pa represent the total pressure on the surface when it is set at an angle of incidence A to the air stream, — referring to diagram on chart No. 2, p. 60. It would be possible to express the variation 01 Pa, with changes in the angle of incidence a, as a percentage of P90 = K S V^. This would necessitate determining the ratio Pa/Pgo, which is called the "ratio of inclined to normal pressure." Then Pa = Pa/P90 K S V^ where K is chosen tor the particular aspect ratio used (see p. 50). This is the system ordinarily employed, but for our purposes it is consider- ably more convenient to return to the conception of having values of K tabulated for each separate item. So that we may call Ka the value of the constant in the expression Pa = KaSV^ where Ka is similar to K the shape coefficient (Chapt. IV), but is here the coefficient for total pressure on a flat surface; and proceed to investi- gate the values of Ka for different angles of incidence, on the various surfaces. Thus, if we desire to determine the total pressure on a surface set at an angle ot incidence, a = 6°, our system of notation becomes quite clear, in stating Pe = Ke S V^ Lift and Drift. It is a fundamental fact of aerodynamics, that, at all angles in flat planes Pa is very nearly perpendicular to the chord. This sim- plifies the consideration of inclined pressures on flat planes, since at any angle of incidence we know the direction in which the air pres- sure acts. Thus, a flat plane, set at an incidence of 10°, is acted upon by an air force, the line of action of which is pointed 80° above the direction that would be taken by the normal pressure. This uniformity in the direction of Pa, with reference to flat planes, enables us to obtain very simple rules for finding the Lift and Drift of flat sections. Obviously from the resolution of forces Lift = Pa cosine a,= Ka S V^ cos a Drift = Pa sine a,= Ka S V^ sin a But we could disregard this if we were to have data on the K's for Lift and Drift instead of figuring them out from the total air force Pa on the flat surface. This cosine and sine rule cannot be applied to curved surfaces, so we must ]ook for measurements for all the K's, when we come to consider them. 60 c zinA .. / CuKVE 2: \ 1 1 <. tP / "■etis 4> ^Vi . — 1 N < / -♦' •'/ V S ^»^ I 1 1 NOHMAL ^ .005 / i' ■X; ■ -. _ . fneisotre -^ 1 / 5-=" . — -^ ' r. '"« ,< 1 f .-^ 1 j! /,. / fOHCE P/^KAff III... !•/ , / 'y' 10' JO" 40" ^O" to' AnieiLES or jNeiDBNCC 10' 90' This Curve shows the varia^tion of Ka, for aspects of 1/3, 1, 3, and 6. .«- .20 .zs .X .^sr CCNTtK or PKB3SUKE. PCSITION •" % C*or-rf This Curve shows the c. p. movement for the various aspect ratios. 61 The variations of Pa are affected by Aspect Ratio, and a very remarkable distinction between squares and rectangles in the man- ner in which Pa varies as the incidence is changed was discovered by Eiffel. At angles in the neighborhood of 40°, on square planes. Pa was found to have values very much greater than P go- Values of Ka for several different aspect ratios are given in Curve No. 2, p. 60. It will be seen from this graph, that an increase of aspect ratio above 1 is accompanied by increases of Pa, at low angles. But there is a general falling off of this at 15° to 20°, as the aspect ratio is in- creased. For efficiency, at low angles, on flat planes it is advisable " therefore, to use the higher aspect ratios. In all cases, S is the plan area of the surface. Center of Pressure. Although the direction and magnitude of forces on flat surfaces have been considered, these forces are not fully defined until determi- nations are obtained of their point of application. The nature of the air reactions on a surface consists of a series of. small impact pressures and friction rubs all over the surface; but their total effect can be represented graphically by a single force, in the resultant direction, and applied at a point about which all pres- sures balance. This center of balance of air reactions is termed the "center of pressure," and if we draw thru it a force proportional to Pa, and in direction normal to the surface, we have completely defined the air reaction on that particular flat plane. On flat surfaces it is indicated by Curve 3 that, as the incidence is decreased, the center of pressure moves forward until at 0°, it is very near the front edge, and at 90° it is at the center of surface. The representation of position of center of pressure, c. p., as a percentage of the chord, is a convenient one that has become quite standard. Example. A typical example of -the use of the data given for flat planes may prove of interest. An aileron, flat in section, measuring 2 ft. chord by 12 ft. span (aspect 12 -^ 2 = 6), is pivoted 4 inches back of the leading edge. The aileron is moved to an incidence of 10° and the air speed is 60 miles per hour. It is desired to find the corrective force on the balance of the ma- chine represented by the lifting force of the aileron, at 10° incidence. 62 Lift, L = Ka S V^" cos a. From the chart we find that for a plane with aspect ratio of 6, Ka = .00175, and Cos 10° = .985, S = 24 sq. ft., V^ = 3600, so that L = .00175 X 24 X 3600 x .985 = 149 lbs. In addition it is desired to know the moment of the total pres- sure Pa, about the pivot, at 10° incidence. From the graph it is found that c. p. position = .33 chord = .33 x 24 = 8 inches from leading edge. It follows, therefore, that the lever arm of the total pressure Pa about the pivot is 4 inches. Pa = Ka S V2 = .00175 x 24 X 3600 = 151 lbs. Therefore, the moment about the pivot is, = 151 X 1/3 = 50 foot lbs., which would enable the pounds pull on a control mechanism to be determined, and leverages suitably arranged. Curved Surfaces. Although the general characteristics of the action of air on flat planes had been known more or less accurately for some time, the nature of air reaction on curved surfaces was not well appreciated until the pioneer work of Lilienthal and the Wrights disclosed it. LUienthal discovered that, at low angles, surfaces slightly cam- bered gave very much more lift and less drift, than did flat planes, at the same incidence, and that the restdtant total pressure was not necessarily perpendicular to the chord, as on flat surfaces. In fact, he found that at certain low angles of incidence the total pressure on a curved surface was leaning considerably in front of the normal to the chord line, which meant that a smaller proportion of this total pressure was drift, and a greater portion was lift. The Wrights, in their gliding experiments, discovered that the center of pressure on an arched surface of Lilienthal type, did not change its position, in the same way as the c. p. on a flat plane, but that in- stead of moving steadily forward as the incidence was diminished the c. p. on the curved plane ceased to move forward at about 10°-15°, and retrograded, moving rapidly past the center of surface, towards the trailing edge as the angle grew smaller. This feature rendered Lilienthal's measurements somewhat inaccurate, but the corrections, readily applied, were used to make the old results applicable to the modern aeroplane. 63 Kl and Kd Since the total pressure on the curved surface, which we will call Pi, is not necessarily perpendicular to the chord line its resolution by trigonometry into Lift and Drift is not possible unless we know its inclination with respect to the chord line. But, since this neces- sitates knowing its components, it becomes, at once, more convenient to study curved surfaces directly from measurements and data on Lift and Drift. This is done throughout the study of curved surfaces and aero- foils (aeroplane sections), and in the laboratories it is customary to measure the vertical and horizontal forces on curved surfaces. The resultant of these determines Pi, in magnitude and direction. But since we are rarely concerned with Pi, where data is already available on L and D, its consideration is not so important. To define L and D, it is most convenient to consider that L = KlSV2 D = Ku S V^ and information on curved surfaces resolves itself into .a study of the values of Ki, and Kd for the various angles and shapes, where these coefEicients are in every way analogous to K (at sea level) in Chapter IV, excepting that they represent the Lift and Drift values of a wing, instead of the simple head resistance of a body. In this chapter, the simplest geometrical curved sections only are considered, as it is desired merely to bring out the main distinctions between flat and cambered sections. The standard practice is adopted of referring to the camber of curved sections, as a fraction or percentage of the chord. Forces on Cambered Planes. The resolution of Pj into L and D is fully indicated in the diagrams on p. 64. The angle of incidence of the chord with the air stream is called i. But, since in cambered planes Pi is not necessarily normal to the chord, it follows that the angle between Pi and L is not neces- sarily equal to the angle of incidence i, as it is on flat sections. This angle of the resultant with the vertical is called r, and from the con- struction of the triangle of forces it is apparent that tan r = diift/lift and the ratio of L/D = cotangent r, although this relation is rarely used. The nature and determination of "Lilienthal's Tangential" is not considered necessary in this study, although it is frequently dwelt upon in elementary aerodynamic treatises. * Influence of Aspect Ratio. The manner in which a change in Aspect Ratio affects the forces on circular arcs is shown in the charts on p. 64. It is found that not only the magnitude of L and D, but the movements of the center of pressure are influenced very much as on flat sections. For a circu- * See "Monoplanes and Biplanes" Chapt. IV, p. 47. 64 To find D at any angle, divide values of L by corresponding values of L/D. Miil ^_ s .A S«f r = 1 / V / CURVE iir s- ruLL. l/p- PC TtP 4- ■IHE. UNE. S -9 1 <*w Ai Pier 6s, t \) I / AiFU r--j ■N / 1 / \ Y Y / \ \ / 1 Is V \ / 1 1 c \ s \ ■ / / ■-■X \ \ \ \ ■ooz 1 1 Y ' /I / / 1 "^ ^ ^ \ \ 1 \ / 1 1 / V ^ ^ ^Asi'ccr-. /3 1 J / \^ ^ — ' 9 1 / \ \ h / ^ \^ ' 3 1 '!i I y ^ 1 / ^ .-£ 76«r=^ 001 / / / \ r 1 1 ^aml>cr= J^gc/jord / / / 1 =<-5V^ r /'\. ^v 'v wJ/ 'CCT= 6 = 3 /1/K;if r~~-— V ^- "^-~. :: . / ^ — — , tf'^^Xfc^ / /s 1 1 ..=rd' ■ :3s c ^NCH£ OF TlVCip E/Wf >~" ere Ho 15 10 ^ /o° Jo° "^P "W' '^° Zo^ W^ 5^^ 90° AlVCiLES or IrtCIPENCB. C. p. PeSiTlorV (fK'VTIoV of OIOKP) efiicients Lift and Ratios of L/D for ' Curve 5 shows the c. p. movements for the same surfaces. Curve 4 shows coefficients Lift and Ratios of L/D for changes in aspect ratio, of a circular arc. 65 lar arc in which the camber is 1/13.5 of the chord (an average camber for good efficiency), the values of L and L/D steadily increase as the aspect ratio is increased from 1/3 to 6. The difference between aspects of 6 and 9, however, is not very marked, and it is indicated from these results that there is not much gained by increasing the, aspect of an aeroplane wing above 6. With reference to the total pressure Pi, it is found for cambered planes as it was for fiat ones, that when the aspect ratio is 1 (a square), Pi rises to a value more than once and a half times the normal pressure Pgo at an angle of about 40°. The center of pressure chart shows that as the aspect is increased the reversal of movement at low angles becomes sharper, and the angle at which this reversal takes place falls from 45° for aspect 1/3 to 13° for aspect 6. Effect of Depth of Curvature. Alterations in the camber, i. e., in the depth of curvature of cir- cular arcs, greatly affect the magnitude of L and L/D, and the move- ment of the c. p. In the charts, on p. 68 there are plotted, the curves, showing the values of Kl and L/D for arcs of 1/27 and 1/7 camber, with an aspect of 6, which are to be compared with the curve for a 1/13.5 camber, aspect 6. It will be noted that the magnitude of L increases with the in- crease of camber, but the ratio of L/D is decreased by camber increase and the point of maximum L/D varied. This indicates that deeply cambered sections would prove to be inefficient wings for aeroplanes. For the smaller camber, the c. p. movement is sharper, and the reversal point ftuther forward. The Reverse Curve. Sections of cambered surfaces may, of course, be other than cir- cular. Combinations of straight lines and circular arcs, parabolic curves, spirals, and the like, have characteric pressures that differ from each other, but in so small an amount that it is hardly necessary to give them separate consideration — excepting in so far as they are taken up later in studies of aeroplane wing sections. The reversed curve, however, is a distinctive geometrical section, to which attention should be given. These sections, as illustrated in the diagram, have the important property that the center of pressure continues to move forward as the angle is decreased, the character- istic retrograde movement, as found on circular arcs, being apparently absent. As will be explained later, this retrograde movement tends towards instability, and although the ratio of L/D and L are very greatly reduced, by a reverse curve, it becomes necessary to bear in mind that for aeroplane wing sections the loss in efficiency may be worth whUe, in order to gain in stability. 66 Aerofoils. Only flat and the simplest geometrical sections have been con- sidered here. In aeroplane wings, it is necessary to have spars and ribs of considerable depth, ii\ order to obtain suitable strength and rigidity of wing. This leads to sections of surfaces in which two cur- vatures must be considered — the top face and the bottom face. An aeroplane wing section is, therefore, distinct from geometri- cal sections, and it is customary to refer to aeroplane types of sur- faces as Aerofoils. They are treated of fuUy in Chapter VI, since a knowledge of their characteristics, advantages and disadvantages enables the first essential step in the design of an aeroplane to be taken — the choice of a wing section that will give the weight-lifting, strength and speed combination desired. Examples of the application of curved section pressures are taken up in several instances in the consideration of aerofoils, and many of them are deduced from actual practice as applied in well known types of aeroplanes. Summary. I. In flat surfaces. The total pressure Pa is usually normal to the section, and can be resolved into. Lift = Pa Cos a Drift = Pa Sin a The center of pressure moves forward as the incidence is decreased. II. In cambered surfaces. The total pressiu-e Pi is not always normal to chord, and the pressures on a cambered plane are, therefore, more easily ex- pressed as Lift = Kl S V^ Drift = Kd S V=i In the terms Kl and Kd, the same applies as in the previous chapter, the values given being the experimental ones determined at sea level, where actually Kl = kLd, and Kd = k^d, d being the density of the air. This must always be kept in mind. And at 18,500 feet, d being one half of its value at sea level, Kl or Kq would be one half the values given. The center of pressure, on curved surfaces, moves forward up to a certain low angle where it reverses and moves rapidly to the rear (except for reverse curved surfaces). CHAPTER VI. CHARACTERISTICS OF AEROFOILS. The manner in which air pressures vary on flat and cambered surfaces has been considered fully enough to enable us to proceed with the study of aeroplane wings themselves. As already indicated, the necessity of having spars and ribs of considerable depth for the rigidity of an aeroplane surface, makes it necessary to use a section of a cer- tain thickness, and consequently two curvatures — the top face and the bottom face — must be taken into consideration. Aeroplane wing sections are ordinarily referred to as aerofoils, and the study of the various aerofoils and their characteristics is of very real importance. WhUe a good deal of the data on air pressures has been given by way of explanation and general information, the wing characteristics referred to here are of the greatest practical sig- nificance, and are every day being put to use and verified, on the avia- tion fields of the military world. The connection between the character- istics of a wing section and the operation of a great war would seem remote, but when it is appreciated that superior speed and climbing ability enables a hostile aeroplane to gather information quickly and escape from attack and pursuit, primarily because of the greater effi- ciency of its wing section, the importance of this study becomes ap- parent. The development of wing sections has been along several lines. Originally geometrical sections were made thick enough to give room for spars and then rounded at the edges. Other pioneers, after de- ciding on the size of spar and thickness required, adopted a certain camber for the mean center line, and then proceeded to fill out a sec- tion that would streamline the spars. Still other investigators adopted parabolic and circular curve combinations, crescent shapes, etc., and finally the great laboratories took up the matter and systematized its study. The Eiffel, N. P. L., and Goettingen results are complete enough now to give a very firm basis for aeroplane design, and to en- able the effects of any changes in aerofoils to be quite accurately an- ticipated. In general the features of an aeroplane wing that may be varied are: 1. Shape of Section, curvature, thickness, etc. 2. Shape in Plan^ contour, aspect ratio. In addition, the manipulations of the wing by warping or moving flap sections that are connected to it, modify the pressures, and there are further modifications of the air forces when the proximity of some other wing or body affects the air flow and interferes with the paths 68 .004^ 6' S' /o' fji' 14' /g /a' ><»• -a?" ^4-' -fls" ;ps' Jo' 90- SO" V (o' / ?U/^\i B 7 / / ►5 '=^ / i u 5 ■*' fferi 'KC / 1 ^?' J /CP y ^ 'y ^ ■if *rtve ftlMT. y^ F ' «««»»\ V ^ Distribution of pressure on an aerofoil shows high suction at i chord. Typical Lift, Kl curve and L/D curve. Geneial Characteristics of Lift. The Lifting forces are usually determined by a curve giving the measured data on Ki, for various angles, and also by a curve giving the numerical ratio of L/D. The values of Kl are merely supplied in the formula, L = Kl S V^ as explained, and dividing L by the value of L/D for the corresponding angle, gives us D,* Although the pressures, on the various sections differ consider- ably from each other, there are certain characteristic features that are common to the majority of the aerofoils. At 0° incidence there is usually a certain lift A, and at a negative angle, anywhere from —2° to —9°, there is a point of no lift, H (see above). The manner in which the Lift and L/D curves are plotted, on a basis of angles, is the same as in Chapter V, the Lifts being defined" by values of Kl in the formula. Lift = Kl S V. From A to C, on the Lift cur\'c, is more or less of a straight line, the curve bending over at C, which point is called the point of maximum lift. From C to D, instead of continuing to increase, a critical state of flow has been reached, where further incidence increase is accompanied by a drop in the Lift. This is an interesting portion of the cur\-e, and we will again have to refer to it when we take up the control of the aeroplane in this reversed pressure region. Where lift decreases in this way, it may be stated briefly, that the controls on an aeroplane would have to be reversed for flying in this region. To go up, it would be necessary to reduce the angle of incidence, and to descend, the elevator would have to be pulled back so as to increase the angle. On the L/D curve from the point of F at 0°, the ^•alue of L/D increases to a maximum E, corresponding to a Lift of value B. From E to G, the L/D ratio again falls off. The ordinary' regions of flight are limited to the peak region of the L/D curve. Having, in a general way, considered the nature of air reaction on aerofoils, we may proceed with a study of the effect of alterations in shape and plan form and interference. The values of Kl and L/D in the curves, refer to the combined action of whatever compression or suction is generated, unless otherwise noted. It is also well to re- call that L/D represents the ratio of the Lift force obtained from an *We are consistently usinK resistances, lifts, etc. in lbs. and recording the proper K's for Surface Area S, in sq. ft., and speed in m. p. h. It must be borne in mind therefore that these units must be used, otherwise values of K will be different. For example in Eiffel's metric units, we must multiply his K's by .041, in order to get "m. p. h., sq. ft." units, unless we use S in sq. meters, and V in meters per second. 71 aerofoil at the expense of the Drift D, a resistance that must be over- come. "Efficiency" refers to L/D, and is higher, the greater the Lift obtained for resistance overcome. ALTERATION IN SHAPE OF SECTION. 1 . Camber of Upper Face. Increasing the camber of the top face from 1/40 to 1/6, on a form with a flat under face, shows that the maximum lift increases up to a camber 1/15 and then decreases. The ratio of L/D steadily im- proves up to a camber of 1/20. This camber appears the most efficient, as deeper cambers show a steady decrease in values of L/D. On an aerofoil, having the under face arched considerably, when the camber of the upper surface is increased above 1/15, the Lift hardly varies, while the drift steadily increases with camber increase. For very thick sections, however, just as in spheres and cylinders, there is a critical flow, which, due to increases in speed, tends to smooth out and reduce resistance. 2. Camber of Lower Face. Increasing the camber of the lower face, for a fixed upper face, shows that L/D does not vary very much, and that L increases ap- preciably with camber increase. Since the depth of spar is very greatly enhanced by keeping a flat underside, there is every reason for con- sidering rather flat under surfaces as advantageous. The increased depth of spar reduces the weight of framework in the wing necessary for a given strength, and would about compensate for the lift increase obtained by camber of the lower face. The upper face, furnishes most of the lift and variations of lower face have very little effect on the upper side. 3. Thickness and Depth at Rear. By keeping the same mean curve of a section, and adding to the top and bottom faces at the same time, the drifts are found to remain about the same, and a decrease in lift is found as the section becomes more and more a streamline body — due to the progressive bulging out of the section, both top and bottom. For any particular curve a thickening of the rear alone to permit of a deeper rear spar, shows a decrease in L/D with increase in thick- ness and a slight decrease in Lift, but this is not so very marked, and sections can be deepened at the rear with ease, thus permitting of hav- ing the front and rear spars of the same depth. 4. Bluntness and Streamlining of Nose. Substituting a blunt for a sharp leading edge, causes the ratio of L/D to fall off, but since L remains about the same there is indicated a pronounced increase in D. Bluntness of the nose may, therefore, 72 be considered a disadvantage. Pointing the nose of an aerofoil, to a streamline shape, designed to divide the air easier, often called a "Phillip's Entry," is frequently used. It is advantageous in decreasing the drift slightly at high speeds and low angles, but otherwise has little effect. 5. Changing Position of Maximum Ordinate. The fraction of the chord at which the camber is the greatest is termed the position of maximum ordinate. It can readily be varied on aerofoils, and it is found that L/D increases as it is moved from the center of the surface, or .5 chord, towards the leading edge imtil it reaches the position of 1/3 chord, when further movement forward greatly reduces the efficiency of the section. The Lift is not affected very much at low angles by changes in the position of the maximum ordinate, but at high angles the lift of the section falls off when the greatest camber is at a point in front of 1/3 chord. 6. Reverse Curvature. Reversing the curve of either face of an aerofoil, has a pronoimced effect on the c. p. movement. The lower face of a deeply cambered aerofoil is readily made to reverse at the rear and meet the upper face. This is often done, and is distinctly beneficial. More pronounced reverse curves in which both faces, at the rear, are turned up, have a very great influence on the air pressures. The advantageous feature of having a stationary center of pressure posi- tion for the various angles, is obtained by raising the trailing edge about .037 of chord — the curve starting from a point about .2 of chord from the trailing edge. But in doing this the maximum lift is reduced, and the range of lift restricted. There is also a speedy decrease in L/D, and, in general, this change leads to inefficiency, reducing lift by about 25% and max. L/D by about 15%. 7. Warping the Aerofoil. "Warping" an aerofoil consists in twisting it in such a way as to have the various sections presented to the air at uniformly vary- ing angles of incidence. The section of wing remains constant, and since its characteristics for varying angles are known, the amount of pressure at the different sections could be found. In the ordinary range of warp in practice, on aeroplane wings, where one side is moved up and the other down equally at the same time, the Lift remains the same, as does also the position of the mean center of pressure, and tests show that computations on a basis of appl}dng the ordinary data for the section to the different regions at their various angles gives correct results. 73 r D The Sharp Corners of +he Rec+angle Creole Eddies '--Leading Edge . i ] TheOriginal Shape used by+he Wrights r-^^-Body ^^UadhgEd^ Tapered Wing used on Monoplanes +0 improve+hestreng+h The"V"Shaped Wing has advantages for stability ''- Leading Edge This Shape- Mos+generallyused- Is efficien+and Airworthy Shape of Plane. Cuttixig away the trailing edge at the tips, and rounding off the ends of the plane, is often resorted to for reasons of construction and appearance. It is found that this does not appreciably affect the pressures, and cuttiag away the tips slightly reduces the weight of wing. On the other hand, it is found that raking the ends of a plane or having the trailing edge of greater span than the leading edge, does appreciably affect the pressures, the Drift being considerably reduced and the ratio of L/D im.proved. The gain in efficiency is due to a better utiUzation of the sidewavs flow of air, in escapiag past the edges. Air Si ream Chord] Low Aspect Ratio Span/ ^Qg High Aspect Ratio Span/ = 6 /Chord Aspect Ratio. Referring to our Life charts, we can sum up the influence of Aspect rati J as follows: A high aspect ratio reduces Drift for the same Lift and therefore improves L/D. But a wide wing span makes the trussing of the aeroplane more difficult, and requires more struts and wires, up to a point where the added air resistance of the added struts and wires is greater than the reduce4 air resistance due to less Drift of the wings. So we must compromise, and make the wing wide enough to be efSicient, without having too much head resistance of the supporting bracing. This is where the designer's cleverness comes in, and the difference in judgment as to how to balance and compromise such features causes different types of aeroplanes. 74 In general, however, an aspect ratio of about six is a good com- promise and the wing section data on our charts is for a wing of aspect six. We shall see that the aspect of a biplane can run up to eight, and a monoplane down to four, without spoiling the necessary compromise too much. A convenient way to present data on aspect ratio is given in the accompanying table : Values tabulated are the ratios of L and L/D at given aspect to values for an aspect of 6. -'= ANGLES ASPECTS 3 O 6° 9 L L/D L L/D L L/D 4 .84 .73 .85 .77 .90 .83 5 .94 .86 .95 .90 .96 .92 6 1.00 1.00 1.00 103 1 00 1.00 7 1.05 1.06 1.04 1.10 1.04 1.09 8 1.08 1.09 1.08 1.16 1-08 1.15 We can now refer to our wing section charts, pp. 76, 77. For example, at 3°, the L/D of N° 36 Eiflfel surface is found from the graph to be 14.7 for an aspect of 6, and the corresponding lift co- efficient, Kl is .0014. It is desired to know what the values would be for an aspect of 4. From the table, we find that L/D will be 73% of the value of 14.7, which is 10.7, and the value of Kl will be 84% of .0014, which is .00118. If it is desired to know the values for angles between 3° and 6°, it is easiest to plot the values of 3°, 6°, 9° on the chart, and draw thru them curves entirely symmetrical and of the same character as the ones for the aspect of 6. Effects of Speed and Scale. In stepping from model tests to full-sized machines, the best ap- proximation at present made appears to work out quite well in practice. Lift values, of coefficient Kl, are applied directly without any cor- rection. Friction effects on Drift cause it to decrease with increase of speed, and, therefore, at speeds higher than the wind tunnel speeds, the value of L/D will be greater. The Eiffel results, however, were obtained in winds of 50 to 70 miles per hour and require no correction, and in or- der to bring the other restalts presented in accord, correction for speed has been made wherever necessary. The values given, therefore, may be applied without further correction to full-sized machines, at ordinary speeds, by supplying the values of S and V^. Presstu-es are, of course, functions of V^ of the aeroplane, and the corrections mentioned apply only to the values of Kl and L/D tabu- lated. Pressvues are also functions of areas, and therefore vary as the scale of the model squared. In the wind tunnels pressures are meas- ured in pounds, let us say, and a partictilar pressure on an aeroplane model to 1/10 scale is found to be 1 pound, in a wind of 30 miles per 75 hour. It is desired to know what the foTce. on the aeroplane will be at 60 miles per hour. The observed value must be multiplied by 602 3600 - X lOSor X 100 = 400 pounds. 30^ 900 Typical Sections of Aerofoils. Twelve aerofoil sections that represent a wide variety of actual practice are tabulated. The sections are drawn out all to the same scale, and the center of pressure graph is drawn for a distance of chord equal to that used in the drawings of the sections. This enables a rather more graphic conception to be obtained than has been possible heretofore. The values of Kl and L/D are given in groups of four sections. The graphs look complicated, but they are merely con- venient methods of tabulating the results, and the curves can readily be distinguished with a little practice in reading off the values. Leading Edges Front Spar., ,Rib ,-'Camber of ; Upper face .-Rear Spar '^'Angle of Incidence '-Camherof Trailing Edge--- Lowerface A TYPICAL AEROFOIL SECTION. Among the sections given the Eiffel No. 13 bis, the one used on old Bleriot monoplanes, was a very widely adopted one, and because of its high lift and good efficiency it was one of the few of the older types of sections remaining long in use. Many of the R. A. F. biplanes, the Bristol biplane, several Germaa and Italian aeroplanes, and several bi- planes in this country, used a section of this type. Its most serious disadvantage was the lack of spar room, necessitating either a wide shallow and, therefore, heavy spar, or a lesser factor of safety on a well loaded wing. The efficiency at very low angles was not as good as in some of the newer types of sections, which permit of a greater range of speed though not possessing quite as good a maximum efficiency. It is regrettable that a strict censorship forbids the publication of data on the latest types of wing sections — which differ, however, only in being more efficient. The Eiffel No. 31 section, of crescent shape, is Eiffel's most ef- ficient all-around wing, although its maximum L/D is exceeded by many other sections. The Lift at low angles is very high, and the wing is well adapted for load-carrying aeroplanes. No. 32 Eiffel is essentially a speed range wing, for fast speed scouts, lightly loaded and with high-powered engines. The high value of L/D at low angles is particularly favorable to high speed. 76 S^ Sy 5 ^a 1 i i ^ ^ « 3jf/ja>^fjy ='" cj'Sa'K 77 ^ i V ^ St ■Q vi ^ «y ■t-i J / .^ ^ / 1 o n / / '" / / ( \ / /^, H \ \ /, / <1 \ \ / 1 1 1 / s (^ a ^ ^ Q < v\ / i_ / ^ \ \ / 1 1 'a / T? \\ \ /'' T / t— 18 H-1 S ^ » ■/ / / / ^■ n \ \ _ / y ■So- fe v ■'X X ^ <^ H ^ ^ V T \ 1— ( ^ ^ / /^ ^^ \ ^ . ^ u ^ / / / ' \ s\ \ 5 / / / \\ sN \ o u ( / /» ^ \^ ^ \ \ . 5 I o V / \ \^ \. ^ X \ Cfi T -^ \ ^^N \ ^^ ^ X UJ \ ■>«. S^ '. \ ^ N^ ^ ^ N ^ a J3 "^ V > ^ \N ^ s '- \ ^1 1 1 1 1 ■ 1 '1 f 1 ' i ^ 1 ' 1 — ^ 1 1 T /f r^ - 7 'mun ijbs 'ifi^'M ,,, ' ry C/5 < W CL, \ w H w u h b*) ^- ^ / ^ "^ ^ y f/y n i^ y : f ^H o S} ^ ?> ^ !?. 78 /f/iT/o Of lirr/pff/fT u a, u, o a! w H W O fa O o c/5 O I § 5 € S § / i f , \ X (/ c> ^x '*■ »-V; -^ V V} 5 ^ \ 79 No. 36, Eiffel is used on several military machines, and is a partic- ularly good wing for a medium speed, military scout. The Lift is not run up very high, but the range of angles thru which a high L/D is maintained is favorable, not only to high speed, but also to climb, as will later be explained, when consideration is given to the complete aeroplane as a unit. The Dorand wing, Eiffel No. 35, is similar to the Wright wing, and gives a very high lift, with a high L/D at angles from 3° to 6°. The small thickness of the section, however, does not make this wing very favorable from the standpoint of construction. In general, thinner wings are the more efficient, but spar room is a very necessary element, and efficiency and strength must be compromised. The Nieuport and Deperdussin are two standard wings, the lat- ter designed particularly for racing aeroplanes. R. A. F. 6 is one of the more modem sections that has been widely used. The effect of a reversal of the trailing edge on this section is shown also, and is of interest in connection with flaps on the trailing edge. A new type of section, with a movable rear piece, is also shown, as a suggestion of improvement by the writer. The combination of low Lift and good L/D of a flatter section, at low angle, with facilities for changing to a deeply cambered surface, which would have a high Lift and also a high L/D at larger angles, could be made very greatly to extend the speed range of aeroplanes. In general, it has been found that the values of L/D for most wing sections, at low angles, 1°, 0°, etc., are actually on the aeroplane, much better than measured in the labora- tory, and therefore very much higher speeds are actually obtainable. 1' I WT.c C. P. Moved Forward--^ and CP. K afsamepoinf ! W C. p. Moved Backward-. CENTER OF PRESSURE MOVEMENT COMBINATIONS OF SURFACES After considering the ideal wing section and then its more practical form in the shape of a wing itself, we can quite, logically go to the con- sideration of wings as used and fastened to the aeroplane proper.. When the wing equipment of an aeroplane consists of a single wing, on each side of the body of the machine, the structure is said to be a "Monoplane." The resemblance of a monoplane to a bird is very striking, and when the bird is soaring on rigid wings, the monoplane and the bird are in fact aeroplanes of exactly the same type. 80 But unfortunately we have not yet succeeded in devising a type of construction as strong and as light as the wing of a bird, with its marvelous beams of hollow bone structure. So that while the mono- plane is without doubt the ideal type of flying machine, consideration of the strength and weight involved in the bracing of the aeroplane's wing structure, has led very naturally to the development of combina- tions of wings which offered better arrangements for bracing. It was quite appropriate therefore for a skilled Railway Bridge Engineer, Octave Chanute, to be the first to construct an aeroplane structure in the form of a double deck truss with strut and cross wire bracing, exactly of the type of bridge, known as a Pratt Truss. This combination of an upper and lower wing fastened to each other, rigidly, is called a "Biplane" and is illustrated herewith. The upper wing Is less interfered with than ihelowerwing and remains more efficient THE BIPLANE The biplane structure has distinct advantages over the monoplane, except in the case of the very smallest machines, because distributing the required wing area in the form of a biplane results in reducing the span of the machine, making the chord less, improving the aspect ratio and with a more convenient arrangem.ent for bracing. The superimposing of planes over each other has not stopped at the biplane, but triplanes — three deckers — quadriplanes — and even five plane structures have been successfully used on aeroplanes. In each case the designer had decided that his gain in having less span, better aspect ratio, less chord, etc., fully justified the disadvantages. In the case of large machines, the triplane arrangement has an added structural advantage, in that the length of the struts between the planes is reduced by the bracing effect of the center wing. Other than complication, the essential disadvantages of biplane or triplane wing structures are the proximity of the wings to each other, causing an interference in the air reactions on the wings. 81 A study of the streamline flow about the biplane combination readily shows the nature of this interference, and reference is made to the diagram explaining this. It has already been indicated how the air where it strikes a cambered aeroplane wing, causes a wave of air which divides into a compressed region flowing under the wing, and a very high suction effect on the top of the wing. When both surfaces are brought one over the other, as in the biplane, the suction, region on the top of the under wing and the compression region below the upper wing both find themselves confined in about the same space, so that they react on each other, the compression finding an easy escape in filling up part of the suction on the lower wing. It follows, therefore, that the total air reaction in the biplane com- bination is actually less than the total of the two wings separately, so that an interference has been caused. Of course the closer the surfaces are brought together, the more serious does this become, and therefore a great deal of attention has been given to studying what the proper spacing between the two surfaces should be. H- Chord The spacing betweenihe wings of a Biplane is fhe'Gap". o Stagger Is usually expressed as a fra^fion off he chord. ->\Sfagqer\<- The distance between any two wings is called the gap and in general it has been found that a gap, equal to the chord of the wing is about the best compromise, because if it is made greater the added length and head resistance of the struts and wires, between the planes begins to outweigh the advantages of increased efficiency, due to a wider spacing apart. The actual effects on the air reactions due to this biplane interference when gap equals chord are about as follows: If the aspect is the same, the lift of the combination becomes reduced to 80% of the Lift of the two surfaces separately, and the L/D is reduced to about 83% of the value when uninterfered with. When the gap gets as high as 1.6 X the chord, the Lift and L/D are reduced only to about 90% of their original values. But the biplane in general has a better aspect ratio, and this im- proves the L/D and tends»to balance the interference loss. In fact a biplane of aspect 8 to 1, with a very large gap, has about the same efiiciency as a monoplane of aspect 5. 82 However, since the biplane has a lower lift coefficient it requires more surface area than a monoplane to carry the same weight and the same speed range. So the wings themselves actually weigh more in a biplane, because they are larger and have more fittings, so that the whole matter becomes a compromise in designing which is still causing a great deal of difference in opinion, between some of the best designers of experience. There is a distinct trend to the monoplane in speed scouts because for high altitude work, the monoplane is more eflScient — - due to the fact that its Lift coefficient is higher, and therefore it can actually travel up into air of less density — - because it loses its lifting force less quickly. The biplane or triplane, on the contrary, due to the generally' higher aspect ratio has a lower Drift, and therefore near the ground its speed is likely to be higher than the monoplane, but these very char- acteristics will make the biplane -or triplane land faster. The bracing of the monoplane, however, is difficult and for larger machines the practical constructional advantage is altogether with the biplane or triplane. Stagger There is one way of avoiding the interference of the surfaces of a biplane with each other. And that is to stagger the top surface, ahead of the lower surface, so that instead of being directly over one another, their leading edges in projection are out of line by an amount or per- centage of the chord, which as indicated in the diagram is called the stagger. This feature is also used in designing, in order to improve the range of vision from an aeroplane, particularly' from the front seat of a tractor. And backward stagger is likewise used on small scouts, to improve the upward view of the pilot and his machinegun range. But forward stagger, distinctly gives a gain in efficiency, and when it amounts to as much as one-half or fifty per cent of the chord (which would be called 50% stagger), then the lift is increased by about 8%. ,-\° Angle of incidence Ihis is a minus decalage of 5-becausefheiop wing is S°less incidence than ihe lower wing. ''6° Angle of incidence DECALAGE We have up to now been considering that the angle of incidence of all these surfaces was the same. But this is not a necessary require- ment. In fact, the stability of a biplane is Balanced Rudder By moving the hinged portions up or down, the air flow is modified, so that the fixed and hinged portion together form a surface in which the camber and also the angle of incidence are altered, to give greater or less lift. This is illustrated herewith, and it is obvious that it is in- correct to consider the change in angle of flaps alone without considering the flow of air in its relation to the fixed surfaces, to which the flaps are hinged, be they wings, stabilizers, or fins. It is also important to consider that this very relation of flap sur- face to its fixed surface may readily result in blanketing the control power of the flap if the angle of incidence of the fixed surface in front of the flap becomes so great that the flap is working in disturbed, swirling air, which having no direct flow over the flap, gives very indeterminate pressures. The determination of the distance of the elevators and rudders from the main planes, and the functions of the stabilizer and fins are considered later. Let it suffice here for us to understand how a movable flap, fastened to a fixed surface, gives a desired variation in Lift. So that with the usual disposition on an aeroplane — the wing flaps out at the ends of the wings, when moved one up, the other down, simultaneously, will roll the aeroplane laterally — the elevator flaps, permitting the machine to be pitched up or down longitudinally and the rudder movement swerving the machine to right or left, directionally. Pulley forCdble., Wing fid p.-, (Aileron) On-some Aeras wing flaps are on the upper wing only THE " STICKXGNTROL THIS IS USED ON LIGHT. FASTMACHINES AND HAS BEEN ADOPTED. LARGELY.BECAUSE OF ITS SIMPLICITY AND INSTINCTIVENESS. Theoffsi^ position shown dotted.wQuld correspond to pulling Elevator Flaps up foraclimb.and ,ot the 5ametirr,e banking the machine upon the Itftsidewitii the foot bar pushed over for aright hand turn "' Elevator Flaps tfS> BALANCED CONTROLS All controls are not necessarily always of the flap and fin type, although this is now the most usual. The control surface may be a pivoted unit in itself, the angle of incidence of the entire unit being varied, as shown in the accompanying diagram. If the pivot is at the leading edge of the surface, the surface is practically a flap, without any fixed portion. In this case, as in the case of a flap, the air force is behind the hinge axis, and there is a tendency for the surface to trail in the air stream at 0° angle of incidence. But this very characteristic means that in order to move the control surface, hinged at its leading edge, it is necessary to overcome the air force created by the movement to a larger angle of incidence — because the center of pressure of this air force is behind the hinge axis. Obviously, though, if the hinge axis were in the line of the center of pressure, this force would be almost eliminated. And greater ease of operation of controls might make this desirable. Where a surface is pivoted somewhere near the center of pressure, so that the air forces balance, as indicated on the diagram, the control surface is called a "balanced control." Due to the movement of the center of pressure forward, when the angle of incidence is increased, however, a point may be reached where the center of pressure moves in front of the pivot, and the surface then "catches" the air in a very undesirable manner, tending to remain offset, because the air pressure in front of the pivot is greater than the air pressure behind. PARTIALLY BALANCED CONTROLS So we again have a compromise, and the unbalanced flap hinged to a surface, which resists being offset to higher angles of incidence, is so constructed that it has a balanced portion, illustrated p. 18, not great enough ever to permit the air pressure in front of the pivot to be greater than the air pressure behind the pivot; but of enough consequence to so greatly reduce the forces required for control, that this partial bal- ancing makes it possible for huge 20,000 lb. aeroplanes to be controlled, readily, by the touch of the pilot's hand. Angle gradually reducing on this side--— WASH OUT 90 WASH OUT AND TORQUE CORRECTION For reasons of stability, and particularly in rendering the wing flaps more sensitive, the angle of incidence of the wings is frequently reduced very gradually from the body out towards the tips. This may be as much as 3°, where the angle of incidence at which the wings are fastened to the body axis is 5°; at the tips the wings will only have an incidence of 2° This reduction of angle is called the "wash out." A wash in, an increase in angle towards the tip, is frequently given to one side of the machine, in order to overcome the turning tendency of the torque of the propeller. If the aeroplane has no grip on the air laterally, instead of the propeller turning, the propeller might remain stationary, the path of least resistance being for the body to turn. Pro- peller torque is very powerful, on high powered scouts. And to over- come this, it is merely necessary to give a greater angle of incidence and therefore greater lift to the left side, if the propeller is turning clock wise from in front and vice versa, the machine thus tending to rotate the same way the propeller is rotating, thus overcoming the opposite rotation, which the torque reaction would produce. THE PROPELLER AND ITS POWER We have thus considered the general disposition of the wing surfaces, stabilizer and controlling surfaces, as they are grouped around the fuselage, all of which in tum. are mounted on the landing gear. There is still to be considered the source of the power that over- comes the Structural Resistance, the Drift of the wings and the Drift of the control surfaces — nam.ely, the propeller, whirled rapidly by the marvelou.sly light power plant it is fastened to. To be very frank, propeller theory is in a highly unsatisfactory state, and it is the one element of the aeroplane that tends to resist calculation, and to retain queer whims and characteristics, which lead to "cut and tr}-" methods. It is a fact, however, that in delivery of actual power, air propellers are generally far more efficient than marine pro- pellers. And the reason is that we have ceased altogether to consider the air propeller, as a screw propeller, in designing it. Instead it has very sensibly been realized that a propeller is virtu- ally a small aeroplane moving in a rotating path. The thrust of the propeller is nothing different from the Lift of the aeroplane wing, and the Torque resistance to turning, which the motor overcomes, is nothing less than Drift plus a great deal of friction. The element in the propeller proposition that is confusing is the fact that the blade travels in a spiral path instead of in a straight path as does the aeroplane wing. The accompanying diagram explains this, 91 and the air flow, as it ' is drawn into the blade from the front and the periphery, is then discharged at much higher speed in a swirling cone, the air on fast propellers, usually discharging at speeds as high as 200 miles an hour. That is why it is so important to have a smooth, sym- metrical body near the propeller, in order to reduce vibration. Around the hub of the propeller the air flow is very confused and the actual pulling power of the blade is not great near it, so that the blade shape is largely determined by the amount of material necessary for proper strength to resist enormous centrifugal and bending stresses set up by the rapidly turning blades. It is in the region of the tips that most of the work is done. The propeller blade has a cross section, not unlike that of a cambered wing. The blade itself has a certain surface area, and it attacks the relative air with a certain angle of incidence, just like an aeroplane wing. In addition to that, the aspect ratio of the blades is important just as if it were a wing. Since the blade as an elementary wing is traveling in a spiral, the angle of the blade with relation to the axis must increase as the hub is approached, in order, in one revolution, for the relative travel of any blade portion through the air to be the same. This travel of the blade through the air would of course be the same as the pitch travel of a screw, if the air were solid. And in fact the angle of the blade to the plane swept by the propeller is called the pitch angle. g5St»^y!SSS^^v.s:f:^ygg O/n ^^A THE TOTAL PRESSURE ON THE PROPELLER, P, MAY BE DIVIDED INTO L AND D, AS DONE ON AN AEROFOIL, BUT WITH THE DIFFERENCE THAT THE ACTIVE FORCE L IS REALLY THE THRUST FORWARD AND THE DRAG D, IS THE TORQUE, WHICH IS OVER- COME BY THE MOTOR. Any point on the blade actually travels through the air a distance equal to its circumference. But being set at an angle, it moves forward at the same time. So the combination results in a spiral path, and the actual distance between the beginning and end of this spiral depends on the angle of the blade. 92 Now the horizontal distance, parallel to the propeller axis, to which this spiral is stretched, is the distance that the propeller would have pulled the aeroplane in one revolution, if the air had resisted like a solid. This distance is called the pitch. If a propeller 8 feet in diameter has a pitch of 6 feet, we have this proposition. The tip, 4 feet out from the center, in each revolution, covers a circumference of 12.56 feet. And in doing this the advance per revolution would be 6 feet. So the pitch angle if shown graphically would measure about 283^°. This is the pitch angle of the blade to a plane perpendictilar to the axis. But as a matter of fact, the air does not act like a solid. It "slips" so that the propeller most of the time screws through the air only about 80% of the pitch distance, because of the "give" of the air. The use of this term pitch is really confusing as it takes no account of the angle of incidence of the blade, and in reality means nothing more than to help in describing the blade. It also does not take into account the fact that the whole body of air around the propeller is being carried and pulled ahead by the aeroplane, so that it sometimes happens that an aeroplane travels faster than the pitch of the propeller x the r. p. m. of the motor (which gives the speed of advance in feet per minute). The answer to unsatisfactory propeller theories is to conduct full size experiments, and particularly in Europe a great deal of this testing is done. The data obtained records the actual power given out by the propeller for the motor power put into it. Due to the friction loss, the loss in overcoming drift of the blade and the unsatisfactory air flow, the propeller gives much less power in actual thrust than is delivered to it by the motor. The ratio of Power delivered by propeller ^. _. „ ^,.. . ^-^ — = the Propeller Efficiency Power of motor If the aeroplane were traveling as fast as the air thrown back by the propeller, the efficiency would be 100%. And when the propeller is working but the aeroplane is standing still, the efficiency is nothing at all. In between these two limits lie the practical values, and there is given opp. p. 104 a typical efficiency curve of a tested propeller, which gives the per cent, power of the motor, which the aeroplane can get in pro- peller thrust at the forward speeds of the aeroplane given. It is apparent that in general the slower the speed the more inefficient the propeller, although there is a falling off at higher speeds. This is because the blade begins to work at too low an angle of incidence to the relative air passing it. Various designs have different efficiency curves, and the theory in- volved is too unsatisfactory and elaborate for us to consider here any- thing more than the actual results. 93 It is well to bear in mind that the actual thrust in pounds, T, is obtainable from the thrust horse power of the propeller H. P. at any speed V, in feet per second, by solving T = H. P. X 550 + V. And for actual flight, T in pounds must be a little in excess of the Structural Air Resistance plus the Drift of the wings and tail. In general, there is only one very good proportion of diameter, pitch, and r. p. m. of propeller for the best results, and on slower machines there is a great gain in efficiency by the use of large diameter, geared down propellers. The nearer the ratio of pitch to diameter approaches 1 the more ef&cient is the blade. But for very fast machines, gearing down is not so necessary and propeller speeds of as high as 1,800 r. p. m. work out very efficiently. BALANCE OF THE AEROPLANE The subject of the balance of the aeroplane, although touching on the realm of the designer and engineer, must be understood funda- mentally by the aviator in considering the aeroplane's elements. The forces acting on the aeroplane, when it is in flight, must all be in equilibrium, and they are as follows: C. G. First, we have the center of gravity, aibout which all rotations take place and it is most important to know where this is. C.P. Then we have the center of Lift acting at the center of pressure at the particular angle of incidence used. The center of Lift is usually behind the c. g., so we have a slight negative pressure on the stabilizer. C. R. The center of head resistance is the point about which all the little structural air resistance forces balance. This center is important to consider, particularly when the power is shut off, because, as an example, if the C. R. is below the C. G. — then when the aeroplane speeds up on a glide or dive, the increase in resistance acting below the c. g. will make the machine dive still more — a dangerous condition. C. T. The center of thrust is the center of the propeller; The thrust is usually a little below the center of head resistance and the c. g., and therefore introduces a couple, tending to pull the machine to a higher climbing angle when the power is put on, and permitting the machine to head into a glide, when the power is suddenly shut off. The various effects that changes of these centers have on stability we will consider later, desiring to understand here merely what the various forces are. In a very quick acting machine all of these centers are brought very close together, and the system is then called "centres confondus." On the next page is a diagram showing all these forces. 94 rTx;, i>^ 1 "0 \i // */ csa: 1 X 3o \ M\ ^ ^ !\ O^D M' ^ J) , H 1— \l-/ \\\ ' ' ^4! 0-- n s?o 1 \ I i \ 11 1 *C ^:=^==4f^ o 0)0 ^Jt ^ "05 U CD / X /' 1 if ^r *' J^ f fc' ^ O *!^ K ^ r ' a • 7k ;» 4s S' es^ 4o is' "TO 7S' eo as- 5»>tta5 IN M. R M. RESISTANCE CHART The heavy line Curve is the Total Re- sistance to motion of the aeroplane, and is the sum of the Drift Curve (light line) on which are indicated the angles of in- cidence, and the Structural Resistance (light line). On the right of the chart is a scale for reading the gliding slopes, represented by the Glide Curve (dashed line). PBW£R ReOUJRED A«B AV/ULABIE ^0 i eo .'--- -■ — — t ,, / ^ „.k N-- u^ — r y ^ ^-y t'^ '/• C Co m ^r>*^ f ^ • :^ 1 N li^"- ^ ts. .<^ ^ ■-r / i"^ ' I ^^ [ -' y ?• «^ \ [ ^/ N v- — ^f- — ^ I -^ V y ^ 4" - Id er "ir /mJix-iW c^'. 'Xho^-^ .^ — f' ^ "C^ ^/ <^ V J ,/■• u / /^' y ^ 3 •>,<, -^ / c / ia ■fis sot gm uu im iK. P. n I) li 9 5 r? o X < Speeds ="d '''"■>m.i imic Jut /*« 'MO ENGINE CHART The heavy line is the Curve of horse- powers delivered by the engine, for dif- ferent values of the revolutions per min- ute. The light line is a Curve indicating the results of tests on the amount of gasoline, consumed by the engine at var- ious r. p. m., the scale of gallons per hour, being given on the right. Sc : 70 H-^o f^,?.« c-C* 1*^^ ^-^ ^ X / ^ f / ,nM& /o iitde tf ■frttss a iJAO * kc4^ ftn^ion MOMBffr VMCflAM A' fmr/Anr cRPannw - J6%, sofnurl ynyj ea-nm ic %'f Itiat). fTcAK CKfllsinM=s^ \ ^ So retrr-fross carrtes \lZ° o'l '''^'f*^ '°"* wmMumm CMOKPs JTfr: SUFFfO: SKTIM y^/TT /f/iL/' 5FAN OF AempiMe STRESS ANALYSIS FOR BIPLANE TRUSS The loads at the connecting points U, U', U", called panel points, are indicated on the diagram, and are due to the air load on the wings. The heavy line wires are the "flying wires," taking the stresses due to these loads; and the dashed lines in the truss diagram are the "landing wires," taking the weight of wings on landing. In the graphical stress method, each panel point is considered in order, and for each one a closed triangle or polygon of forces is drawn. The force polygons must all close, since the point is in equili- brium. First, a "sense" of rotation for the diagram is chosen and indicated by the arrow as clockwise and a scale to which to lay off the forces is chosen. Then, on the truss diagram, the regions between forces are lettered A, B, C, etc., the forces considered being only the forces carrying the truss load. That is why the compression in L"U" is not considered, since it is carried to U" and from there over the truss. Taking the first point U", we have the force between A and B, called ab, = 205 lbs. total, and the force of compression in U"U', called be and a third force, the tension in the wire ca. Thus, there are three forces at this point. The magnitude of one is known and the direction of the other two, so that a force triangle, as given on the stress diagram abc, may be drawn, such that ab = 205 lbs. to scale, be, is parallel to U"U', and ac is parallel to U"L'. Their point of intersection establishes the closing point of the tri- angle, thus determining ac and be in lbs., merely by reading their lengths to the same scale to which ab was laid off. Panel point L' is now taken, the forces being taken in the same order going around the point clock- wise. First, we have ac, already solved and then cd, the strut compression, the direction of which we know, so we draw cd thru c, parallel to U'L'. To obtain the rest of the polygon it is now necessary to consider ea, acting upwards at L', which is laid off on the vertical, and then to close the polygon the other force line de, may be drawn thru e, parallel to L'L, the point d being located by the intersection of the lines of action of the two unknown forces thru e and c. Thus, with d found, cd and de are readily read to scale. A similar process is employed for the rest of the truss. CHAPTER IX. STRESSES AND SAFETY FACTORS. Having "flown to the aeroplane's ceiling," we may proceed with a consideration of the next feature — the study of the required strength of construction of the machine. While the consideration of the stresses caused by flying is really a study for the aeronautical engineer, our understanding of the funda- mentals of aviation is not complete without a clear idea of how the air loads on the wing surfaces take hold of the main weights. In gliding, the lifting forces on the wings are slightly less, and in climbing slightly greater, than in horizontal flight, but only in a small degree. When attacked by sudden puffs, the air forces are in- creased in various ways; banking on turns introduces extra stresses, due to the centripetal force, and in various maneuvers such as a sud- den recovery from a steep dive, looping the loop, flying with full power at very high angles, etc., additional loads are imposed on the structure of the machine, which must be withstood. Safety Factor The ratio of the breaking strength of any structural part to the load imposed upon it, is termed the safety factor of that part. Thus, if a wire requires a tension of 3000 lbs. in order to break it, whereas the load it carries is only 300 lbs., it is said to have a safety factor of 10. In ordinary engineering practice, the load that it is considered necessary for any part to carry is taken as the maximum load that the particular part will ever have to stand, and, in designing it, a safety factor is applied to this maximum possible load. Contrary to all good engineering practice, the structural parts of an aeroplane are gener- ally designed to have a certain "safety factor," with reference to the normal fl3^ng load, determined by the weight of the machine. The excess stress due to some additional maneuver is taken account of in the "safety factor" itself, so that in the engineering sense it is not 110 a safety factor at all, but merely an allowance for extra stresses, in- duced by conditions other than ordinary horizontal flight. It is pos- sible to estimate what the maximum possible stresses are, and to deter- mine whether or not the aeroplane will collapse when they are im- posed. And in general an aeroplane is so designed that the strength of its weakest structural part will at least be great enough to with- stand a reasonable value of this maximum stress, without breakage, the real safety factor being very seldom as much as two. In most other branches of engineering a safety factor of at least ten is required. The object of a safety factor is to provide against the increased stresses of sudden impact shocks, which are difficult to estimate, and to take accovmt of defective material and workmanship, so that, at first sight, it would seem odd that intelligent engineers should permit this gen- eral conception of "safety factor" in aeroplanes to survive, thereby apparently still further increasing the dangers of aviation. It is useless to deny this element of danger, or to attempt to excuse it, on any ground, excepting that it is a well considered compromise of opposing features. An aeroplane, constructed with a high safety factor, on the maxi- mtun stresses to which it can be subjected, would actually prove so poor and dangerous a flyer and so difficult to land, due to its enormous weight, that ever-present dangers and limitations in its operation would far outweigh the possible dangers of its not being quite strong enough to stand some very unusual and remote maximum stress, to wliich in the hands of a well informed aviator it would never be subjected. The justification for building aeroplanes as light as possible, and cut- ting down to the limit of simplicity and necessity all the structural features, is exactly what makes a well-built aeroplane one of the most refined of engineering structures. The fact is only too often lost sight of, that increasing the strength of an aeroplane for ffight, by thicker spars and struts, heavier wires, cables and larger fittings, immediately reqiiires a landing gear much heavier in proportion, all of which results in a very much heavier machine, which for the same flying character- istics will require a more powerful engine, not only heavier in itself, but requiring more fuel, larger tanks, etc., until the final result is a machine in which the higher safety factor is largely lost by greater stresses due to the increased weight — with nothing gained. In aero- plane engineering there seems to be a remarkably nice balance be- tween flying capacity and limitations of strength due to allowable weight of machine. And the degree in which strength has been gained by lightening up a machine, thereby improving its fljing capacity, is a better criterion by which to judge of an aeroplane. Ill Maximum Stresses. The greatest source of danger in flying, due to imposing great stress on the wings, is, without question, given rise to in flattening out sharply after a long dive. Modern aeroplanes have compara- tively low structural and drift resistance, and when pointed earth- wards the gravity force of the weight is opposed only by the air re- sistance of the machine, so that in diving steeply the aeroplane read- ily acquires a velocity through the air very much greater than its maxi- mum high speed in horizontal flight. If, after acquiring a great speed, due to a steep dive, the aeroplane is turned, to flatten out and fly hori- zontally, a centripetal force must be exerted on the wings in order to make the turn. For any given radius of turn r, in feet, an aero- plane of weight w, pounds, having acquired a speed thru the air of V feet per second, will have to have exerted upon it a force equal to wvV32.2 r (see p. 30), in order to flatten out at this rate. As an ex- ample of the magnitude of this force, let us take the case of an aeroplane, weight loaded = 2000 lbs., which dived a few hundred feet and ac- quired a speed of 75 miles an hour (110 f. p. s.), and which the pilot rather quickly flattens out by turning up on an arc of radius = 100 feet — a quick recovery to be sure, but not at all unusual. The centri- petal force exerted on the wings, is, wv'^ 2000 X 12,100 = — = 7520 lbs. gr 32.2x100 a stress almost four times as great as the weight of the machine. The magnitude of this force for greater speeds and sharper turns would seem enormous, but there is a definite limit, since, if this force, which makes the machine take a curved path, exceeds the maximum pressure corresponding to the angle with the highest K of the wing surfaces for the particular aeroplane speed, the aeroplane will "slip" and refuse to take this curve, since the air pressure on its wings cannot be made greater than the maximum pressure. It becomes quite easy then to determine the limiting stress. The maximirm speed attain- able on a glide is the one for which the air resistance becomes equal to the weight of the machine. This limits the speed of falling. A simple way to estimate it is to determine from the Resistance Chart, the minimum value of the quantity K S in R = K S V^. Then sup- plying this same K S, and R = Weight of machine, a solution is ob- tained for V^, the maximum diving speed. Thus, it is found, p. 96, that at 85 m. p. h., on the Resistance Chart, R = 365 lbs., and V^ = 7225; it follows that K S = 365/7225 = .0505. The assumed total weight is 1800 lbs., so that 1800 = .0505 V2, and V = V 35,600 = 189 miles per hour. 112 The maximum value of K for the wing (about .003), would indicate that if the machine after diving several thousand feet vertically, could suddenly be turned up, the wings would "bite" the air with a force K S V2 = .003 X 335 X 189^ = 35,650 pounds, which is almost twenty times the weight of the machine. This is the limit that is approached, and it is clear that the lower the head resistance of a machine and the greater the surface and weight, the greater does this become. On the other hand, the greater the longitudinal moment of inertia, the more difficult does it become to flatten out sharply. In tulming, the additional force on the wing, caused by banking the machine, and required in order to hold the machine to the turn, may be determined in the same way. Other excessive stresses, such as those induced by sharp upward puffs, are not as easily evaluated, but careful observation indicates that the forces of sharp puffs, or sudden changes in wind direction, may easily give stresses three to four times the weight of the machine. Although the stresses in the main wings are the most important ones, the other parts of the aeroplane also are subjected to great pres- sures. The effects of sudden maneuvers, or of gusts, in snapping the tail around, not only introduce great pressures on the tail, but subject the fuselage to severe twists. The proper proportioning of parts to resist vibration, due to variations in the engine and propeller, is almost entirely a matter of experience. And the stresses introduced by landing shocks are a separate class, requiring careful considera- tion and much experience, to be properly taken care of. In taxi-ing on the ground on some aeroplanes with tail skids, enormous twist- ing stresses are induced in the fuselage, by sharp turns, that every careful pilot avoids as much as possible, since all such stresses are un- necessarily racking and fatiguing the aeroplane structure. The maximum stresses in an aeroplane may become very large but, in the hands of an expert pilot, they can be kept under control. Supported in the most perfect pneumatic fashion imaginable, and operated with skill and caution, an aeroplane is not likely to receive impact shocks of dangerous magnitude, and at the present time a break- ing strength factor of 5 times the stresses due the weight on a very large slow machine, and a f3,ctor of 8 on small agile, high speed machines appears to compromise all opposing features properly and to give a suffi- cient "safety factor" for military purposes. Kinds of Stresses. In an aeroplane, distinction can be made between six different kinds of stresses : (1) Lift stresses on the wings due to the lifting force equal to the weight, and carried by the main struts and wires. 113 (2) Drift stresses on the wings, taken account of by the inte- rior cross-bracing of the wing. (3) Stresses on the control surfaces, transmitted thru the frame or fuselage of the aeroplane. (4) Stresses on various small items due to their air resistance. (5) Stresses induced by the pull or push of the propeller and secondary effects of gyroscopic action or vibrations on the engine bed. (6) Landing stresses on the entire machine, due to the shock of alighting. In view of the variable nature of landing fields and of air conditions near the ground, estimates of these stresses are difficult to make, and are largely a matter of experience for any particular machine. As a general rule a strength of 6 to 8 times the actual weight on these parts will give enough strength for practical use. The thrust of the propeller is the largest single air force acting at any point on the machine, and necessitates proper distribution over the frame. But it is definite in magnitude, and easily taken care of. The consideration given stresses here, is not for the purposes of design, but rather to enable the military aviator more readily to under- stand the information on stresses supplied by the manufacturer. The most important stresses are occasioned by the load lifted on the wing structure. Stresses in the Wings and Bracing. Since the consideration and method of determining the lifting stresses in the main supporting wings may be extended, readily, to other stresses in the machine, it may prove beneficial to take up an example. The process of determining stresses consists, of (1) Finding what proportion of the load is carried by different parts of the frame; (2) Determining what stresses these loads induce in the mem- bers of the framework. Since a biplane involves practically every feature requiring con- sideration, we may take as an example the aeroplane assumed in Chap- ter VIII, in which the full load weight is 1800 lbs., the surface area 335 sq. ft., chord 5 ft., gap 5 ft., and span 36 feet. Let us assume that the bracing is of the familiar strut and cross-wire type usually termed a "Pratt Truss." 114 A moment's thought on the manner in which the air force on the wings lifts the rest of the machine, will lead to the simple conception that an aeroplane is virtually a swing bridge, turned upside down, ■with a uniform static load of the simplest kind, equal in average in- tensity to 1800/335 = 5.4 lbs. per sq. ft. (a factor often termed the "loading" of the wing). The complicated stress determinations for steel bridges resulting- from "live loads," such as moving locomotives of 300,000 lbs. weight, are happily in another realm, and as for the actual consideration of the aeroplane structure itself, it is well to real- ize that it is the simplest kind of a bridge. For the purposes of this example reference is made to only one- half of the machine, since the other side is symmetrical, and it fol- lows that the upper and lower wings under consideration together carry half the load. The load actually carried by the structure is the total weight less the weight of the wings themselves, since the latter pressing down b}' gravity directly against the air pressure, relieve the struts and wires of having to transmit any stresses due to their weight. If the weight of the wings is taken at 240 lbs. — a reasonable figure — the load on the side of the machine we are considering equals (1800 — 240) -^ 2 = 780 lbs. This is the distributed load over the upper and lower wings. But, due to the biplane effect (Chap. VII), the upper wing may be expected to carry a considerably greater proportion of this load. In general, on a biplane the upper plane carries about 60 % of the load and the lower plane 40 %. From this it is indicated that the upper plane on one side, carries 780 X .60 = 468 lbs., whereas the corresponding lower plane carries 312 lbs. This load is transmitted by the cloth covering to the ribs, each one of which, acting as a beam, transmits the load to the spars, which in turn are suitably braced to the body by struts and wires, so that lbs. weight in the body are carried by lbs. per sq. ft. air pressure on the outstretched wings. But, since this is distributed between the sparF,of which in this case there are two, it follows that separate stress determinations must be made for the front and rear truss. This at once necessitates determining what portion of the load each spar carries. The position of the center of pressure determines this readily, for if the c. p. were midway between the two spars, obviously they would each carry half the load, and if the c. p. were directly in line with a spar, the entire load on the wing would be carried by it. Since the c. p. moves, and we are here interested in the maximum stresses due to carrjdng the weight, the next step is to determine the max. rear 115 position of c. p. applying the greatest load to the rear spar, and max. front position for the front spar. This is done (p. 108), and from what information we already have on the aerofoils and the aeroplane, we may recall that the former condition corresponds to a high speed and low angle of incidence, and the latter to a slow speed and high angle of incidence. Since the data indicates that the rear spar carries a maximum of 75 % of the load at 0° incidence, it follows that the upper plane rear spar, which spans 16.5 feet, carries (468 X .75) + 16.5 = 21.3 lbs. per foot run, and the lower plane rear spar, carries (312 x .75) -=-15.5 = 15.1 lbs. per foot run, — the spans being taken to include allow- ances for the rake and reduction of pressure of the ends of the planes, and for the body section. Knowing the spans we can, as has been done on p. 108, indicate the load at ea,ch panel point U, U', U", etc. This load, which is the force carried thru the truss, results from the uniform loads on adja- cent spans. For example, U' carries half the load on span UU' = 21.3 X 3.125, plus half the load on span U'U" = 21.3 x 3.625, which to- gether give 144 lbs. The other panel loads are obtained in the same way, and since the slopes of wires and depth of truss are outlined to scale, the graphical method explained, p. 108, is readily made use of to determine the stresses in the members of the truss. The tension stress on any wire, as determined in the stress dia- gram, may be compared directly with the breaking strength of the wire, to determine the safety factor. Thus, if L U' indicated by dg, as having a stress of 730 lbs., consists of two 5/32" cables each with a breaking strength of 3000 lbs., the "safety factor" is more than 8. The strength of struts is not as readily found, since struts usual- ly fail by bending. Only in the case where a strut is very short and thick is it possible to find its strength by multipljdng the compres- sion strength of the material in pounds per square inch by the cross- sectional area. Failure from bending makes it necessary to introduce standard engineering formulae*, which vary greatly among them- selves and are largely based on experiment. Their object is merely to determine a reduced value of the allowable compressive strength of the material, to take into account the weakening due to bending. As an example, spruce, ordinarily, stands 5600 lbs. per sq. inch in direct compression, whereas one of the most practical strut formulae taking into account the average dimensions of aeroplane struts, reduces this * These formulae and data are ordinarily furnished by the manufacturer, and if need be are readily checked by actual breakage test on a strut. A typical formula is the RAF strut formula. FA Crippling Strength^ ■ — , in which F = allowable compression stress, 1 + 6500 IVk A = area of section 1 = length of strut in inches, andk = least radius of gyration. 116 to about l/5th, giving 1100 lbs. per sq. in., as the ultimate strength to be expected. If U'L' is made of spruce, with 2.3 sq. in. cross section, it may be expected to have a strength of about 2.3 x 1100= 2500 lbs., and since the stress induced is 310 lbs., there is a safety factor of 8 (see p. 108). .Frorrh RearSpar Wing cutout for movable flap jt'\ hinged to rear spar Strut Rib-, for Drift gracing The constiuction of a wing showing the spars to which ribs and cloth covering transmit the load, and also showing the Drift wiring. The stresses in the Spars may be divided into the following elements : (1) The stresses, due to their part in the general bracing of the wing truss as found by the stress diagram, p. 108, which indicates at once that as members of the rigid truss the lower spars are sub- jected to tension and the upper spars to compression. (2) The stresses, due to the loading of the spar as a beam carry- ing the air pressure loads transmitted by the wing covering and ribs. X The result of the application of these stresses to the spar may be taken up as follows : (a) Compression or Tension Stress in Spar. — The allowable breaking load in lbs. per sq. in., for the particular material used, mul- tiplied by the area of the cross section of the spar in sq. inches, gives the breaking strength, which, divided by the load as determined from the stress diagram, determines the factor of safety for that stress. (b) Bending due to the pull of wires of the frame, attached un- symmetrically with reference to the neutral axis of the spar. This feature on some machines is of considerable magnitude, but fittings are so readily made to bring the pull of wires, etc., all together at any one point symmetrical with the beam's center line, that they should 117 Simply ji/ppoktej) Cl -A Sections OF B£flM5 UH U4JU MonENTS ef INEHTIA^I. nNcenTK/^m low Max.B.M.^ fj/^ M/ix.Pen.^ Mak. 8M= M/)X. PEFL a Jf UmFCKM LOAD */ lbs. per/aei run MAjr.&M. = PJ Mw pen- "^/rj I^Vm OF ^YKATIOU, r = Vi//\ . *. To FitJp r, piyipe r»B MOMCfrr cf ManiA OF THE SeCTUN BY T»e CItOSi StCTItU AKFA . b4^ JlZ Bending Moments and Sections of Beams THESE AKE UliEMCD^ C IS /{AISED Bf TICiHTENlf/e, CF,Ce;^Ca, EflUALiY. Diagrams for Alignment. Assembly and Alignment by Cross Distances. The several steps in the assembly are indicated by referring to the sketch above. To begin with, the center section of wing over the body is set over the body on the four small struts. The first step in alignment is to make this center section parallel to the body and centered over it. Since the body is lined up, and the section aff'a', is a parallelo- gram, it follows that the cross distances, indicated as af', may be made equal in order to center up. When this is done for both front and rear trusses, the center section is bound to lie parallel to the body axis, provid- ing, of course, the distances were measured between symmetrical points. It then remains to adjust the front and rear wires until the section has been pulled forward or back, so that one measurement f"a agrees with the similar one on the other side of the body and with the data on the machine. But these wires should not be tightened up until the wings are on, in order to give play for the spar fittings of the wing section. Unless the center section is somewhat near centered, however, difficulty will be found in fitting the rest of the wings. The next step is to fit the lower wings on either side to the body, and to hold them up by means of their landing wires, fastened to the proper fittings at aa', but not tightened up. The top wings next to the body are then fastened to the center section and held in place by hand until the struts de, d'e', are inserted, when the landing wire ae, a'e', will hold both wings in place. If the wings have no dihedral and the fittings are symmetrical, the distances ae and bd, should be equal and can readily be made so, by tak- ing up the landing wire on both sides, front and rear. This will then give the proper setting laterally. If a dihedral is employed, there will be differences in the measurements, ae being shorter than bd, but for proper, alignment it is merely necessary to have similar wires on the other side, the same length. Of course, it is assumed that the struts and the size and position of fittings on the spars are unalterably correct. The outer sections may now be put on by the same method, the lower one first, held by the landing wires, and then the top one, supported on the struts, and the cross distances made equal similarly by taking up on the landing wire. The entire wing structure is now assembled, attached to the body, which is resting on the chassis. It is assumed that, laterally, the wings 130 are symmetrical to the body and properly transverse thereto. This is readily, checked by measuring the two distances, ah and cd, as indicated. The cross wires running from front to rear between the struts are next adjusted, just to tautness, and the alignment of the struts as viewed from the side is checked by eye. Measurements of these cross distances from top front to lower rear, and lower front to top rear, at the body, are then carried out to the tips, and thus the angle of incidence is checked. The "fl}ang wires" are then all tightened up, just so as not to give the landing wires more than the strain of carrying the weight. Final meas- urements are then made from the rear point of the tail to panel points h, h', out at the ends of the wings, in order to determine if the transverse wing axis is symmetrical with the longitudinal body axis. The machine is then lined up correctly — providing that the distances measured are all taken to some points on fittings or marks on spars and struts, that are absolutely symmetrical for the two sides. Alignment by Sighting. The process of assembly, as outlined above, would be the same. After the wings are attached to the frame, the trueing up process proper begins. The method consists merely in doing by eye what was done in the previous example by extensive measurements. The first sight is taken, from below the body, up to the center section, so as to get the points a, a', over the points b, b'. Both sides, front and rear are sighted and the positions averaged up by the wires. In assembling, however, no lining up is done, until all the wings are on, held by the landing wires. The observer then stands at s, to one side, and sighting along the top plane, establishes the line across the bolt heads or fittings at a, a', and proceeds first, to bring up d, d', by means of the wires a e, a'e', and then g g', on either side, are brought up by taking up their landing wires until they are in line with a, a', d, d', etc. In other words, the transverse line, across the top of the center section, is projected to either side. The same is done for the rear spar, and then the load wires for the front spar only are tightened, just enough so that when the point h, for example, is al- ternately raised and lowered no wires are seen to slack or sag, — the alignment held by the landing wires being the correct one to be held. The final and important element in the sighting is to establish the correctness and uniformity of the angle of incidence, which is the main object of the alignment. To do this best, the observer stands 15 to 20 feet in front of the fuselage, taking care to center himself, by sighting along the center struts, shaft, axle center, tail piece, etc. The observer then chooses a height, or tilts the machine, so that, when sighting along the top plane, he can see just a little of the under side. This permits him to see a certain point of the rear strut sockets showing against the lower side of the front beam. Then, by holding the head, central, and just high enough to see these points, and moving only the eyes, to right and left, he can note any lack of alignment of the rear spar, parallel to 131 the front spar. This is corrected by means of the rear spar landing wires, if necessary, after which the rear load or flying wires are also tightened. The fore and aft cross wires are now set, so that when standing 10 feet or so from either end of plane aU struts will He in Hne and parallel with each other and with the center struts. These wires are then set no tighter than necessarj^, for if too tight they merely tend unduly to com- press and buckle the ribs. A check on the perpendicularity of the transverse wing axis to the longitudinal body axis is then made by measurement, and if necessary, adjusted by the "drift" wires running from the nose of the machine to the front intermediate struts. The tail pieces are then lined parallel to the wing axis, by merely sighting and adjusting them until they are parallel. It is well to sight from behind and below, so as to get the taU Hne just below the front edge of the top plane. The last wires to be tightened are any auxiHary wires from the chassis to the wings. This method, in the hands of one who has had some experience, is the quickest, easiest, and accxu-ate enough for field work. A judicious combination of the sighting method and the method of measuring cross distances, gives the best results in the aHgnment or trueing up of aeroplanes. Particular attention is caUed to the systematic manner of doing the aKgning with the landing wires, leaving the tightening of the "fljnng" wires to the very last thing. On the diagram, a note is given relative to the itnportance of loosen- ing up the proper wires when a local adjustment of one panel point is niade, on a machine already aU wired up. j Propeller Diagram and Balancing stand. 132 Propeller Balance. After the machine is assembled and lined up the propeller may be mounted, but before doing so its balance should at least be checked up. A propeller "out of balance" is heavier on one blade than on the other, and when run on the engine will vibrate. Any vibration of this nature is, really, a severe strain on the machine, and particularly on the engine. A propeller may also be troublesome in vibrating if the blades are warped, and lacking in symmetry. This may be checked up by measurements of offsets on the blade. The accompanying diagram shows a method of propeller balancing that is effective, and also shows the manner in which the useful data on the shape, section and angles of the blade may be presented. If the propeller is slightly out of balance, a little more varnish on the light side is the best way to equalize it. Metal tips along the entering edge of the tip of the blades are a great protection against both water and shrubbery, to prevent cracking and splitting of the edge of the blade. These, however, must be very firmly attached and because of the centrifugal force should be made as light as possible. For water work, it is necessary to bore a few small holes in this metal tipping, in order that the water, that has soaked in by impinging so hard, may be freely thrown off by centrifugal force, instead of tending to work in under and finally to split open the metal tipping, and for land work such holes will prevent "dry rot." Attention should also be given the propeller bolts to make sure that they are properly proportioned as to thread, that the nut fits and shows no sign of having been forced, and that the bolts are properly locked by a wire, which is not likely to be cut by the nut of the bolt "backing off." Details of Construction. Examination of the details of construction, to make sure of the proper fitting of parts and "follow thru," is most important, and special training in the proper inspection of machines is next in importance to training in flying. No matter how well built or how reliable struc- tural features appear to be, there is always the possibility of breakage. Just because an aeroplane has flown very successfully is no excuse for being any the less careftd in inspection of its construction. It is well, first, to go over the entire machine and make sure that all the bolts are locked, and while doing so the material of the bolt, whether special steel or "commercial" iron bolts, should be examined, and also the thread of the bolt, and fit of the nut and its locking. If iron bolts (stove bolts) are found, with deep threads, in places taking any vital stress, they should be replaced. Bolts may be locked in four ways : 1. By a lock washer, or cut washer, fitting under the nut and "biting" into it when the nut turns backwards. 1. Control with cables and pulleys on ball bearings. — 2. Same with friction leads. — 3. Detail of rubber shock absorber bridge. — i. Steel Spring chassis, with central skid. — S. Softer rubber chassis with no skid. Both of them are typical chassis for exactly the same work. — 6. Fuselage details. — 7. Details of wing frames, ferrules and lumber. 1, 2, 3, 4. Various single and double pulley arrangements for control cables. — 5. _ The Curtiss double U bolt fitting. — 6. The Burgess clip fitting. — 7. The Curtiss single U bolt fitting. — 8. Signal Corps, pin and plate fitting. — 9. The steel block and eye head strut bolt fitting used on German aeroplanes. — 10. The Wright hook fitting. — 11-12. Hinge details. 133 2. By a pin, or lock wire, passing thru a hole drilled into the bolt, and fastened in such a way that vibration will not permit "backing off" of the nut, to break the locking wire. 3. By riveting the head of the bolts. This is the most positive lock. 4. By painting the bolt head. This is suitable only where a small, relatively unimportant fitting is concerned. The practice of "spoiling the thread" of the bolt for locking is not a reliable one. Knowing the comparative strengths of various bolts in shear and pull, as outlined in the table p. 140, the inspection will intelligently reveal the uniformity of "safety factor" and "follow thru." After attending to the bolts, the pins in the fittings and the turn- buckles may be examined at each panel point, one by one — the pins for proper locking, unless already riveted, and the t. b.'s for the purpose of making sure that enough threads are ever3rwhere engaged in the barrel and that the t. b. is, in each case, locked so that the safety wire will not wear or tend to break at any point. The general inspection of the wires, struts and remainder of the machine can then be made, special attention being given to the controls, so as to make sure that they are connected up to work properly, and that all t. b.'s and pins are suitably locked, with no possibility of a cable binding by running off its pulley, or of parts of the control "catching" anything. To assist in the detection of flaws in construction, improper propor- tioning of parts for a uniform strength and "follow thru," and for gen- eral information on the construction of aeroplanes, some tables and data are presented. It is perhaps necessary to state that the strength values are largely based on tests and experiences of the writer relative to aeroplanes, and may be taken as at least a beginning of a handbook for Aviation, to which new data of value should constantly be added. The illustrations of details of construction, with examples of ap- parently rehable and unreliable features, should receive particularly close attention from mihtary aviators. The small variety of details shown must, of course, be taken as serving merely as examples, since no attempt has been made to present all the structural features that might be found on a various assortment of types of aeroplanes. In order to avoid the inconveniences of cross reference, notes relative to the various features have been incorporated on the illus- trations themselves, and should be read and digested as carefully as any emphasized text. 134 Steel. Steel is obtained from iron by many processes, differing in ore treat- ment, expense, etc., the most extensive ones being Bessemer, open- hearth and crucible. All refer to the original method of obtaining the steel, and have little bearing on the quality of the steel, excepting in the amount of carbon, alloy, etc., in it. There are many instances, however, of Bessemer process steel proving less reliable than the others. The crucible process is used to obtain the most uniform tool steels. The percentage of carbon in steel largely determines its hardness, strength and ductility, and ranges from .05 % to .25 %. The higher the carbon, the harder, more tenacious and less ductile is the steel. The lower the phosphorus or sulphur, the less likely is the steel to develop flaws and cracks. The word "temper" is used by manufacturers to represent the amount of carbon in steel. Thus, a "high temper" steel is a "higher carbon" steel, and therefore hard, tenacious, but brittle. Steels may be "tempered," after manufacturing by applying various degrees of hard- ening and softening — that is, most uniform steels can be made as hard and tenacious, or as ductile and soft as desired. "Hardening" is done by heating the steel — • with particular attention to uniform heating of the metal — and then quickly immersing in brine, oil or water; the amount or nature of this qviick uniform cooling, or of the heat to which the steel was brought, being determined by the kind of hardening desired (all of which requires personal skill and experience) . "Softening" of steel is designed to make its texture more uniform, easier to manipulate, and less brittle. This process is termed "anneal- ing," and consists merely in heating steel up to a desired temperature and then letting it cool very slowly, the slower the cooling the softer the steel. As in any other treatment of steel, uniformity of heating or cool- ing is of the utmost importance. Practically all high grades of steel come from the mills in annealed condition, but if not, and if it is de- sired to bend the steel sharply, great care must be exercised in heat- ing it in a forge for annealing to make sure that the steel is uniformly heated, otherwise its grain and texture will be uneven and weakened. In this connection, it is important to point out that steel has as marked a "grain" as wood, only not as easy to see. Steel is always weak- est across the grain. Alloy steels, by various heat treatments, can be made to give various strengths, but increased hardness or elastic limit is almost always obtained at the expense of ductility. In the annealed condition, which, because of the reduction in brittleness, is desirable for aeroplane work, steels do not show much variation. A table is given of the strengths -Fuselage Longeron Rubber Cord, "i ihock obsorber\ holding skid j by a wire J Two way wiring taking flying load li-Steel ^■f/PlaieXlip' Bolts holding fitting to spar 1 ;. "^''Tvv^'-^ ..t, a ,' ■-'^ ^■i ':;:h-4' , ^ ■ife p P^' IC^\M W^^^^^"^ The engine mounted on its bed. This whole construction must be kept clean and tightened up and frequently examined for fittings strained by the severe shocks of engine vibration. Shackle- Cable Loop-' , Welded Steel Arms '.Held by thru Bolts (■■■Strut Longeron--^'' ^ , r-j_L- ^ Fuselage Fitting Landing Gear Strut- Grease Cup---._ Wheel Hub Axle Rubber Cord Shock Absorber' Collars to hold Rubber Cord in place The landing gear is a very light construction that roust be kept in perfect alignment to develop it strength. No wires must be over tight and no fittings pulling oflE at eccentric angles. The wheels mus have plenty of grease and the tires at least 50 lbs. of air. As much as possible the spokes should be watchec for equal tension, and the shock absorbers and axle slots examined for wear after every few flights. The pins holaing the wheels on the axle should also be examined as they are most important, anc if they sheer oiT due to wear, a wheel will run off the axle and cause a disastrous smash. SUtchei.. ■ Clotty held to ribs by sewing Section of Rib wifhCloth sewed on >-r~i Control Arm Typical control lever arm. The thru bolts should be examined for rust. The wing construction — showing the doth covering over the ribs and spars and the Drift wiring, which should be examined from time to time, as it is inconveniently out of sight, but very important, particularly on an aeroplane used for "stunting." 135 of various grades of alloy steels, and the elongation or per cent that any length will stretch before breakage is given, and is an indication of the ductility. In aeroplane work, it is essential to have the maximum of reliability, and since local thoughtless heating may have robbed a "special" steel of its special qualities, it is the best practice to proportion all parts for a ductile, easily bent, mild carbon steel, with the strength given in the table. Then, if any advantageous alloy like Vanadium steel is used, its greater resistance to fatigue is an added and much needed safety factor. The copimercial names of "tool steel," or "drill rod" (bars of tool steel), refer to a specially uniform and reliable grade of rather pure steel, particularly adapted to being heat treated, tempered and hardened for special tool purposes. Tool steel and drill rod, in annealed condition, are good, mild steels for bolts, pins, etc. Bolts, pins, tumbuckles, and particularly wires and cables, may often be of heat-treated special chrome nickel or vanadium steel, and care must be taken not to heat unequally any of these parts, and thus reduce the added safety factor they furnish. This is particularly important in the case of steel wires and cables, in which the material and method of drawing of the wire have been designed particularly to give a high tension strength, which any local heating, for the purposes of bending or attachment, may very seriously weaken. For example, a tension brace of a particularly fine grade of piano wire, received undamaged from the manufacturer and properly put into place, may be relied upon to give its average tested breaking strength. But let this same wire come into long contact with a torch flame, being used to bend, solder or braze some fitting, and it may well have been reduced in strength to one- third of what it is supposed to be. Cold rolled steel (abbreviated c. r. s.), which is used so largely in aeroplane work, in fittings, ferrules, clips, etc., is steel that has been rolled out to the sheet or bar in question, but in doing so the grain of the steel becomes more marked. This steel is harder and more tenacious than mild annealed steel, but works very easily and has splendid wearing qualities. Bends in c. r. s., however, should not be made too sharp, and when plate more than 1-8" thick is used, care should be taken to anneal before bending, or else to bend slowly in a vise in which the jaws are protected by thick copper pads, to avoid nicking the plate. Other Metals. The table on p. 140 gives the strengths and weights of other metals, but they are rarely used in the parts of an aeroplane carrying the main stresses, excepting the bronze barrels of tumbuckles. Aluminum should never be used in any important fitting, and its alloys, though at times exhibiting remarkable characteristics, are almost 136 as unreliable as aluminum itself. Many of them, Jiowever, are ad- vantageously of use in castings, sheet metal coverings, etc., requiring a metallic construction, but carrying no great stress. Duralumin has very nearly the strength of mild steel, in spots, and is somewhat more weather and water-resisting than any of them. Aluminum sheeting should never bei used on coverings in sheeting of less than l-16th inch thick, as it eventually flakes and cracks. Tin and copper are used for the ferrules of wire joints and for tankage. "Monel" metal, an alloy of about the same qualities as mLld steel, is extensively used on metal fittings where particular rust resisting quali- ties are desired. Crystallization and Fatigue. The wearing down of the resisting qualities of a material by constant vibration and jar, is a familiar phenomenon met in practical engineering of all kinds — so much so, that a certain "life" is assigned to metal parts, after which their strength is considered unreliable. This should be followed in relation to aeroplane metal fittings, but a great error is made in attributing so much danger to "crystallization" in the failure of parts, since the vibrations on aeroplanes are neither sharp nor excessive. "Fatigue," is the destruction of the resisting qualities of a material by repeated strains of bending or twisting, exceeding what the "springi- ness" of the material will stand, as illustrated by the ease with which a wire can be broken by repeated twisting or a steel plate by repeated CABLES niMBU JPUCfO CABU ne jniAt/Ds /mt ipkud MP riltiP W/THMLPf/J 16 FOKM APlUi;. 7» PCVCIOP FULL WiKl jrKfNdTH Pf-t^ Of V^iKt , ISO " . too" ■ oeo " ■ OiS" LcNcm or f'e/riruLB Wlf^ES Hard Wmc CiOi AND rm mj' £V£N (i LoMii ^ \ SOLDtn HAi ntlMlO in FOLLY BeHD 'FEKKVLt roe iHoni mine Mm 81 scKAmi Cable and Solid Wire Ends. 137 bending. It is of the utmost importance, then, to make sure that the structttral details are not such as to permit the pull or flexing of a part to result in bending or twisting strains on details not suitably made for them. Attention to some examples of this is given in the illustrations of structural details. In the construction of military aeroplanes it is desirable to eliminate brazed and welded fittings as much as possible, not only because of the added difficulty of replacement, but because a welded joint does not always reveal a possible flaw to the naked eye, and, though apparently satisfactory, might actually prove dangerously inadequate for its stress. Practically all aeroplane fittings may be made of simple and effective steel plate clips, as light and as strong as more "refined" and elaborate arrangements — refined only in that they are harder to make, replace, and pass on. Aeroplane Woods For use in the construction of aeroplanes wood has peculiar virtues, one of the best of which is the ease with which flaws can be detected. In this connection, it is a great mistake to paint wooden parts on aero- planes, since varnish, or "dope," will give as good preservation and yet bring out clearly in evidence any defective features. Among the woods used in aeroplane work attention may profitably be given to Spruce, Ash, Maple, Hard Pine, Walnut, Mahogany, Cedar and Hickory, strengths and weights of which are given in the table. Spruce, of clear silver grain, straight, smooth and free of knotholes or sap pockets, is the lightest, strongest and most generally satisfactory material for aeroplane construction available. It must be properly ferrtded, where fittings are attached, however, to prevent splitting. As a material for spars, ribs, struts, etc., it gives a splendid combination of flexibility, Hghtness and strength. Ash is springy^ strong in tension, hard, and very tough. Its weight, however, is considerably greater than spruce, which, when properly ferruled, can for the same weight be made stronger than any other wood. Maple has excellent qualities, in strength and reliability, for very small wood details reqturiiig imusual resisting powers — like the blocks connecting rib pieces across a spar. Hard Pine is a tough, tmiform wood, particularly applicable to members like the "longerons" of fuselages (longitudinal members). Walnut and Mahogany are used extensively on propellers, their uniformity in finishing and ^strength giving excellent results for this purpose. Cedar is often used as planking of hulls, or fuselage covering, is readily obtained in the boards, and quite tmiform and easily worked. 138 In this connection, fuselages, particvilarly "monocoques," are some- times made of veneers, or glued layers of wood, with the grains crossing for added strength. Tulip wood, bass wood, cedar, alder and mahogany, are used for veneer covering work. There are innumerable trade makes of "veneers," some of them very satisfactory in aeroplane work. Hickory, which is tough and springy, and with a hard surface, is a favorite material for skids, control levers, etc. For the preservation of wood several coats of spar varnish, or of aeroplane dope, should be used, after an original "filler" of oil or shellac. Laminations in wooden members are designed to make splitting of the member more difficult by having different layers of wood with the grain running in opposite directions, glued firmly together. Weather- ing, however, is apt to affect the glue and open the laminations, and it is good practice to wrap the members with linen or paper, or to freshen up the paint or varnish from time to time. The wrapping of wooden members with linen may be made to increase the strength against splitting, if the linen is wound very tight and treated with "dope" or glue in such a way that it will forcibly tighten up. The "dope" should be renewed from time to time. Due to the necessity of ha\ing a certain least width to a strut, so that the ratio of the length of a strut, 1, to its least width, r, will not exceed by too great a margin the 1 /r of 45 that engineering practice prescribes as a limit, wooden struts, particularly of spruce, are better than steel or any other material, — for the saving in width and therefore head resist- ance of a stronger material, would sacrifice strength against bending. In a wood strut, however, inspection must carefully be made for any signs of initial bending, as the use of improperly seasoned lumber, or some eccentric load, may give a bad bend. This of course greatly weakens a strut, and becomes particularly important to watch for in the case of light, interior fuselage struts. Experience in being able to pick out good lumber and detect flaws is of great benefit, and should in a measure be acquired by any aviator who is interested enough in his machine to desire assurance as to its strength. Wing Covering. The general practice in wing construction is to cover the rib and spar framework with an air-tight cloth, giving a smooth finish to the surface and some degree of resistance to deterioration by exposure. 139 Rubbered fabrics were used for several years, but it was necessary to tighten them by hand in stretching on the frame, and the cloth would sag in dry, sunny weather, and tighten in damp weather. An improvement in covering was made by the adoption of fine, unbleached linen, which is stretched rather loosely on the wing frame, and is then treated with "dope." "Dopes" are of several kinds, but they are almost all cellvdose or "collodion" compounds, some soluble in ether and some in aceton. "Cellon," "Novavia," "Emaillite," "Cavaro," "Titanine," are but a few of the trade names, all with some particular virtue — some fireproof, others lacking in bothersome chemical odors, but all designed to accom- plish the same purpose, i. e., to tighten up the linen on the frame, and after a few coats, applied with a brush, to give to the surface a smooth, weather-resisting finish. Skill in applying dopes and various "formulae" for the processes, give varying degrees of finish, but in general four or five coats of a tightening solution, followed by three coats of a thicker finishing solution, will give a good finish. It is customary to varnish this covering with spar varnish, after the dope has set, but, in view of the difficulty of patching and "re-doping" over the varnish, the advisability of this practice is questionable. To clean most doped fabrics, some soap and water will be found better than anything else. The linen fabric used for this covering is woven in the customary way with "warp," the yarn running lengthwise, and "weft," the yarn running across the cloth. Good aeroplane linen should test to a tension of at least 50 lbs. for 1-inch width strip of cloth undoped, and should be difficult to tear and rip. When doped it should show a strength of at least 70 lbs. per inch. Cloth with a fine thread is not quite as strong as cloth with a coarser thread, but the latter absorbs very much more "dope" for a good finish. Aeroplane linen, doped to a good finish, weighs approximately 0.10 lbs. per sq. ft. of svu-face, inclusive of tape or batten rib-covering and varnish, for both top and bottom faces of a surface taken together. WEIGHTS AND STRENGTHS OF METALS Weights Elastic Limit Ultimate Modulus of per cu. in. Tension Tension Compression Shear Elasticity Steel c. r. s 283 35,000 50,000 50,000 40,000 29,000,000 Steel, piano wire 280,000 300,000 30,000,000 Aluminum 096 10,000 15,000 12,000 10,000 11,000,000 Duralumin 103 29,000 45,000 50,000 40,000 Tin 265 3,000 3,500 6,000 4,000 4,000,000 Brass 310 20,000 25,000 30,000 30,000 9,000,000 Mn. Bronze 319 50,000 50,000 80,000 70,000 14,000,000 Copper 320 12,000 20,000 30,000 20,000 16,000,000 All strengths are in lbs. per sq. inch and averages. 140 WEIGHTS OF SHEET METAL B&S Thickness Steel Gauge Inches lbs. per sq 2 .258 10.5 5 .182 7.4 8 .128 5.24 10 .102 4.16 12 .081 3.30 14 .064 2.62 16 .051 2.07 18 .040 1.64 20 .032 1.31 22 .025 1.03 24 .020 0.82 Tension of c. r. s. steel plat e in lbs. pe Aluminum Brass or Copper bs. per sq. ft. lbs. per sq. ft. 3.59 2.53 1.79 1.42 1.13 0.89 0.71 0.56 0.45 0.35 11.6 8.2 5.8 4.6 3.65 2.90 2.3 1.83 1.45 1.14 0.91 Bearing strength of wire in plate = diara. wire X thickness plate X 50,000. STRENGTHS OF VARIOUS GRADES OF STEEL Elastic Ultimate Kind of Steel Limit Strength Elongation Softest Low Carbon Steel 25,000 45,000 28% Commercial Mild Carbon Steel, annealed 35,000 55,000 20% Chrome Nickel Steel, annealed " 55,000 80,000 25% Type "D" Vanadium Steel, annealed 67,000 100,000 26% Chrome Nickel Steel, tempered 134,000 150,000 15% Type "D" Vanadium Steel, tempered 195,000 210,000 10% STEEL BOLTS Single Diam. No. of Threads Diam. at Tension Shearing Inches to the Inch Root @ 50,000 @ 40,000 1/8 40U. S. St. .092 320 256 3/16 32 U. S. St. . 147 880 704 1/4 20 U. S. St. . 185 1,350 1,080 1/4 28 A. L. A. M. . 205 1,650 1,320 5/16 18 U. S. St. . 253 2,500 2,000 5/16 24A. L. A. M. .271 2,865 2,292 3/8 24A. L. A. M. .321 4,050 3,240 1/2 20A. L. A. M. .435 7,500 6,000 5/8 18A. L. A. M. .553 11,900 9,520 3/4 16A. L. A. M. .669 17,650 14,120 1 14A. L. A. M. .907 32,500 26,000 STRENGTH OF MILD STEEL RIVETS AND PINS Diam. Lbs. Strength Inches 1/8 3/16 1/4 5/16 Double Shear 1100 2400 4400 6900 Diam. Inches 3/8 1/2 3/4 1 For single shear take 1/2 loads given. Lbs. Strength Double Shear 9,500 17,600 39,000 70,000 CABLES Diameter Inches No. of Wires 1/32 R 7 1/16 R 19 1/16 R flexible 3/32 R . .091 MS. 7/64 R.. .118 MS. 1/8 R... .138 MS. 5/32 R. . 3/16 R.. .158 MS. .209 MS . 1/4 R. 19 19 19 19 19 19 Wt. Lbs. per 100 ft. 0.35 0.96 13 R = "Roebling" MS = "Morane Saulnier" Breaking Strength in Pounds. 200 50,0 400 899 1000 1400 2100 2300 3000 3000 3600 4000 6000 8300 141 SOLID WIRES Breaking Diameter Gauge Wt. lbs. Strength in Inches or descr per 100 ft. Pounds. .032 20 R .264 225 .040 19 R .436 340 .051 16 R .718 540 .055 ASW .78 530 .064 14 R 1.13 830 .065 ASW 1.21 680 .080 ASW 1.80 1000 .081 12 R 1.82 1300 .090 ASW 2.26 1300 .100 ASW 2.90 1500 .102 10 R 2.91 2000 .130 ASW Van. 4.50 3000 .250 ASW Van. 16.00 5000 R and No = gauge RoebUng. ASW = American Steel and Wire Co. STEEL TUBE TABLE Outside Area of Wt. per Diam. Section foot Moment of Rad. of Lbs. Tension Inches Thickness sq. in. length Inertia I Gyr. r. @ 30,000 1/2 20 ga. .051 .17 .0014 . 165 1,530 1/2 1/16" .086 .30 .0021 . 156 2,580 3/4 18 ga. .108 .37 .0067 . 248 3,240 3/4 1/16" .135 .46 .0080 .244 4,050 1 20 ga. .106 .36 .0124 .341 3,180 1 1/16" .184 .63 .0203 .332 5,520 1 1/8" .344 1.17 .0336 .313 10,320 11/4 20 ga. .134 .45 .0247 .430 4,020 1 1/4 1/16" .233 .79 .0412 . 420 6,990 1 1/4 1/8" .442 1.50 .0708 .400 13,260 1 1/2 1/16" .282 .96 .0730 . 509 8,460 1 1/2 1/8" .540 1.84 .1287 .488 16,200 1 1/2 3/16" .773 2.63 .1699 .469 23,190 2 3/16" 1.07 3.63 .4431 .644 32,100 2 1/2 1/4" 1.77 6.01 1.132 .800 53,100 3 1/4" 2.16 , 7.34 2.059 .976 64,800 STANDARD GAUGES Diameter of Amer. Steel & Wire Go's No. of Gauge Birmingham Brown & Sharp United States Gauge 00 .380 . 36480 .34375 .3310 4 .238 . 20431 .23437 .2253 8 .165 . 12849 . 17187 .1620 10 .134 . 10189 . 14062 .1350 12 .109 .08081 . 10937 .1055 14 .083 .06408 .07812 .0800 16 .065 . 05082 . 06250 .0625 18 .049 . 04030 . 05000 .0475 20 .035 . 03196 .03750 .0348 22 .028 . 02535 .03125 .0286 24 .022 .02010 . 02500 .0230 Birmingham, used for steel tubes; B and S for sheet metals. WEIGHT AND STRENGTH OF WOODS Weight in Tension Extreme Kind of Wood Lbs. per cu. ft. Strength Fibre Stress Ash 50 14,000 6S0O Bamboo 22 6,000 1000 Cedar 28 5,000 3000 Hickory 48 13,000 7000 Hard Pine 45 12,000 11,000 6000 Mahogany 51 7000 Maple 46 10,000 8000 Oak 52 10,000 6000 Spruce 32 10,000 5600 Walnut 42 9,000 5000 AH strengths in lbs. per sq. in. 142 AREAS AND VOLUMES Triangle. — Area equals one-half the product of the base and the altitude. Parallelogram. — Area equals the product of the base and the altitude. Irregular figure bounded by straight lines. — Divide the figure in triangles, and find the area of each triangle separately. The sum of the areas of all the triangles equals the area of the figure. Circle. — Circumference equals diameter multiplied by 3.1416. Circle. — Area equals diameter squared, multiplied by 0.7854. Circular arc. — Length equals the circumference of the circle, multiplied by the num- ber of degrees in the arc, divided by 360. (Useful for tanks, partly filled.) Circular sector. — Area equals the area of the whole circle multiplied by the quotient of the number of degrees in the arc of the sector divided by 360. Circular segment. — Area equals area of circular sector formed by drawing radii from the center of the circle to the extremities of the arc of the segment, minus area of tri- angle formed by the radii and the chord of the arc of the segment. Prism. — Volume equals the area of the base multiplied by the altitude. Cylinder. — Volume equals the area of the base circle times the altitude. Pyramid or Cone. — Volume equals the area of the base times one-third the altitude. METRIC CONVERSION TABLES 1 kilometer = 0.6214 mile 1 meter = 3.2808 feet 1 centimeter = 0.3937 inch 1 sq. meter = 10.764 sq. feet 1 sq. centimeter = 0.155 sq. inch 1 cub. meter = 35.314 cub. feet 1 liter = 0.0353 cubic foot 1 kilogram = 2.2046 pounds 1 mile = 1.609 kilometer 1 foot = 0.3048 meter 1 inch = 2.54 centimeters 1 sq. foot = 0.0929 sq. meter 1 sq. inch = 6.452 sq. centimeters 1 cub. foot = 28.317 liters 1 U. S. gallon = 3.785 liters 1 pound = 0.4536 kilogram Typical streamline wire used in the aeroplane bracing — threaded at each end for adjusting the tension, and fastened with a universal forked end joint to prevent crystallization. CHAPTER XI MARINE AEROPLANES. Hydro-aeroplanes and aeroboats involve all the features of aero- planes that we have considered, and in flight, whether land born or water born, no distinctions can be drawn. But in the replacement of landing wheels by watertight pontoons for flotation, there is intro- duced an important feature worthy of special attention. Because of the continuous and broad expanse for alighting, and the generally smoother air conditions, large .water courses offer par- ticularly practical inducements for flying, whether it be for the pur- poses of coast defence and naval operations, or for travel and sport. And for preliminary instruction in flying there are many who hold — and justifiably — that flying should first be taught over water, be- cause of its greater safety, more uniform conditions, and continuous facilities for practice in alighting. The general care and maintenance of aeroboat hulls, or pontoons, differs in no way from that of high-class boats, excepting that in be- ing hauled out and in, with more or less abuse, the light structure neces- sary is apt to suffer rather severe wear and tear. The necessity of strongly braced construction, the best of lapped and copper-rivetted planking, the elimination of metals liable to rust, the use of the proper wood and its protection, so as to avoid water soaking, protective keels and coating, all with a minimum of weight, are but applications of good motorboat practice. In aeroplane features there are many incompatibilities, but few are more formidable than the proposition of making an aeroplane a seaworthy boat, or conversely making the latter light enough to fly. It is this difficulty in "material" more than anything else that has made Naval Aviation apparently progress more slowly than on land. A few brief notes are presented here, so that the military or naval aviator may understand the mechanics of water- flying machines suffi- ciently to detect difficulties in balance or "planing," and be able to judge of the suitability of various units of flotation for any particular machine. Air Resistance. Attention should be given to having as little disturbance as pos- sible to flying characteristics, by the addition of pontoons. Of ne- cessity, the floating members must be low, and being bulky, more or less additional air resistance is introduced. The addition of this weight, so low, appreciably lowers the center of gravity. 144 Pontoons have, generally, a considerable expanse of side surface, which by being low and at the front, brings the directional center of a surface forward, and also introduces large fin effect below the c. g., a condition ordinarily giving serious lateral instability. Both of these features must be cared for, preferably by adding a fin at the rear and giving a slight extra dihedral to the wings, or by rebalancing the ma- chine, unless the design was originally made for water flying. Refer- ence is made to Chap. XII, on the significance of these features. The difference in resistance of pontoons and wheels is not nearly as great as commonly supposed, excepting at cabre attitudes or large angles of yaw. Some values of K are given on p. 146 for several dif- ferent pontoons. When the fuselage and hull are combined,* as done in the aero- boats or flying boats, efficiency in fljang may actually be gained by the elimination of the resistance corresponding to the chassis. Al- though the seaworthiness of this type is not necessarily greater or less than other types, the compactness of design and gain in efficiency that may be obtained by placing the crew, motor, etc., in the hull, — which of itself has the proper strength and form to serve as the fusel- age — is considerable, and the entire craft becomes more boatlike in design, passing from the "aeroplane with floats," to the "boat with wings." Flotation. In order to support the weight of the machine on the water, the pontoons or hull must displace 1 cubic foot for every 62 to 64 lbs. of weight. The number of cubic feet necessary for the total weight of the aeroplane, loaded will then represent the volume of the pontoons or hull "under water." The center of flotation (merely the center of this volume) will be under the center of gravity. The total available amount of flotation, for any kind of prac- tical use, should be, at the very least, two and one-half times as much as this, and the subdivision of the pontoons or hull into water-tight compartments, is as necessary for reasons of safety in flotation as it is to prevent any water that has leaked in, from acting as a shifting ballast to the detriment of the flying qualities. The distribution of the flotation used and the extra flotation pro- vided must be such that there is : 1. Ample flotation at the rear of the c. g., in order to prevent the craft, when at rest, from being blown over backwards by a wind from the front. The amount is largely a matter of experience but depends on the size and height above the water of the wing surfaces and air-resisting parts. * Several years ago the author proposed this feature, and was the first to put it into actual practice in his aeroboat, publicly exhibited in 1912, after months of pioneer experimenting. 145 2. An excess of flotation forward, to give plenty of lift over on- coming waves, and to prevent upsetting by a wind under the tail. Ordinarily ample flotation is given forward, because of the necessary forward position of the pontoon for hydro-planing. 3. Sufficient flotation on either side, to prevent side gusts from pushing a wing into the water, the construction of the wings being so fragile, ordinarily, that contact with the water may result in dam- age. This side flotation is usually obtained by using either a twin- float or a three-float system, the latter consisting of a large central float and smaller side floats placed on the tips of the wings Even where twin floats of large size are used additional floats on the wings are some- times fitted. The provision for excess flotation, as indicated, is of the utmost importance, since high winds out on the water exert a most power- ful force in tending to upset the craft when it is at rest, drifting or anchored. When anchored in a severe storm, it has often happened that the wind blowing on the wings has lifted the entire craft bodUy out of water, upsetting it. In this connection the feattire of folding back the wings, when on the water, is a particularly advantageous one. Hydroplaning. The action of a surface at an angle of incidence, moved in water or "hydroplaning," is the same as "aeroplaning" in air, in that a Lifting Force is generated at the expense of a Drift or Resistance. The plan- ing surface on pontoons or hulls is obtained by suitable conforma- tion of the bottom, the sides of the hvdl causing this action to be very similar to the action of a surface of the "wetted" area and shape of the bottom on which the water is impinging when immersed. The various shapes of the bottom of aeroboat hulls, or pontoons, arched, flat, or double concave "V" aU appear to have very nearly the same hydroplaning power in lifting force. The contours and dis- position of these planing surfaces, however, differ greatly in efficiency. In getting under way, the marine aeroplane ploughs thru the water as a displacement boat for some time, until the speed thru the water becomes great enough to cause the hydroplaning action of the hull to take effect, after which, as the speed increases, the "planing" surface lifts more and more of the hull out of the water, at the same time reducing its own surface and resistance. Meanwhile, the wings are acquiring speed enough relative to the air to acquire their lift, and, finally, the amount of surface "planing" on the water is reduced to a fraction of an inch, and the speed of the wings thru the air being sufficient for support, the craft leaves the water. In the acquirement of flying speed on the water, the greatest power is required at just that stage where displacement travel ceases and "hydroplaning" be- 146 /C'. 00137 %MW^ flOTTEm FABKE "Fiorrrun TriLicK" SIHC,Le POVTOaH ANO Jiff FU/ftS ^ i>ouBLE Pornoot/ FLAT PorroM CftWitH B6V1I FLAT Bem>M eujvr Jaw BOW fiitumrr FlMr HyPKOPLAUCS Al ■ TIEK OFJTCIB t aikikdV 'y boHaH TUTTLe BACK ' VIPER' pamM MONOPiANt AemeoAT PLAHINC, • TKACTOK HYDKO Diagrams of Floats or Pontoons, and air resistance values — Aeroboats and Pontoon Hydro-aeroplanes, showing centers of forces and balance. 147 gins, and unless enough power is available to overcome the drift on the hull, necessary to obtain this lift, "planing" will not be attained. Any suction tending to hold the craft down, or to add to the hull's resistance, may render "planing" at speed difficult, so that everything should be done to make the bottom of the hull or pontoon a good ' 'planer. ' ' This is secured primarily by having a high aspect ratio to the planing area of the bottom — as important in hydroplaning pontoons as it is in aeroplanes. So definite a factor is this in determining "planing," that it may be laid down as a general rule, regardless of laboratory results, that for every 500 lbs. weight of machine there should be at least one foot width of bottom. If this be obtained in two pontoons, the increased side resistance would give slightly more drag than if a large central float were used, with the small side pontoons lifting readily out of the water. The angle of incidence of the flat bottom that gives the best re- sults is about 4° incidence; any greater angle than this gives too high a resistance and is, therefore, wasteful of power. The contour of the bottom, so as to obtain this angle on the wetted surface and with the area and center of lift properly placed, is worthy of extensive study. Centers of Forces and Balance. As indicated in the diagram, the proper balance for planing is de- termined by considering the thrust, the lift on the tail in the pro- peller stream, the c. g., and the c. h., or center of the hydroplaning pressure on the bottom. The thrust, being so high above the water, exerts a powerful moment about the point of support, i. e., the water surface. This moment may be overcome by turning the tail up, giv- ing a downward pressure and moment opposing that of the propeller. This is actually used at the very start, before the planing action on the hull is appreciable in order to prevent the propeller push from forc- ing the bow in too deep. When the planing comes into effect, however, it is possible to do away with this negative tail moment, — which is both slowing down and adding weight to what the pontoons must lift — by having the wetted hydroplane surface far enough forward to have the c. h. in front of the c. g. The Shape of the Bottom. The contoiu- of the bottom of the floats must be such that the c. h. is well forward, when planing, and yet with sufficient planing surface aft to feather on the water and prevent the craft from jump- ing back too easily on its tail, since the latter condition, causing sud- den changes in the angle of the bottom and its planing pressure, is 148 what gives rise to the disagreeable effect of "porpoising" • — a fore and aft rocking and jumping, which is, at times, difficult to stop. At the front the contour shotild be such that there is a large ex- panse of hydroplaning surface in front of that wetted in ordinary opera- tion, in order to give ample lift at the bow for proper recovery when alighting on the water at a steep gliding angle • — otherwise the nose of the float might catch in the water and upset the craft. It is interesting in this connection to point out a feature on many floats or hulls that defeats its own purpose. It is assumed by many designers that a bow gradually ttirning up steeply, presenting a greater hydroplaning angle, will be the most effective in recovery charac- teristics on a "nose down" landing. As a matter of fact, the recovery moment is dependent not only on the size of surface and speed of landing, but also on the lever arm of this pressure afthe bow, about the c. g. As indicated on the diagram (the pressures being normal to the siu:- face) a flatter angle at the bow gives a much more powerful recovery moment. This is fully verified by actual practice. Steps. In order to break up the contour into the various areas at dif- ferent positions and angles, the practice of building the bottom in "steps" is resorted to. A reduction of friction resistance and splen- did effect in dividing up the surface is obtained by this feature, if the steps are made from two to five inches deep, with ample ventUation, i. e., large air tubes or air slots in the hull, to feed air into the corner of the steps, for the relief of the suction created there by movement of the water.* In considering the contour of a float, the fact that the water wiU acquire, and for a time hold, an acceleration downwards, produced by passing under an inclined surface, is often lost sight of. And the friction resistance of long surfaces is very great. Seaworthiness. Perhaps the most difficult incompatibility (excepting that of "stability and controllability" on an aeroplane) is to make a hydro- plane type of hull seaworthy. The fact that the hull, when it gets up to speed, is supported by dynamic water pressure, means that any increase or decrease of angle or surface wetted, caused by choppy water, will result in terrific bumping and pounding, and the old sajdng about the hardness of water, if hit hard enough, becomes uncomfortably evident. If the angle at the bow, as the craft goes into a wave is very * The surprising magnitude of this suction is illustrated by the fact, that, in the early development of hydro-aeroplanes, a single J/^-inch air-tube was considered suflS- cient ventilation for a step, which today would be required to have at least three 23^- inch tubes. 149 Steeply upturned, the bump is felt with unusual force, since it also tends to slow down the craft. Where a hull is used in which the upturn at the bow is kept as flat as possible, very little bumping is felt, in com- parison, the hull riding over the waves instead of pounding into them. However, in the latter case, since considerable depth to the bow is necessary to avoid "tripping" on waves, a freeboard is obtained by a "cruiser" bow construction quite readily, or by use of a "turtle back" bow. The cruiser bow cuts thru very large waves, throwing a great amount of spray to be sure, but the speed of the craft is not stopped as suddenly as with an upflare bow, and spray is readily protected against. The shape of the bottom is of importance for seaworthiness, since a V bottom is found to give much less pounding, an easier entry, a softer landing, and much less tendency to bounce on alighting. In addition, tendency of the craft to skid outwards, when being turned on the water, is somewhat provided against. Pounding on the bottom causes very great strains on the seams, by spreading, and a V bottom by relieving this, of necessity reduces the possibiliby of leakage. The long dragging hull in the rear, on some types, greatly increases the length of run necessary to get off, because of its added and unr necessary resistance. The hull at the rear should be given a positive action that will lift it out of the water, but this may become too great, resulting at the start in digging the nose in too deeply. All these features require careful compromise and balance. Sev-r eral outlines of hulls and floats are given. Many features, such as self-bailing cockpits, and thorough water protection of the motor, etc., require attention for increased sea-worthi-r ness. But the most seaworthy characteristic of marine aeroplanes has been, and possibly always will be, ability to rise out of the water quickly and with the shortest run. The greater excess of flotation of the aeroboat type is a feature of considerable importance. On a marine aeroplane, consisting of a land machine mounted on pontoons, a very large pontoon at the rear is required, to give anywhere near the excess of flotation obtained with the aeroboat. In this — ■ and in the greater ease with which the centers of flotation, hydroplaning, thrust and c. g., may be brought closer together — there are found the only real advantages of the "boat" type over the "hydro" since flying characteristics and even "planing," on either one, are governed by the same limitations. Structurally, the aeroboat type can be built stronger for the same weight than a "hydro," or pontoon aeroplane, and when the great stresses induced by "side swiping" in landing across wind are considered, the boat is decidedly advantageous in being so well self-contained. 150 The relative merits of the single pontoon and twin pontoon sys- tems are not yet well defined. The single pontoon is handier in a sea, but twin pontoons, on a large craft, give a wider expanse of bot- tom, thereby improving the planing by a higher "aspect ratio," but at the expense of more frictional resistance. The twin pontoon system is apparently well adapted to launching devices. Elements of seaworthiness found in the larger sized marine aero- planes are distinctly advantageous, and indicate that for real work in the open sea, seaplanes will become huge in size, and will have to pos- sess great range of action and excess of power. The Porte flying boat, developed by the British Admiralty — an enormous craft of over a thousand horse power, weighing several tons, and capable of carrj'ing great loads of bombs and fuel. It is freely predicted that a large machine of this type will soon cross the Atlantic. CHAPTER XII. FLYING, STABILITY AND AIRWORTHINESS. The characteristics of resistance, lift, speed, and power of the aeroplane having been studied, and attention having been given to the construction and adjustment of these machines, it is appropriate now to consider the actual flying of the machine. As already outlined, it must be borne in mind that the aeroplane is supported in a perfectly free fashion on a medium that is, at times, very treacherous, and the most efficient aeroplane in the world, as to speed and power, and the very best and refined in construction, is more or less worthless unless it embodies "controllability" and, above all "airworthiness." For the military aviator, the importance of acquiring a very sound and intelligent grasp of the principles of stability and operation involved in the notion "airworthy," cannot be overestimated. Actual instruction in the manipulation of controls on the ma- chines, thorough practice in acquiring the "feel" of the air, and de- velopment of unerring judgment on landings, form the major part of the practical work in the training of aeroplane pilots. But unless this is accompanied by an intelligent understanding of the actions of aeroplanes in the air, the pilot is little more than a somewhat in- stinctive automaton. No mathematics, or formulae, need be involved in the consid- eration of the stability and operation of aeroplanes. But there is required a continued and judicious use of "common sense." The subject may be divided into the three broad generalities of considering : 1. The stability of the machine, which may at once be defined as the degree and manner in which the aeroplane tends of its own accord to keep a certain relative "even keel" attitude to the air stream. 2. The airworthiness of the machine, or degree in which com- fortable stability is obtained without too much sensitiveness to air disturbances, and controllability, is obtained without making the aero- plane too easy to upset. 3. The flying of the machine, the assuming of different atti- tudes, unsafe positions that may be taken, and proper methods of oper- ation. 152 The absolute opposition of inherent stability to controllability is always met in flying characteristics, and it is a fact that an inherently stable and safe aeroplane is stiff and apt to "fight" its controls, while it is sensitive to and moved by air disturbances — whereas a "neutral" stability aeroplane, with powerful controls and no tendency to hold any position relative to the air, is more handy and precise in answer- ing its helm. The popular notion, held by many intelligent people, that "sta- bility" means "steadiness in flight" is very erroneous. The least air disturbance causes a "stable" aeroplane correspondingly to adjust itself to keep the same attitude relative to the air so that its position relative to the ground is changed by air movements, and so percep- tibly that on a rough, puffy day an inherently stable aeroplane ap- pears to roll, pitch and sway in a most alarming fashion, while, as a matter of fact, it is merely answering to the air billows. It is much more correct to conceive of an "inherently stable" aeroplane, pri- marily as "non-capsizable," and not at all steady in its flight. For that reason a neutral stability aeroplane, with powerful controls is much preferable. CENTERS OF FORCES It has already been pointed out in Chapter VII, pp. 93 and 94, that the aeroplane, in flight, is subjected to the action of four forces: (1) The Thrust — acting at the center of thrust, C. T. — which is merely the line of the propeller axis. (2) The Total Resistance — acting at the center of resistance, C. R. — which is determined by balancing the air resistances of all the separate structural parts (see Chap. IV) with the drift, and finding the resultant point at which a force equivalent to the total resistance would be applied. (3) The Lift — acting at the center of pressure, C. P. — which is the center of pressure of the lifting forces for the particular angle of incidence, at which flying is taking place, and found from the sur- face section data and tail lift data. (4) The Weight — acting at the center of gravity, C. G. Center of Gravity. It is of fundamental importance, before studying this subject further, to know how and where the center of gravity of any machine is located. An aeroplane is suspended in the air and rotates about its center of gravity, so that it is proper to consider the path of the center of gravity, in considering the trajectory of any machine. An aeroplane distinctly does not rotate about any center of lift or resistance. The center of gravity, therefore, must be known, and should be measured and marked on the machine. 153 The manufacturer furnishes drawings and data, indicating the proper position of the center of gravity. The aeroplane user, after fully loading the machine for flight, should determine whether or not the weight of the machine is properly balanced. There are several methods of finding the center of gravity.* (1) The machine could be swung, by flexible suspension from an overhead point, and a plumb line dropped from this point, would intersect the body at the c. g., no matter what the position of the machine. (2) The machine could be supported on a large pipe, or knife edge, and moved until balanced on either side. The c. g. fore and aft, and side to side, may be obtained readUy by this method, although it is, at times, awkward to support a machine in this way. In this method the height of the c. g. above the bottom of the body is not so easily obtained, and the total weight is not measured. (3) The method of moments — ■ in which the measurement of weight is made at any two points, and the distance between them measured. The total of the weight at any two points of support is the total weight of the machine, and as indicated on the diagram below, the center of gravity is very easily obtained by solving the suitable lever arms. This is an exceedingly quick, simple and accurate method for a combined determination of the weight and balance, and at anj' large aviation field, where platform scales are available, this method is particularly convenient. To determine the lateral correctness of the c. g., it is merely necessary to see if weights measured at either end strut, lifting the machine about the opposite wheel, are equal. The lateral c. g., however, is Bala/^ce rarely variable enough to require checking. To determine the actual height of the c. g. above the wheels, it would be necessary to repeat the operation for the horizontal balance, with the tail very low and the front as high as possible, thus establishing an intersection where the two c. g. lines cross each other. Or, if the chassis permits, the machine may be tilted up at the rear, until a balance is obtained over the axle, and by projecting a plumb line above this an intersection point is also obtained. The longitudinal position of the c. g. is the important one, and consideration of the accompanying diagram shows that if the total weight is reasonably well known, the single measurement of the weight carried Jay the tail skid, and its distance from the axle, will at once determine how far back of the axle the c. g. is situated. For this only a 500 lb. spring balance is necessary in the field, and data on the cor- rect weight the tail skid should carry is given by the manufacturer. * It is necessary to note that correct results in balancing are apt tb be upset if a draught or wind blows on the aeroplane when being balanced. Still air is a prere- quisite. 154 The Equilibrium of the Forces. These four forces of Thrust, Resistance, Lift and Weight, acting at their respective centers, must be in equilibrium when the machine is in steady flight. It is desirable again to emphasize that an aeroplane may be so designed that the line of thrust passes very nearly thru the center of resistance and the center of gravity is made in line with the center of pressure. The aeroplane is then said to be balanced on the principle of "coincident centers" {centres confondus). But there are notable excep- tions to this practice. For reasons of handiness in taking proper angles, as already explained, the center of thrust is often placed below the center of resistance. This couple, tending to turn the machine, as indicated on the diagram, p. 88, is overcome by the couple obtained by having the center of Hft in back of the center of gravity. This can be obtained either by having the center of pressure of the surface slightly back of the c. g., or by Introducing a small lifting force on the tail. We are at once led then to consider, The Effect of Tail Lift on the Center of Lift. Up to now we have considered that the center of the lifting forces on the machine, was found at the center of pressure of the main wings. This is only true if the tail surfaces are perfectly neutral, as found in the majority of well balanced aerbplanes. If the tail surfaces re- ceive a negative pressure — a downward air force — the center of lift of the aeroplane will be in front of the center of pressure of the main wing, and if the tail actually exerts a lift, then the center of lift will be pro- portionately behind the c. p. of the wings — so that the lift of the tail X its lever arm back of the resultant center of lift = the lift of the wings X the lever arm of the mng c. p. about the resultant center of lift. This, then, is the nature of the position of the Total Lift force (tail + wings) acting at the center of pressure, C. P., of the entire machine, and is the point referred to, in considering the four forces in equilibrium. c.e;. LIFTINCi TAIL Lateral and Directional Centers. There are two other centers to be considered. The center of sup- port, or pressure, may shift slightly laterally as the aeroplane takes different positions in the air, due to differences in the lift of either win"-. Attention is given to this under "rolling." 155 The aeroplane with fins, covered body and wheels, rudders, etc., presents a sidewise expanse of surface to the air. It is necessary to know the position of the center of surface of all this side area. Of course the areas can be computed and the center of area determined, but it is much easier to cut out a paper pattern to scale of the side ele- vation of the aeroplane, and then by balancing this on a pin point, finding the center of gravity of the paper. The center of side sur- face, or directional center, as it is sometimes called, may then be taken as slightly in front of this point, and may be marked on the machine. The centers having been defined, we are free to proceed with the study of the relative movements of the aeroplane and the air. Their classification into Pitching, Rolling, Yawing, has already been out- lined in Chapt. II. CHARACTERISTICS OF PITCHING OR LONGITUDINAL MOTION. The longitudinal motion of an aeroplane in rising or descending, corresponding to changes in the angle of incidence, is controlled by the elevator, but is subject to inherent effects in the aeroplane itself, due to the disposition of stirfaces and the magnitude of the longitu- dinal inertia. The action of an elevator in merely steering the machine up or down, in its trajectory, is remarkably powerful, and only a fraction of a degree change in the angle of the flaps is sufficient in normal fly- ing, to direct the machine to a different angle and path. For any one speed there is only one elevator setting and balance, corresponding to the one particular angle of incidence for that speed, and any change in the elevator manipulated by hand, changes the incidence, and for the same initial speed will cause the machine either to climb or point downwards. Flight at the different angles is effected by changes in speed obtained by throttling of the engine, combined with a more or less unconscious setting of the elevator to give the proper balance. The necessity of introducing lifts or depressions by the elevator, to keep the relation of the center of lift about the c. g., in the form that wUl balance the action of the centers of thrust and resistance, is al- ways present for flight at any angle of incidence. The pitching control can be varied greatly in delicacy and power by alterations of the size, movement and leverage of the elevator flaps. The general charac- teristics of this control, however, and the movements, positions and limits of equilibrium longitudinally are common to all aeroplanes. The angle ranges that have been studied in Chap. VIII now assume a more particular significance. 1. There is a "normal flight" position of the aeroplane — gen- erally when the body axis is in the line of flight — where there is the desired combination of speed, power, glide and climb characteristics. NOKMAL ■, — - , . TAIL HIC,H T=Si^ '""• '■''*' 156 2. There is a "low angle," or "vol pique," position, generally corresponding to the attitude for highest speed and least angle of in- cidence at which the machine is said to fiy "tail high." 3. There is a "high angle," or "vol cabre," position, correspond- ing to the attitude for slowest speed and large angle of incidence, at which the machine is said to fly "tail low." The "regime" of flight, at different speeds and angles, is all the way from the "tail high" to the "tail low" position. At any of these positions, governed longitudinally by the elevator and the throttle, the machine has a certain climb, depending on the excess of the Power Available over the Power Required, a glide governed by the total resistance, and a certain fuel consumption, all as outlined in Chap. VIII. But in its path through the air, if the machine does climb or glide, it must always be borne in mind that the angle of incidence is the angle between the chord and the flight path. Just because a ma- chine is pointed up very steeply, does not mean that it will necessarily climb steeply, since it might have much more excess power at a very much lower angle, and actually climb more feet per minute by the application of this excess power, with the machine on an apparently level keel. As a corollary, a machine does not necessarily glide best the more it is pointed down and speeded up. By holding a machine apparently pointed up, the actual glide slope would be flattest if the particular high angle of incidence, corresponding with this attitude to the air flow, was actually the angle for best glide. Speeding up a machine by nosing down on a glide may increase the Total Resistance so much as to cause the glide to steepen, greatly. All these charac- teristics may be studied from the Power and Resistance charts, and any aviator can profitably acquire familiarity with them. Although its significance is often overestimated, consideration of the "reversed flight" region may be given. Referring to chart opp. p. 104 it is seen that at speeds below angles of 10°, increase of angle of inci- dence, corresponding to slower speeds, involves a pronounced rise in the Power Required, due to increased resistance. And above 17° the lifting power of the wings actually decreases. The effect of flying at these high angles is to cause an inversion of controls. Increasing the angle of incidence, when in horizontal flight, without giving the engine "more throttle," actually causes the machine to sink, and whereas if flying at 14°, let us say, the machine's incidence were to be reduced to 10°, with the same engine power, the maneuver would result in a climb, due to gain in excess power. The old conception, then, of pointing a machine up for climbing, and down, for gliding, is not al- ways correct; and the aviator at some angles may, much to his surprise find himself climbing when he points the machine down, and sinking when he points it up for a climb. Such phenomena are only too often blamed on "puffs and uptrends," when, as a matter of fact, a glance 1S7 at the power chart would show the reason for apparent inversions of this kind. This leads at once to the realization that higher power is often of advantage in attaining slower speeds, since flying can then be done at angles where the resistance would prove too much for a lower-powered machine. Thus, see chart opp. p. 104, it is seen that the slow speed attain- able at 800 r. p. m. is 43 miles an hour, whereas an increase to 1400 r. p. m. would very Ukely permit of flying at 39 miles an hour. The "regime lente" or "slow," at which the aeroplane is flying, in cabre atti- tudes, is perhaps, the most difificvdt one to negotiate, and there are not many pilots expert enough to get the very slowest speeds out of their machines. Flying at the high speeds is merely a matter of giving the engine all the power it has, and being on the alert for the uncomfortable, quicker action of puffs. Flying a machine at its attitude for best climb, or best glide, is, of course, a matter of systematic practice, but information from the Power Chart is particularly of value for this. Different types of aeroplanes, of course, vary widely in their pitch- ing characteristics, but in practically all aeroplanes with deeply cam- bered surfaces, at low angles of 1° or 2° flying becomes exceedingly uncomfortable. The machine apparently loses much of its handiness in the control of pitching, because the surfaces at these low angles are flying at low values of Kl, and greatly varjring values of L/D, so that slight variations in the wind direction cause rather large and sudden changes in the pressures, and consequent "jiunping" of the machine. Large surfaced, deeply cambered machines, with any con- siderable excess power, always exhibit this characteristic when flown with fuU power on the horizontal. And the angle on some sections may come so perilously near to the angle of no Lift, and rear most c. p. position, as to introduce the danger of a sudden dive, .which the elevator may not be big or powerful enough to negotiate. Aeroplanes with deeply curved wing sections and an excess of power for climb, should therefore be flown carefully on the horizontal, with "all power out." Longitudinal Stability. The attitudes assumed and limits of control reached in pitching having been considered briefly, attention may be given to the natural characteristics of the longitudinal equilibrium of aeroplanes quite in- dependent of manually controlled pitching. The center of pressure on practically every type of aeroplane surface extensively used at present, see Chapter VI, moves back- ward as the incidence is decreased, and forward as the incidence is increased, within the ordinary range of flight angles of 0° to 12° (see Chap. VII). This means that the main lifting force has a pro- 158 nounced tendency to make a machine dive still more when the angle of incidence is decreased, and to stall when the angle is increased. This is, clearly, a condition of instability, i. e., any pitching is accentuated by the air pressure. For stability — that is, a tendency for the ma- chine to right itself — it would be necessary to have the center of pres- sure move back on increase of angle, and move forward on decrease of angle. Many attempts have been made to attain this by the use of reversed curve sections of wing, but up to now they have all been at a great sacrifice of efficiency. A c. p. position that is almost sta- tionary, thru the range of angles, has been closely approached by some of the newer flat section wings, and by use of a washout in the angle or the upturned tip as in the "Taube." But for actual positive stabil- izing action, it has been necessary to rely on the action of a tail plane or auxiliary stu-face, called the stabilizer. MAIN pLnm TAIL PLAfE coNWKCBnr rAHotKi O/VEKdENT TANOErt This brings us to the consideration of, perhaps, the most import- ant and essential inherent stability characteristic of an aeroplane — • the powerftil corrective action, on disturbances of longitudinal equi- librium of the convergent tandem arrangement of surfaces. The definitions of the convergent tandem system, often called the "longitudinal dihedral," which is so desirable for longitudinal stability, and sketches of several systems of tail and main surface combinations, are given in the accompanying diagram. -T^ "Txnobm" monoplane. is T(VICTaM MONOPLANE TKACTOR BIPLANE TOKPEDO" MONOPIANB 01 PlliHCr\ B)FLANE For most practical purposes, on the average present day aero- plane, the complete tail surface, situated at about three chord lengths from the main surface, is made to have an area of about ith of the main surface. This is inclusive of the flaps, which are merely a means of altering the camber and pressures on the tail surface for purposes of control. The error should not be made of considering the fixed tail pieces as separate from the flaps, because of the continuity of the two, except in the extreme case of "masking" already considered. The main and auxiliary surfaces could be of various different proportions, 159 such as the tandem disposition of equal surfaces, as in the old Langley machines, or the "Canard" arrangement with the smaller surface in front. Whatever the relative size of the surfaces, if the angle of the front one is positive and the angle of the rear one negative,* the system is said to be a "convergent tandem"; and its characteristic is that when the angle of incidence of the aeroplane is decreased, the air force on the tail becomes more negative, acting downwards, thus tending to force the nose of the machine up, while if the aeroplane assumes a cabre position, the rear surface lifts more, thus pointing the nose of the ma- chine down. This action is accentuated, in addition, by the slowing down of the machine at high angles and the speeding up at low angles. Practically all Lift values on aerofoils increase at a much steeper rate at low angles than at high angles. So that the rear surface, as in the divergent tandem, will actually change its Lift in less proportion than the front one, for changes in incidence, — • and this accentuates the action of the pressures on the main surface alone, tending to make the machine nose over still further of its own accord when it pitches forward, and to make it nose up still more when the incidence increases. The "di- vergent tandem," then, is naturally an unstable system. The con- vergent tandem is often spoken of as a "longitudinal dihedral," because the surfaces are turned up relative to each other. It is not always necessary to have a negative tail in order to obtain the desirable pitching stability, since the main surface may be set at + 3° and the tail surface at -I- 2°, with an interference on the tail caus- ing a 1° negative flow, which would give a longitudinal dihedral of 2°, and still leave the tail a lifting one, at an incidence to the air of 1°. This leads to the consideration of the effect on the balance of speed variation of the air passing the tail surfaces. Varying the r. p. m. of the propeller by the throttle varies the speed of the air thrown back by the blades. In every type, excepting the "torpedo" type, the pro- peller is in front of the tail surfaces, and therefore changes in the pro- peller stream affect the pressures on the tail.** In machines with a neutral tail, neither negative nor lifting, the effect of stopping or speed- ing up the propeller is not felt. But on a lifting tail machine, sudden stoppage of the propeller will relieve the lift on the tail, and give a tendency to stall just at the wrong time, while sudden starting again will nose the machine over. This can, of course, be offset by having the center of thrust below the center of resistance. Where the tail is a negative one, with a large longitudinal dihedral, sudden stoppage of the propeller stream causes the negative tail pressure partly to be relieved and the machine to nose over to a proper gliding angle. And, * In all this discussion the angle of the tail surfaces with the air is meant, i. e., interference of the air flow by the main surface is taken account of by the usual reduc- tion of a degree or two. ** It must be borne in mind that, due to "slip," the actual velocity of the air thrown back by the propeller averages 20 to 25% faster than the velocity of the aero- plane. 160 when the propeller is speeded up, there is introduced an increased nega- tive tail pressure, tending to make the machine climb at just the right time. On overpowered machines this tendency of a negative tail surface to make the machine climb when the full power is applied is an air-woithy feature, but may become uncomfortable to hold if too powerful. The most dangerous feature of a pronounced lifting tail is in the acquirement of higher and ever-increasing speeds on a steep dive. The lift of the tail is directly increased as the square of the speed, but its lever arm about the center of gravity remains the same; so that, as the speed increases and this tail lift moment increases, an unbalanced force is introduced. The speeds attained on dives increase so greatly and this tail lift action may become so powerful that the maximum exertion on the part of the pilot on the elevator control may not be enough to overcome it. This exceedingly dangerous feature of the lifting tail has resulted in some very severe accidents. It is seen, then, that a longitudinal dihedral givin^ the "converg- ent tandem" system favorable to inherent stability is far preferable to a lifting tail for safety, stability and airworthiness. Their compari- son on a basis of efficiency is not favorable to the negative tail, because the machine must constantly carry double the negative air load and extra resistance, whereas a large lifting tail will add just that much area for the load lifting capacity and give very great improvement in Climbing Rate, Speed, Range, etc. At times it is necessary to compromise stability and safety for efficiency, and for special performances in the hands of an expert a powerful lift on the tail is often used. Rarely, however, does this ex- ceed 50 to 60 pounds. The effect of having the Center of Thrust below the c. r. and the c. g. is to introduce a tendency for the machine to assume a glide angle when the engine is shut off, and to climb when the power i? applied — characteristics that are certainly more desirable than a high thrust, which, when the power is shut off, would tend to stall the machine. •C boat" 01 PLANE HK,H THKUir J^ »j» — n PARAiOL MONOPLANt (INIMC, KAIiEJl) c"f]p '^ — ^=^ Low THRUST L '•^- , Q J^ "^ 7fZ^ — ^ i'r^-^ 'Vw^ au|u_j;:3- • h =^ _ mCiHTHKUST 161 All these forces, however, are so balanced in amount that for the normal condition of flight, the aeroplane flies along with practically no effort on the part of the pilot required to keep those forces in their re- quired relation. It is only when the aeroplane's balance is seriously dis- turbed by a wind gfust that the pilot steps in to return it to its proper angle. As a matter of fact, the stabilizing forces are constantly acting on an aeroplane, although the pilot hardly realizes it. If they are made powerful enough the pilot could let the aeroplane correct itself, but it would follow an oscillating path, gradually damping out the movement, which would be bothersome to the average pilot. In addition, the large amount of stabilizer surface necessary to give complete inherent stability would be inefficient. Too much stability would make an aeroplane very stiff on its con- trols, because the air forces would tend to keep the machine in a fixed attitude with respect to the air, and would fight any change of angles, so that if the pilot wanted of his own accord for a maneuver, or for landing, to change the aeroplane's attitude, he would also have to over- come by physical effort the aeroplane's own stabilizing forces. The Adjustable Stabilizer One way out of this is to have an adjustable stabilizer, permitting the pilot to change the angle of the stabilizer and thus reducing or in- creasing its stabilizing pressure. It is most important for the center of gravity to be kept at its required position, for if it is not the pilot will have to introduce the proper compensating force with the elevator, and since he would have to hold this force all the time he is flying, it would become very tiresome. Where varying loads of bombs are carried, the adjustable stabilizer is a very desirable feature, for if after dropping a load of bombs the aeroplane, for example, were to become nose heavy, the pilot would merely turn the stabilizer to a slightly greater negative incidence, thus relieving him of having to hold up the nose by a constant pull of his elevator. A feature also considered, in reation to the size and setting of the stabilizer, is that there are many military maneuvers in air fighting where a vertical dive is necessary. But we have already pointed out the effect that speeding up on a dive has in increasing the negative pressure on the stabilizer, tending not only to limit the speed but also to flatten the dive. An adjustable stabilizer, as explained in the next chapter, gives a desirable control of this and this is a typical example of how stability, highly desirable for safety, fights controllability required for a certain maneuver. The Final Compromise All of these various features are therefore gathered in a compromise on a good aeroplane, such that the machine has a very definite tendency of its own to keep its balance longitudinally when in normal straight flying, and yet not so stiff that it makes control by the pilot difficult. 162 ROLLING AND LATERAL BALANCE. The lateral balance of an aeroplane is understood to refer to the balance of the wings transversely across the flight path. And rolling is the movement about the longitudinal axis, caused by alterations in lateral balance in distinction to pitching, which is the movement along the longitudinal axis. The lateral balance of an aeroplane may be varied by air disturb- ances and by the torque of the propeller (assuming that the wing setting and weight are symmetrical). The Torque of the Propeller, is an air force due to the pressure of the propeller blades on the air, which on single propeller machines must be resisted or else the propeller might stand still and the motor turn about it. The tendency of the machine is to turn opposite to the propeller, so that the efEect of the torque is to unbalance the aero- plane laterally, — in so much as it is necessary to introduce a lift on one side by a slight increase in the incidence, which will have a tend- ency to make the machine roU in the same direction as the propeller turns. Of course, where two propellers are used, working in opposite directions, the torque is neutralized. When the engine is suddenly turned on or off, on single propeller aeroplanes of high power and small surface, the torque is a very perceptible force. It is interesting to note that the torque of small, high-speed propellers is very much less than that of large, slow, geared-down propellers. The effect of air disturbances on lateral balance is merely to tip up one side or the other, or to throw the entire machine sideways, thereby affecting its transverse attitude. Since the actual attitude of the aeroplane to the air that is pass- ing it, governs the stability characteristics, it follows that we are con- cerned here with the effect on the wings of a sidewise flow of air, and of a difference in the angle of attack on either side. The latter, on any type of aeroplane, merely makes the air force on one side greater than on the other, and for the preservation of the balance requires a correc- tive effort. Lateral Stability and Instability. Pitching requires control for the attainment of different angles of incidence and altitudes. Yawing requires control for the steering of the machine. But, independent of the necessary feature of bank- ing on turns, the lateral control of an aeroplane is primarily for the pre- servation of lateral balance. "Lateral stability" may be defined as a natural tendency for an aeroplane to keep an even keel transversely. If a machine departs 163 from an even keel laterally, it may roll over and fall sideways, and it is well for any pilot to realize, that of all conditions of instability, lateral instability is the easiest to acquire and the most difficult to eliminate, without sacrificing controllability. The lateral stability characteristics of an aeroplane are consid- ered before taking up the study of lateral controls, so as to acquire a better understanding of their function. The effect of side winds, or, what amounts to the same thing, a sidewise movement of the machine, is not necessarily destructive of lateral balance, as will be explained presently. On the older type of open-bodied aeroplanes, with the wings straight across the span, and at constant incidence, a side wind would pass thru the machine with very little effect in tipping up one side more than another. But as soon as a large covered fuselage or nacelle is used, it is obvious that a side wind on the body will blanket the wing away from the wind, to a certain extent, so that the machine will have a slight tendency to lift up on the inside wing. This, however, is largely overcome by the effect of the body wheels, etc., which as covered areas below the c. g., catch the side wind and tend to turn the inside wing down. This opposition may be balanced on a machine quite readily and neutral lateral stability obtained, to the degree that the machine will not tip up sideways. The entire machine, however, being acted upon by a sideways flow of air of less velocity fore and aft, has less lift and would tend to stall, were it not that the "weathercock" action, considered later, turns it to meet the side wind. The "side wind" referred to is not of "puff" nature giving an actual incidence difference on the wings and tipping up the side with the greater angle. This must be borne in mind. It is well to realize, at once, that any arrangement for natural corrective effort when the machine moves sideways, relative to the air, makes the same machine roll when hit by a side wind. There are three general ways of obtaining natural lateral sta- bility : 1. By a Dihedral Angle to the Span. The wings are bent up, as indicated on the diagram, p. 165, and when the machine, due to some disturbance, rolls over, the low wing lifts more than the high wing and tends to correct the roll. When the machine moves sideways the dihedral angle of the wings causes a greater area and angle to be presented to the air on the leading wing, thus lifting it up. At the same time, however, the higher resistance on this wing tends to make the machine turn into the relative wind. A side puff will lift up the inside wing that it first attacks and then throw the machine sideways — after which the dihedral causes a greater 164 lift on the low wing, tending to bring the machine back to an even keel. This answer to a side puff, followed by the righting effect, is al ways characteiistic of a dihedral wing, and is uncomfortable. 2. By a Retreating Wing Shape. The shape of wing in the form of a retreat, as indicated, pives clearly a difference in projected entering edge and shape of wing, which, without quite as much sensitiveness to sharp side puffs, at the same time gives considerable difference in lift and strong recovery. Like the dihedral, however, the difference in wing, laterally, causes a dif- ference in resistance, tending to turn the machine into the side wind, and the great leverage of the difference in lift and resistance about the c. g. makes both systems exceedingly sensitive. , Airflow I I \ \ \ \ \ Span A. When the air flow is side ways, due io the refreafing wings. Span A lifts moreihan Span B Retreating Wing Effect. 3. By the Double "High Fin" System. As indicated (diagram p. 165), the rudder is placed high and a fin above the c. g. is placed forward. The action of a side wind on this system tends to roll the machine up on the inside wing, but while the dihedral and retreat are exceedingly sensitive to the least sideways deviation of the air flow from its direction along the axis of the ma- chine, fins of this class require a most pronounced sideways attack of the air before any considerable effect is created. Ordinary deviations of the wind direction in flight (which would cause a dihedral or retreat to roll the machine) have very little effect on this fin system, and the small leverage of the fin pressures about the c. g. rob them of sen- sitiveness. At the same time, when the machine itself moves sideways to any great extent, the high fin action resists the movement and tends to bank the machine up properly, and to overcome lateral instability. If the fin surfaces were below the c. g., or if the angle across the span is made catedral (turned down) instead of dihedral, a side puff would press down the inside wing, and a side movement of the ma- chine would introduce a force tending to roll the machine o^'er and to upset it, i. e., lateral instability. 165 vmepRAL CAreOKAL ^l/ie»/.3>itDiii^ anfk di/ftrenct V0U8LE HItH FIN SrSTtM It is clear, then, that on a machine with provision for corrective effort, tending to right the machine laterally when it is thrown over sideways, it is actually necessary for the machine to be disturbed and moved sideways before this corrective force is created. Every inherent lateral stability feature, as a corollary, has more or less tendency first to permit air disturbances to roll the machine — high fins less so than any other system. It only takes a very slight dihedral to give a desirable amount of lateral stability to an aeroplane, and in fact too much dihedral is partic- ularly bad in stiffening up the controllability of a machine, and in puffy weather makes the aeroplane very disagreeable to land. The position of the c. g. may effect this, in so far as a low c. g. does tend to give a lateral righting effect, although the machine is apt to swing in increasing amplitude if too low, while a high c. g., if above the center of support and displaced, would tend to roll the machine over and upset it. The lateral moment of inertia is ordinarily small, since the weights are practically at the same height, laterally. But on the old Wright aeroplanes, and the Curtiss fl3^ng boats (with motor high and hull low), there is a considerably greater inertia laterally, which makes the roll slower and the resistance to initial movement by air puffs greater. However, this feature causes the machine, after it has acquired a roll, to keep on rolling with considerable force, which is detrimental to controllability. Control of Lateral Stability For the purposes of assuming the proper banking on turns and the preservation of equilibrium, laterally, aeroplanes are provided with transverse controlling devices. Practically all of these take the form of adjustable surfaces out at the sides, in which changes of incidence or changes in camber (as in wing flaps), are relied upon to give a greater lift on one side than on the other, thereby rolling the machine. Depending on the size of flaps and amount of movement, there is usually a slight difference in Drift resistance when the lateral control is operated, the Drift being higher on the wing with the higher Lift given to it. This results in a tendency to turn around the higher side. A change in Lift on either side is thus made use of to control the lateral equilibrium of the machine, in those instances where the inherent features on an aeroplane do not give the required response. It is important to note here that the inherent features of lateral sta- bility are steadily receiving attention and development, and it may 166 well be possible, in view of the great progress already made, that the lateral balancing by manual control will give way to an automatic functioning of the aeroplane itself, thus eliminating one of the con- trols, and rendering flying that much easier. At any rate the assist- ance to lateral balancing given by natural stability features at present is very great and very promising. Yawing and Directional Stability. There remains to be considered the stability of direction, or "yaw- ing." If the directional center were in front of the c. g., a side wind would obviously tend to turn the machine away from the wind and either stall or upset it laterally. Some tendency to head into the rela- tive wind is necessary. This is obtained by having enough rudder or fin surface aft to bring the directional center back of the e.g. and is called "weathercock" stability. Few aviators realize how much ease of flying they owe to this, and how ceaselessly the aeroplane of its own accord is being pointed to its course by its own directional stability. DmecTioML cf/i/reK ■TUfAKfemi/Ami H r/m AfT However, if this feature is accentuated too much, the machine tends to yaw uncomfortably on meeting the least side wind. What is called "spiral instability" may also be developed, i. e., the machine, when making a spiral turn downwards, has a tendency to sharpen the spiral and dive, due to the side pressure on the body. In "nose spinning," see p. 183, it is generally found that too much fin aft makes an aeroplane resist spinning, but if there is not enough fin and rudder, once the aeroplane gets into a spin, it may be diflicult to get it out. INERTIA. We have considered the effects on balance and natural stability of the aeroplane in its form and shape. Equally important effects are given rise to as the machine moves about the center of gravity, due to the inertia of its weights as distributed about the frame. An aeroplane, just as any other body, has more or less inertia de- pending on the distribution or concentration of its weights. In addition to the weight of the frame, wings, and tail, the prin- cipal weights are the motor, the fuel tank, and the pilot. The inertia of the aeroplane is determined by the placing of these weights. If they are all grouped closely together around the center of 167 gravity, as on a small fighting scout, the inertia is very low. And if they are wide apart, distributed some distance over the frame, the inertia is high. It is exactly the same proposition as the weights on the end of a stick. The further the weights are from the center of the stick, the more force it requires to start moving it or to stop its motion. At once we must realize that inertia is persistence in a state of rest or of motion. It is the quality resisting any change of motion in a body. It is "pigheadedness." So that when a body of great inertia is moving it is as difficult to stop it as it was to start it originally. While aeroplanes of small inertia with the weights close together are easy to control, it is evident that too great an inertia on an aeroplane may be very troublesome and dangerous. Low Center of Gravity The erroneous idea is frequently held that a low center of gravity gives stability. If the aero had a constant speed, and did not need to maneuver, it would give stability. But the high and low speed accelera- tions and centrifugal force on turning would cause the low weight to swing and introduce very disturbing forces and, once started swinging, the inertia of a low c. g. aero becomes highly uncontrollable. OF INdKTIA smau mohbnt OF INCPTIA tvrs. CLose tkituck LONGITUDINAL INERTIA. If the motor is placed far ahead, and the pilot way behind on a tractor, the inertia becomes very high longitudinally, with the result that the aeroplane becomes stiff on pitching, whereas if it once gets started on a pitching oscillation, it is difficult to stop it. It should be pointed out that the Flying Boat as a type with the motor high, and the low weight of the hull, has a particularly bad longitudinal inertia, and this is accentuated by the high thrust which, if suddenly shut off, may start a very dangerous oscillation. It is exactly in not taking proper account of the effects of inertia on stability that wind tunnel tests on stability become inadequate and misleading. In fact, on fighting scouts where quick maneuverability is a very essential quality, the importance of low inertia is so great that designers go to any extreme allowable to have the weights so close together that the motor is practically in the pilot's lap. And frequently very excellent 168 motors are condemned for aero use because their designers, failing to realize the importance of this, made them too long. It is important to point out here that the low inertia aeroplane, quicker to answer its controls, is more dangerous to operate, because its very ability to turn up or down so quickly may induce very great stresses on the structure, making it readily possible to snap off the wings. LATERAL INERTIA. The effect of separating the weights laterally is even worse than longitudinally. Due to their spread of wings aeroplanes have considerable inertia laterally from the action of the air itself. And ordinarily with the weights distributed along the center line, the control power obtainable from wing flaps is just about sufficient. But when, as in twin motored machines, the weights are distributed across the span with a motor on either side of the central body, a reallj^ serious state of affairs is encount- ered. The aeroplane becomes stiff laterally, and will ride over many disturbing gusts without ceding to them, but once it starts rolling later- ally there is a most powerful tendency to keep on rolling over, which it requires a very powerful control to stop in time. In fact, in the actual operation of large aeroplanes of this type, it takes an appreciable time for the aeroplane to come back to a normal position when once it has rolled laterally. The Dunne. An examination of the photographs of this type (p. 23) reveals an aeroplane with a very accentuated retreat, with the angle of incidence varying from positive at the nose to nsgative at the tips, and con- trolled solely by flaps on the ends of the wings. WhUe there is no tail, there are virtually what amounts to two tails on this type, and the operation of pitching consists of turning aU flaps up or down for rising or descending. There is the added feature of the large braced panel on either end of the wing span. The "bustle" and change in camber are not considered vital. Studying this type of machine, it becomes apparent that the change in angle of incidence gives the effect of the "convergent tandem" sur- face arrangement, but with an exceedingly powerful negative tail. For a normal flap setting there is no question but that stalling or diving are rendered practically impossible by this inherent stability feature. This might lead to the conclusion that the machine was, in conse- quence, a constant incidence, constant speed machine, with no range, and a climb obtained solely from excess propeller push. This, how- ever, is actually not the case, due to the changes in trim obtained from flap adjustment in flight. 169 The retreat, combined with the change in angle, gives most re- markable effects on rolling and yawing. To begin with, the least deviation of the air is immediately felt, and the machine has a power- ful tendency to turn into any side wind, which results in a great deal of yawing in flight, although the action is slow and deliberate. Yaw- ing and rolling, however, appear to be inseparably combined. Oper- ation of the flaps, inversely, will lift up one side and press down the other, and in doing so the machine will tend to sideslip in. This, how- ever, is met by the presentation of the low inside wing, across its en- tire span, to the relative side movement, which causes the low side to lift and turn at the same time. In being thrown over on one of its sides in this fashion the inside side-panel of the machine receives a considerable pressure, which tends still more to accentuate the turn. A skid is, of course, impossible, since the machine wotild turn into it and the negative tips would keep the wing from rising. Any turn is at the expense of a roll, and any roll, even when caused by a puff, results in a turn. The inherent tendency and power of the machine to hold an even keel, with respect to the air, is unmistakable. Because of its constant answering to air disturbances, however, the machine is not comfort- able and handy in flight, and has lately gone out of use. The safety features of its inherent stability when used over water, where there is a great deal of room for alighting, gave the Dunne type a practical use. But for land flying, where operations in more or less restricted places are necessary, it is apparent that the Dunne in- herent stability features hardly compensate for the dangers of catching a wing or landing across wind, due to the inherent rolling and yawing movements of the machine. These, however, may be capable of im- provement, though they might very possibly lead to this type becoming more and more like the ordinary airworthy, controllable type of "main surface and tail" aeroplane, so widely and successfully used. The Taube The outstanding feature of this old type, a German "pigeon" shape monoplane, is a retreating wing shape combined with upturned wing tips of flexible construction. The upturned wing tips, when warped for lateral control, give a distinctly greater resistance on the side that it is desired to lower, thus helping to turn the machine properly when banked. This, combined with the retreat, does give a strong, in- herent stability action, tending to eliminate side-slipping and skid- ding very much as on the Dunne, but the Taube has rudders which permit of powerful control, near the ground. The flexible, upturned wing-tip feature, renders the c. p. movement for the wing favorable to longitudinal stability by increased negative pressure at the rear 170 of the wing when the incidence is decreased, and reduction of this pressure when it is increased. This feature, however, is wasteful of power and slow acting on its controls. A Taube in flight. The up- turned wing tips are evident. Above — A modern Taube. — The flexing of the wing end is indicated; below a more modern Aviatik. Summary There may be drawn from the consideration of the common ele- vator rudder and laterally controlled "main and tail surface" aero- planes, several interesting conclusions on airworthiness. The most airworthy combination for longitudinal control and stability would appear to be a slightly negative tail on a convergent tandem system of which the flaps form a large percentage of the area, so that ample control is obtained with minimum effort and drag. On the lateral equilibritun, handy control, wind-fighting qualities, natural stability and comfort, seem best obtained by a combination of powerful lateral controls, on an aeroplane with a high fin system and a slight retreat or dihedral. In a high fin system it must be borne in mind that a dihedral in side projection is virtually a fin. The arching of the wing transversely as in the, wing of a bird (see p. 123), appears to give excellent "fin" qualities without being too sensi- tive to rolling in side winds. Since the approach to the critical angle and a stall greatly affect the sensitiveness of the lateral control, thus accentuating tendency to side slip, a very powerful control by large flaps (variable camber) is most desirable. The degree in which many qualities of controllability and inher- ent stability can be combined and accentuated are much more a mat- 171 ter for the personal taste and "feel" of the pilot than has been sup- posed. Some pilots rather plrefer a quick handy machine, while others favor a high degree of natural tendency to a level keel, requiring less attention and being less tiring to operate. The necessity at present of considering the landing and starting conditions as the real limitations for flying, need hardly be emphasized. ■ And the constant effort of designers to extend the speed ■ range, not only to higher speeds but to slower speeds for landing, and to obtain greater climbing rate for rising out of confined areas, must be accom- panied by an equally great effort to make the machines handy, quickly controllable, and devoid of tricks or whims, in order to make operationf under puffy, treacherous conditions as practical as possible. It is un- fortunate that, thus far, every device for inherent stability or automatic mechanically controlled stability lacks the flexibility and quick power of judgment of the human brain, necessary for operations in landing in difiicult places in a bad wind. Flying aloft is, after all, not so very difiicult, on a comfortable, well-balanced "inherently airworthy" ma- chine, but aside irova the advantage gained in relieving the pilot of having constantly to operate the controls, all "inherent" or "automatic" stability features fail to add in safety, unless they first render safer the operation of coming back to earth. In this connection safety is, perhaps, better served by a robust landing gear on a machine that is perfectly controllable, and in the hands of a pilot with good judgment. A few notes in the form of directions may prove of value: 1. If a machine is tail heavy, with a lifting tail, move the entire c. g. of the machine forward. If tail heavy with a negative tail, first reduce the negative tail angle, slightly. 2. If a machine is nose heavy with a lifting tail, thus tending to dive, first move the c. g. back by some weight in the rear, and if the characteristic is still exhibited, take the weight out, and reduce the angle of the tail two or three degrees. 3. If there is a pronounced tendency for the machine to yaw at the least puff and to want to dive steeply into a spiral, there may be too much "weathercock" action, in which case either mount a small rudder or put some fin surface forward. 4. If an unbalanced (flap and fin) rudder is too hard to operate, increase the lever arm. If a balanced rudder "catches" it is a sign that its hinge is too far back. 5. Adjustment of flaps is capable of giving various degrees of sensitiveness and ease of operation, depending on the machine. The best all-around results are given by having the trailing edge of the flap a little below the trailing edge of the plane. 6. Only two maneuvers need be resorted to as tests of the im- portant inherent features. When the aeroplane is flying horizontally, 172 application of excess power without any elevator change, should cause the machine to climb. And in a turn with rudder alone skidding out strongly the machine should display a natural tendency to bank. In all cases the pilot must make sure that a trouble in balance is not due to putting some greatly increased Air Resistance above the c. g. or below it. For example, if a pilot who has flown a machine with covered wheels on the landing gear, were to replace them with a larger size wheel and uncovered, he might suddenly find that the aeroplane has become nose heavy, and wonder why. He has merely introduced an added resistance far enough below the c. g. to give a pronounced nose pitching couple. So that in moimting gujis, and particularly in carrying a military load, ammunition, instruments, etc., all pilots must be careful not to alter the various centers without due thought on how this will afifect the flying of the machine. CHAPTER XIII. FLYING AND "STUNTING." The control of the aeroplane in the hands of the pilot, is by means of certain combinations of wheels and levers, leading to the three con- trols — the flaps on the wings for lateral balance, the flaps on the stabili- zer for pitching control, and the rudder for directional control. All of these may be operated simultaneously, and the first instruction of the aviation pupil is concerned in acquiring all of these reflexes instinc- tively. The descriptive diagrams shown opp. p. 88 of the two main controls now used — the Dep or wheel control, and the stick control, will be worth a very careful study, and much time can be saved if the pupil will but fix these systems clearly in his mind, and mentally practice the maneuvers. The least instinctive element is the foot bar for the rudder, and yet in the air one gets quickly accustomed to it. The Dep control is used on large aeroplanes and the stick control is now practically universal on all small machines. The amount that flaps are moved for corresponding movements of the levers has been developed by test and is about the same on all aeroplanes. THE FOUR AEROPLANE SPEEDS Speed is necessary for flight, as we have seen. Even more so air Speed is absolutely necessary for control. This is the fundamental always to be kept in mind that in actual &ying, no matter what the position or maneuver, controlling power can be obtained only when the air flows past the controls at suflScient speed, and in line with the aeroplane's axis. Aeroplanes have essentially four speeds that the military aviator must become equally proficient in attaining. 1. Speed of maneuvering. 2. Speed of travel on the horizontal. 3. Speed of climbing. 4. Speed of diving. All have their particular military value, and an aeroplane deficient in three, but ahead in any one, can usually get away. The command of the air goes to the aeroplane that is fastest in all four speeds. 174 TAXI-ING— STARTING In starting a flight the aeroplane first rolls along the ground to acquire speed for flight — an apparently simple procedure called "taxi- ing." But in reality "taxi-ing" is quite difllcult, particularly on a puffy day with a large wing surfaced machine. The important feature to bear in mind is that if the aeroplane just starts to turn it will generally acquire a momentum which will keep it turning. This is because in order to prevent nosing over on landing, the wheels are placed far ahead of the weight. This tendency of the weight to swing is overcome by a very quick counter rudder, a trick readily acquired. And in order, in maneuvering on the ground, to avoid acquiring too much momentum, the aeroplane is turned by switching the motor on and off, causing a series of jerky air pressures on the rudder. A steerable tail skid also greatly assists ground maneuvering. It is important for the aviator to practice taxi-ing to the point where he can guide the aeroplane in any direction. TAKING TO THE AIR After a run on the ground suflScient to give some pressure on the tail, the pilot eases forward on his elevator and first lifts the tail off the ground, thus permitting the aeroplane to gain still more speed rolling on its wheels until, by a very slight pull back on the elevator, the machine floats off into the air. At this particular moment, the pilot must be most careful not to be too ambitious to climb, because if he points the ma- chine up too much before proper speed has been gained, he will at once run into the most frequent condition of loss of flying power known as THE STALL. In studying the Power Charts the speed range of an aero was ex- plained, the high speed condition corresponding to low angle of incid- ence, and the low speed to the high angle of incidence. In general, this low speed can be brought down to the angle of incidence where the Lift coeflScient attains its maximum value. But if we go beyond this the air support for flight ceases, the power of the controls is lost and the aeroplane sinks in a haphazard manner, very expressively called a stall. When the aeroplane lands flat, in this way, without support, it is said to perform a "pancake." As a matter of fact an aeroplane can be made to "stall" at any angle of incidence if the power is cut down low enough. But the stall will gradually build up to the same maximum lift angle before loss of control occurs. Af thii poini; forward speed pracHcally ceases, and fhe Aero falls and ii may in falling go over side ways. ^~'- Angle of rnatmum lift, "■^ abouf le'-ZO" Afrequen} source of accident, isdue io trying fopull-fheAero up here,before enough airspeed for control. This tendsfoaccenfuafe fhe stalled condition of settling without control. (At this point the up-pressure all over the tail and rear -\of the machine, becomes great eiough to turn \ the nose down jBut.at this point although pointed down, \ the Aero has not yet gained enough speed {And it is most important, to actually dive it \ still more in order to gain speed for control. A Too sharp upturn here puts great stresses on the Aero. ^, Soastopullthe Aero out and carry on. THE STALL In which 1-he Aero is pointed up beyond the max. lifting angle, and therefore falls. Motor isthrottled here — , By not trying to glidetooflat the Pilcff has l 5^^ ^ to ^ yj ^ tJ cnui > Ztjts ts ^3 0) — n — 'V => ^ c ui.— 5'b ■2 3 SyS 182 LANDING It is most difficult to discuss landing, other than to point out the importance of coming down with plenty of control speed to very near the ground, and then floating along, increasing the angle of incidence, and gradually shutting down the power without climbing, but holding the aeroplane off until the speed is as low as possible before actually touching. Great difficulties in landing result from hitting the grourtd with the aeroplane still going too fast and consequently bouncing into the air again. Or, on the other hand, leveling up at slow speed too high, and pancaking down with no speed. NOSE SPINNING Nose spinning, which has often been considered most dangerous, is safe enough if at a great enough height, and is decidedly the quickest way of losing height, without gaining too much speed. It requires very great skill, but there is much needless confusion between nose spinning and "tail spinning," and "spiral dives." The diagram explains nose spinning, primarily as a rotation on an axis, not in line with the body, but passing thru the nose, merely because the side pressure at the front is the air force, causing the spin. But this movement is quite distinct from a corkscrew twisting of the aeroplane on its own axis, due merely to rolling the wings end over end. Some aeroplanes will go into a spin just after a stall with the engine on, and will come out by shutting off the engine. A dangerous feature of spinning comes in where there is too much side fin surface in front of the c. g., as this stiffens the aeroplane making it harder to get out. As a matter of fact though, the aeroplane can be brought out of any of these positions by merely bringing all controls to neutral and just holding them there, courageously, for a definite time, and then pushing forward into a diving position and holding that for enough time to regain control air speed on the flaps. FLYING IN A WIND It is most important for the aviator always to bear in mind that the aeroplane is flying, so to speak, completely immersed in a large body of air, and should always travel axially thru this air. But if the whole body of air is travelling over the ground in the form of wind, the aeroplane, even tho its flight relation to the air remains the same, moves with this body of air as a whole. Thus its direction over the ground becomes the resultant of its ow:n and the wind velocity. And if, for example, the aeroplane flies at 100 miles an hour against a wind of 40 miles an hour, it would travel actually at 60 miles an hour 183 j^ _- _^ The Aeroplane Is stalled here And then by moving the stick sideways, is helped over Into a side slip. At which time the stick Is pulled back all the way to keep a full stall Axis of Spin And the rudder Is turned' into the spin,- the more is turned the taster the spin. The Aeroplane spins around on the axis shown, with the side pressure on the nose of the body acting as a point of support and continuing to act on the same side throughout the spin. It Is the same movement as the spinning top that Is started unevenly by a side thrust. Note:- The axis of spin is not the same as the axis of the machine. To Recover Put all controls at neutral. Hold them there! Push stick forward until the wind begins to whistle a bit, the/; pull the | stick back gently and carry on. THE NOSE SPIN Variously called TailSpin Vrllle"etc. which is used to descend quickly without too much increase of speed. This maneuver requires plenty of height for safe recovery. 184 over the ground — whereas if the wind were behind it, it would go at 140 miles an hour over the ground. Incidentally, this latter feature brings up an important considera- tion — when the aeroplane flying with the wind turns, the momentum it has acquired in its high speed with the wind causes it to climb^ of itself when it is turned facing the wind, and conversely, when turning from "into the wind" to "with the wind," the aeroplane drops a little, due to the necessity of picking up a new acceleration. SUMMARY In all stunt flying, the main essential is sufficient altitude to give the aeroplane room for recovery, and then a realization that in diffi- culties the best thing is usually to bring all controls to neutral, and then go into a dive holding it until quite certain that enough control speed has been attained. Many accidents have happened by failure to dive the machine enough to gain that very necessary axial flow of air past the various control flaps. On a well balanced machine at sufficient height an aviator need worry about very little, because there is really no unbalanced position of the aeroplane out of which he cannot bring the machine. Whatever the maneuver, the important consideration for the student flyer is always to plan out carefully just what he is going to do, and to be able to tell in flight just what the aeroplane is doing in response to his efforts. CHAPTER XIV. THE EYES OF THE ARMY AND NAVY. A proper appreciation of military aeroplanes cannot be had with- out giving consideration to the manner in which aeroplanes may be used in military and naval operations. But, in doing so, let us not trespass on the special studies of flying officers in the use of aeroplanes in strategy and tactics, further than to state that aeroplanes are used, 1. To see with: 2. To communicate with; 3. To attack with. Superiority in speed, facility and accuracy of observation, com- bined with fighting power to run the enemy's aeroplanes "out of the sky," or to do damage to important points, must be sought for in com- pany with efficiency in construction, equipment, repair and operation. The command of the sea belongs to the ship that can "overtake, observe the most, hit the hardest, and run away" — with the greatest reliability. And the command of the air belongs to the aeroplane that can get up into the sky the quickest and observe the most, with precision and ease, and with sufficient fighting power to prevent the enemy from doing the same — all of which also must be accomplished with re- liability and efficiency. Structural Perfection. For military ptu-poses, efficiency and reliability in the structural features of the machines must be sought in : 1. The utmost simplicity in construction, ease of repair and facility, in rapid assembly. 2. Resistance to deterioration by weathering and hard use, min- imizing the requirements for parking and overhauling. 3. Standardization of parts, requiring a minimum of stores and facilitating interchangeability. There are many different types of metal fittings, wooden parts, struts, controls and chassis (see Chap. X), that differ so slightly from 186 c 3 o Q 187 one another in the use to which they are put that a Flying Corps can readily standardize many of these features for all machines. In general, welded or brazed fittings, or laminated wooden members, requiring special facilities for manufacture, can largely be eliminated, and aero- planes for military purposes with a few rugged, easily accessible and repaired parts, are far more preferable than aeroplanes with delicate construction and countless small parts, clips, pins, bolts and ''gadgets," all differing from each other. The "military" aeroplane is bound to be the one the construction of which is typified by the feature — that only one size of bolt, with the same thread and nut, is reqiured for the entire structure. It is not at all impossible to have an aeroplane so designed, with solid wire braces and simple steel plate fittings, that the crew of the machine need carry on the machine in flight only a few tools, a blow torch, a soldering iron, a roll of wire, and a piece of steel plate, with an extra wheel or two and a few wooden members (and engine spares) for the immediate repair of the machine without outside assistance. How impossible this would be on some types of otherwise sat- isfactory aeroplanes, is evident at the first glance. The more difficult an aeroplane is to repair, and the more extensive the expert labor and equipment required to do it, the less satisfactory is the machine for military work in the field. The necessity that the average "fussy" aeroplane construction has, of requiring most elaborate Repair Depots for service, is a very great and inefficient demand on the personnel and material. Observation. Whether in actually observing the movements of troops, the effect of artillery fire, or in taking sights for and noting the results obtained by gun firing or bomb dropping, the most important requirement in military aeroplanes is that the field of view be as unrestricted as possible. Obviously, the "pusher" type offers a better view and arc of gun fire than does the "tractor," but in the latter type many modifications, such as openings in the planes near the body, the raising of the wing, as in the "parasol" type, and special posts for the observer ("prone" below the fuselage or above the wings), are certain to be incorporated. The ordinary tractor aeroplane is exceedingly difficult to observe from. In this connection the use of suitable periscopes is well worth experiment. The effect of speeds of aeroplanes in rendering observation more difficult is not of as much consequence now, in view of the great height from which observations are made. Although it generally has not been so considered in the design of the more common tractors, it is the writer's opinion that, for mili- tary purposes, the "eyes" of the army and navy should be made to see, and everything that is possible should be done to extend the field of view. A high powered American Tractor climbing The Breguet ''avion Canon," a typical example of a high powered pusher type of bomb-carrying aeroplane, armored and equipped with guns to fight off attacking aeroplanes. 189 SEVERAL MILITARY AEROPLANES 1. The Morane-Saulnier "Parasol" Monoplane, a highly successful French speed scout, later copied in the German Fokker Monoplane. 2. The Albatros — a long range, heavy duty German Tractor, which has proven to be an effective type. 3. The Twin Tractor German Battleplane, with gunners in center nacelle. 4. The Voisin "avion de guerre," a pusher gun carrier. 5. The Bristol Speed Scout used by the British. 190 .'^Observation I ..■^ Blind Spot i ill' A^HA&STROH M V. BLIND SPOTS One of the fundamental elements in which various types of war machines differ is in the obstruction to view or gun fire. For a long time the tractor tjrpe was considered unsuitable for a war machine, because of the obstruction to forward fire of the propeller — a feature overcome by shooting thru the propeller with a timed mechanism. It is not possible to discuss the "blind spot" features of various types without infringing on valuable military information that cannot be published. But an explanatory diagram is given explaining the meaning of the term. CHAPTER XV. CONCLUSION It is much to be regretted that military necessity forbids the publi- cation of the interesting details of the new types of war aeroplanes, and also of the fascinating tactics of air fighting and formation flying. All of this instruction, however, is obtained at the military aviation schools, as well as the details of how aeroplanes are used in photographic reconnaissance, bomb dropping, infantry offensive with machine guns, mapping, artillery control, etc. After the war there is an opportunity on these features to compile a textbook of great value, excepting that the swiff progress of war is apt to make many features out of date in a very short time. But the basic principles of the aeroplane as we have outlined them will never go out of date, though structural details may improve and vary considerably, and in fact have already done so. And the quick grasp of aviation, that the pupil must get in order to speed up his instruction ini the great emergency caused by the glorious fight for democracy, is greatly assisted by the understanding of the simple fundamentals that have been presented, thus leaving him more time to study and practice the great military features that the air service is used for. After all his understanding of the aeroplane itself is only the very first essential in 'a vast study. For military purposes aeroplanes vary from the small agile, single seated scout, to the huge two or three motored bombing and offensive aeroplane, slow in maneuvering but armed and armored to the teeth. Modifications are constantly being developed, many efforts being directed by brilliant minds to improving new types for better view, gun range, speeds and production in quantity. There is an intense rivalry between nations, particularly in fighting scouts, in the four equally important speeds of maneuvering, climbing, diving and travelling. As already pointed out, the master of the air must be superior in all. Speed of horizontal travel alone is by no means sufiicient, excepting to get away, and here this may be limited by too slow a diving speed. While great attention must be paid to streamlining and aerody- namic efficiency, the newcomer does not get the proper appreciation of aviation without realizing that weight is the deadly enemy of flying. After all is said and done, the first thing to seek in a flying machine is 192 light weight. Every extra pound weighs like lead in the hands of the flyer, uses up power and materials, and limits maneuvering. If there is any real basis of comparison at all between aeroplanes it is on a basis of weight. Weight per horse power of the motor ! And weight per square foot of wing surface! In fact on the modem aeroplanes a well informed flyer merely wants to know the "lbs. per h. p." He has learned that all aeroplanes have more or less the same efficiency. And he has seen different types of the same "lbs. per h. p." do exactly the same performance. So much so that at an aviation field 20 lbs. per h. p. means 75 miles high speed and 3,000 feet in 10 minutes climb, and no more, no matter how elaborate the theory or how queer the aero looks. And 11 lbs. per h. p. means 125 miles per hour speed "on the floor," and 10,000 feet in 11 minutes, more or less, depending on the pilot. And so it goes, such that when an aeroplane is announced of 6 lbs. per h. p., it is immediately pronounced "very hot" and awaited with breathless interest. The same with the loading on the wings, which is a criterion of the landing speed. A biplane over 8)4 lbs. per square foot is undersurfaced and will land fast. On the other hand, if only 5 lbs. a square foot, it is likely to be too "loggy" and lightly loaded, and therefore due to too flat a glide, difficult to land in a small field. Another interesting side light on the military use of aeroplanes is that pilots readily take the most unreasonable likes and dislikes of machines. It is said of two squadrons, along side of each other at the war front, that one constantly condemned and criticised the type of machine used, whereas the adjoining squadron using the same machine in exactly the same work could not praise it too highly and would use no other. 193 Prejudices like this are well known, and to experienced men in avia- tion have frequently been not only childishly erratic but so unfounded that a few fortunate circumstances will completely reverse them. It is to be regretted that this is so, particularly as it greatly ham.pers the work of the directing and organizing staff. And in general it is proven now that such likes and dislikes are almost always held by aviators who really do not know their business, while definite recommendations founded on most careful observation and in agreement can always be obtained from the hard-headed experienced flyer who knows how many errors can be made, but, whose knowledge and thorough practice keeps him from jumping to conclusions until very certain of his ground. Whether monoplanes or biplanes, tractors or pushers, with rotary engines or water cooled engines, the most suitable aeroplanes for mili- tary purposes will be the ones that are superior in flight to the aeroplanes of the enemy. And this means that, precisely as in naval work, a "race" is on between nations for superiority in aircraft! In what, then, may we find "superiority?" Simplicity of construction and efficiency in organization for main- tenance of the machines is not all. More is reqmred than numbers, although a Flying Corps is not of much use without plenty of spare machines. Thorough training and great personal skill on the part of the flying officers — as important as the personal equation in any line of human endeavor — may still fail to give superiority, because our aeroplanes in flight must have command of the air, which can be obtained only by ability to start from and alight in more difficult country, higher climbing rate, greater speed and radius of action, better facilities for observation and gun fire, and greater load-lifting capacity. High speed, so desirable for operations in the air, means a re- duction in load-lifting capacity and limitations of landing and start- ing, requiring special aerodromes. Facilities for observation and gun fire may necessitate sacrifice of flying efficiency and simplicity of con- struction. Great radius of action and climbing speed may limit the load capacity in bombs, etc. So that the ingenuity and skill of the engineer officers of a Flying Corps must be exerted to the utmost in compromising properly these opposing features. There are many types of military aeroplanes, light, fast speed scouts, slower load-carrying, gun and bom.b machines, two seater "fighters," aeroplanes especially adapted to artillery observation, to naval coast defense work, to messenger service — but the fact remains, that from all of them the maximum possible view must be obtained, particularly for bombing with fighting quality superior to the enemy's and with the greatest load-lifting capacity and climbing speed possible. Every- thing must be done, therefore, to improve the aeroplane's efficiency for military work in extending the speed range, the climbing rate and the load capacity. 194 Instruments. Although flying is properly taught on a basis of acquiring the "feel" of the air, any instrument of assistance to flying without adding- considerable weight is most desirable. On the dashboard of a well- equipped aeroplane there are found the usual clock, aneroid, fuel gauges, and engine tachometer. But, in addition to these, other devices are mounted to indicate the relation of the aeroplane to the air. For this purpose pitot tube, or pressure plate air speed indicators are used. Angles of incidence to the air may be indicated by a vane floating in the stream, operating a needle on a dial. The inclination of the aero- plane to the ground may be indicated by inclinometers, such as a bubble in a curved tube, or a pendulum. Various simple devices, such as strings or light vanes, may be used to indicate any sidewise movement or skidding of the aeroplane thru the air. In the determination of the speed, climb, etc., for any position, the pilot, having at hand a power chart of the machine, may read the r. p. m. of his engine, thus establishing its power; by reading the air speed or the angle of inci- dence (either one determines the other) he readily notes the power required — so that he can judge what his climbing power and rate are, and what the fuel consimiption is. Or, if he is flying on the hori- zontal and desires to use the minimum of fuel per mile, he throttles to the r. p. m. indicated, and checks the speed of greatest economy by reading his angle of incidence and referring to his power chart. A very extensive use of these charts may be made in flight, the only two instru- ments necessary being the engine tachometer and an angle of incidence indicator. Comparison of the inclinometer and incidence dial will readily reveal whether or not he is flying in up or down trends, since the one reads the "air angle" and the other the "ground angle." Stabilizers or Automatic Pilots. In addition to the instruments giving the pilot information on his flying, there are the "automatic stabilizers," — instruments to relieve him of having to hold the controls. Inherent features of airworthiness in the machines will also do this, but only after answering to disturbances in much greater measure than a delicately adjusted stabilizer. The latter, also, if pendulum or gyrosope governed, holds the aeroplane to a "base line" relative to the ground and not to the air. Level flight is thus obtained, with more or less success, and with pendulum and gyroscope stabilizers it is possible for the pilot to be re- lieved of having to attend to the controls, in that the "stabilizer" or "automatic pilot" keeps the aeroplane on a fixed and steady course. This requires careful adjustment for each particular type of aeroplane, however, and since flying on an airworthy machine, with inherent features not too much accentuated is comfortably possible with con- trols locked, reasons of safety alone do not demand "automatic stabilizers," in view of their added complication. 195 Stabilizers can also be made to bank an aeroplane properly on a turn and hold it, with an accuracy and precision that is remarkable. For night flying an automatic pilot mechanism has very great advantages. And for bomb dropping, etc., in improving the steadiness of the aeroplane as a platform, it is a valuable auxiliary. Performances and Operation. It is of the utmost importance in military operations to have in- formation on the radius of action, the load-lifting capacity and the speeds of the aeroplanes to be used. For the purpose of assisting in these matters particular attention has been given to the prediction of the performances of aeroplanes. In choosing machines to lift a great load of bombs here, or to travel a great distance at high speed on a raid there, or to climb up very quickly and return with information for some other purpose, a study of the Power Charts and data on fuel consiimption and lifting capacity (Chap. VII and VIII) is not merely helpful — it is necessary. And for all An Aeroplane of the Stiu-tevant type, the first built of steel construction, climbing off the ground. intelligent military aviators a study of this kind is of great import- ance. In fitting auxiliary devices, guns, bomb droppers, etc., in- formation on the resistances (Chap. IV), and on proper balancing of the weights (Chap. XII), as well as the strength of parts necessary to do the work desired (Chap. IX and X), may be appHed directly to such problems in the field. The conditions of actual operation of aeroplanes as dictated by the weather are quite variable. Fog is the most serious detriment to flying, next to which may be put the possible limitations of start- 196 INTERESTING WAR LESSONS The Caudron Twin Tractor, with centre nacelle for gunner (upper left) gained excellent climb, at the sacrifice of speed, by the two-motor arrangement. This, however, obstructs the view of the pilot'. Below it is shown the Curtiss Seaplane, with two tractor motors of so called "America" type. This large craft, due to its size, shows good seaworthiness, but at the expense of flying characteristics. At lower left is shown a view of the huge Sikorsky multi-motored machine, used by Russia. In general, huge land aeroplanes are being most highly developed, and their efficiency in carrying large loads of boinbs is excellent. Efforts are being made to arm them sufficiently to fight off the smaU scout types, and the big machine has great future possibilities. On the right are shown some speed scouts. The big aeroplane has still much to prove for itself, although its gradual development is inevitable. The light, fast speed scout, however, is decidedly the success of all war aeroplanes. Operated by one man only, who is expert in both flying and military work, these small machines outclimb and outspeed all the heavier, larger types. Their offensive value has consisted merely of a light machine gun, shooting over or through the propeller. The Nieuport, as seen from a companion machine in flight, is shown at the top right. Below it is a "pusher" type speed scout, built in England, and at lower right is the S. E. 4, a very fast machine, constructed by the British Gov- ernment. Great excess of power gives these small machines a very real advantage in acquiring command of the air, but if the big multi-motored machine can shoot more accurately in all directions, its helplessness against the speed scout, due to less maneuverability, is largely overcome by superior armament. 197 ing and alighting. In certain winds some small fields are not difficult to negotiate, but under different conditions they may prove impos- sible. Here again local conditions bring up questions of suitability of various aeroplanes in such a way that countless problems are pre- sented requiring "heady" resourcefulness. For example, a condition may readily arise where a machine of slow speed, which gets off the ground in a short run but does not climb fast, may be preferable to a very much faster machine of longer run, even though its climb is better. Not only may the performances of an aeroplane be studied on the field, but in their work the technical officers and engineers of a Flying Corps should be able to judge of the probable performances of an aeroplane from charts and drawings — sufficiently to limit the ac- ceptance tests to satisfactory construction and balance, and to choose the aeroplanes needed for any particular purpose before delivery. It is decidedly inefficient blindly to try a machine out for some special performance without first going through all the simple computations and determinations bearing thereon. The operation of aeroplanes in a wind requires consideration of the direction and force of the wind in determining the radius of action. The aeroplane always keeps its particular attitude and speed relative to the body of air it is passing thru, but this entire body in the form of wind may be moving — so that the aeroplane's travel relative to the earth becomes the resultant of its velocity and the wind velocity. In naval work, only speeds on the horizontal need be considered, but in speed thru the air an aeroplane must have superior velocity up- wards as well as onwards. 198 For tactical observation and for artillery work, it becomes of the utmost importance to consider that climbing speed, after all, may prove the most vital criterion of superiority — since a slower machine, superior in load capacity and climbing speed, may dominate a faster machine and climb away from it — so that efficiency may well be strained to the limit to obtain speed upwards. The fight between aeroplane and aeroplane is where the real test of superiority is certain to be found, and both the moral ascendency and actual command of the air goes to the pilot whose aeroplane and whose skill permits him to climb over and dominate or drive the enemy out of the sky. It has been assumed throughout this work that one of the most vital parts of an aeroplane — the motor — was working smoothly and without a miss. If not imiversally the case, at present, the day is certainly not far distant when aeroplane motors may be relied upon exactly as are automobile motors today. Attention has purposely not been g^ven to the military technique of the use of aeroplanes in military or naval operations, neither has any special attention been given to the art of Aying, cross country navigation, etc. — features that are acquired by the military aviator from the officers and instructors of the Flying Corps in their routine work. Consideration has been given to the military aeroplane for the particular purpose of assisting the military aviator or student to acquire a better appreciation of the machine, a fuller knowledge of why it ffies and what he may expect of it in performance, in strength and in flying characteristics. Finally a word about UPKEEP An aeroplane should really be kept in shape like a yacht, all parts polished and cleaned and oiled, with a very definite effort on the part of the crew to have everything spick and span. Particularly should oil be cleaned off of the wings and of wooden parts, as it so readily tends to rot them. Not only is this work desirable for looks, but it is a systematic way to inspect foi flaws. INSPECTION Once in the air, if nothing is wrong with the aeroplane, the flyer is absolute master of his own destiny. On the contrarj', if there is something wrong, he cannot "stop by the roadside and fix it." So inspection of an aeroplane is of the very greatest fundamental importance and should never be neglected. In fact, a good pilot, regardless of any foolish criticisms of "cold feet" on the part of less sensible rivals, will examine his machine 199 carefully every time before going up, taking particular care if he has a passenger. And in addition, in the proper "esprit de corps" of flying, he will take no offense if his passenger does likewise to assure himself. Because there is really no "come back" to something wrong with an aeroplane, and where there are unfortunately still so many little fittings and "gadgets," it is very wise to acquire this point of view from the outset. In general, a quick, thorough inspection covers the following vital points : Is the motor wiring and switch correct ? Does the throttle work properly ? Is it loose or worn anywhere ? Is the propeller locked on ? Is there water in the radiator, oil and gas in the tanks ? Does the gas tank hold its air pressure ? Are the controls all connected up to work properly ? Is there any unlooked for friction or lack of alignment ? Are there any loose bolts or unsaf etied turnbuckles ? Particularly are the wing pins at the body joint properly locked? Is the motor hood fastened down? And the rear fuselage cover? The tail — is it properly fastened down ? Is anything going to jam a pulley or a control ? Is your luggage or military load properly fastened in ? Is your safety belt really safe? Have you a fire extinguisher handy ? Is there grease on the wheels ? Are the wheels held by the pins from slipping off the axle ? Run your motor a few minutes to see that it really "motes." Take a good look around the field for wind direction and other aeroplanes ! INDEX Acceleration, machines of, 29. Aeroboats, 144, 146. Aerodynamics, 36, 41. Aerofoils, 67, 73-75. Aeroplane, defined, 11, 96. Aeroplane types, 11. Aeroplane characteristics, 85, 96. Adjustable stabilizer, 161. Ailerons, def., 12. Air flow, diagrams, 45, 48, 57. Air, nature of, 42. Air resistances, 45, 143. Airships, def., 9. Airworthiness, 151. Alignment of wings, 126, 130. Albatros aero, 13. Altitude effects, 106. America, flying boat, 22. Angle ranges, 156. Angle of incidence, 57, 127. Angular velocity, 31. Areas of figures, 142. Areas and Volumes, Table, 142. Ash, 137. Aspect ratio, 49, 73. Aspect ratio, corrected table, 74. Aspect ratio of aerofoils, 73. Aspect ratio of hydro surfaces, 148. Assembly, 124. Atmosphere, 42. Automatic stability, 194. Axes of rotation of aeroplane, 11. B Balance or aeroplane, 93, 94. Balance, directions for, 171. Balance of hydros, 146. Balanced rudder. 88. Beam sections, 117. Beam, bending moments, 117. Bending moments, general, 118, 119. Biplanes, tractors and pushers, 12, 13. Biplane effect, 81. Biplane interference, 80. Bleriot monoplane, 21. Blimp balloon, 9. Blind spots, 190. Bolts, locking of, 133. Bolts, strength of, 140. Bracing of wings, 108, 116, 123. Breguet Hydro, 22. Breguet bomber, 188. Bristol Fighter, opp. p. 14. Bustle. 23. Cables, 48, 50. Cable ends, opp. p. 132. Cable strengths, 140. Cabre attitude, 156. Camber, 58, 64. Cambered planes, 69. Camber of upper and lower face, 75. Camel Sopwith, opp. p. 14. Ceiling of the Aero, 107. Centers of forces, 88, 154, 160. Center of pressure, 61, 62, 76, 79, 114. Center of pressure total for aeroplane, 154. Center of gravity, by moment method 152, 153. Centers co-incident in balancing, 154. Centrifugal and Centripetal forces, 30. Centripetal force, stress. 111. Charts, 40. Chord, 81. Climbing, 177. Climbing Rate, 105. Climbing on turns, ISO. Complements, forces, 39. Composition of forces, 38. Constants, 27. Construction details, 132. Controls, opp. p. 89 Controllability, 152, 173. Co-ordinates, 40. Crystallization, 136. Curtiss Flying Boat, 22. Cylinders, resistances, 48, 50. D Decalage, 82. Deflection of Beams, 117. Density of air, 35. Dep Control, opp. p. 88. Deperdussin monoplane, 21. Depth of curvature, 70, 71. Depth of aerofoil section, 76. Diametral plane, 49. Dihedral angle, 83. Dihedral arched, 163, 164. Dihedral adjustment, 128. Dihedral, longitudinal, 158. Dipping front edge, 74. Directional stability, 166. Directional center, 166. Dirigible Balloons, 10, 11. Diving, def., 176. Diving speed, max.. 111. Dopes for wing surfaces, 139. Drift, 58, 59, 63, 101. Dunne aeroplane, 23, 168. E Efficiency in power, 38. Eiffel's experiments, 44, 46. Eiffel wings, Nos. 13, 31, 32, 36— p. 76 Eiffel wings, Nos. 35, 48, 53, 54— p. 77. Elastic limit, 35. Elasticity, 32. Elevator, def., 12. Elevator flap, descr., 88. Elevator, rear and front, 14. Empennages, def., 16. Empirical constants, 27. Energy, kinetic and potential, 36. 200 Engine bed, opp. p. 134. Enlargement from model tests, 74. Equilibrium of the air forces, 88, 152. Fairing, 49, 56. Fatigue, 136. Fiber stress, 119. Fineness ratio, 49. Fin system, double high Fins, 164. Fittings, metal, opp. p. 133. Flap and fin rudder, 13, 88. Flaps, def., 12. Flat planes, 49. Flotation, 144. Flying machine, def., 11. Flying in region of inverse controls, 156. Flying in wind, 182. Flying at low angles, 156. Flying Boats, 22. Follow thru, 122. Forces, graphics, of, 34. Formulae, derived and empirical, 26, 27. Formulae for Aerodynamics, 47, 48. Friction def, 44. Fuel Charts, opp. p. 104. Fuselages, 12, 51, 85. Gauges, table of 141. Gap, definition, 81. Gases, mechanics of, 36. German aeros, opp. p. 22. Gliding, 104, 175. Goettingen results, 52, 53. Graphical stress methods, 34, 40, 108. Gyroscope, 31. Gun iriountings, 186. H Handley-Page, 18. Helicopter, definition, 10. Horsepower, defined, 37. Horsepower, available and required, 102, 103. Hydro-aeroplanes, 143. Hydroplaning, 143, 145, 146. I Immelmann Turn, 180. Inclination of Aeroplane, 99. Inclined surfaces, 57, 69. Inertia, 166. Inherent stability, 23. Inspection, 198. Interference of aerofoils, 81. Interference of tail plane, 86. Inversion of Rudder and Elevator, 179. K Kite balloon, 7, 8. Landing, 181, 182. Landing Gears, opp. 133, 134. Langley's experiments, 46. Lateral balance, 162. Lateral center, 153. Lateral control, 163, 165. Lateral stability, 162. Leading edge, 58, 75. Lifting capacity, 98, 99. Lift and Drift, 58. Lilienthal's Tangential, 63. Lifting tail, 154. Linen on wings, 139. Lining up, 125. Looping, 180. Longitudinal stability, 159. M Marine aeroplanes, 143. Master diameter, 49. Mathematical signs, 28. Mechanics, theory of, 29. Metals, weights and strengths, 139-140. Metric system, 142. Metric conversion, 142. Modulus of Elasticity, 33. Moments of forces, 39. Moments of Inertia of Aeroplanes, 166. Moments of Inertia, mechanics, 30, 31. Monocoques, 21. Monoplanes, 21. N Nacelle, 19. Negative Tall, 154. Newton, 46. Nieuport, 2. Normal plane, 49. Nose Spinning, 182, NPL 4, wing 78. 183. O Observation, 187. Opposition of Speed and Climb, 105. Ordinate, maximum, 72. Ornithopter, definition, 10. Pancaking, 174, 175. Parallel normal surfaces, 52. Parseval dirigible shapes, 52. Pendulum, mechanics of, 30. "Pique, vol," 156. Pitching, definition, 11, 12. Pitching, stability, 157. Pitot Tube, 35. Polar Co-ordinates, 34. Pontoon, double float system, 146, 149. Pontoons, sections and details, 146. Power, definition of, 37. Power charts, opp. 104. ^ Power required and available, 102. Pressure, definition, 42. Propeller, general, 90, 91. Propeller balancing, 132. Propeller tipping, 132. Propeller thrust, 113. Propeller diagram and offsets, 131. Propeller stream action in tail, 159. Propeller Torque, 162. Pushers, descr. and def., 13, 16. R RAF 6 wing, 78. Raked end wing, 73. Rectangles, resistances, 50. Rectangular Co-ordinates, 34. 201 Regime of flight, 156. Resistance, Total, to motion, 46, 100. Resisting Moment of a beam, 119. Resolution of Forces on planes, 58. Resultant, 38. Retreat, definition, 164. Retreat, effects, 164. Reversed curve sections, 68, 78. Reversed curve wings, 78. Rolling, definition, 11, 12. Rolling control, 12. Rolling and stability, 162. Rotary motion, mechanics of, 30. Rounded end plane, 73. Rudder, definition, 12, 88. Safety Factors, 109. Seaworthiness, 148. S. E. 5 aero, opp. p. 14. Seaplanes, 40. Shapes of planes, 73. Shapes of Pontoons, 144. Sheet Metal table, 140. Shape coefiicients, 51. Side Slipping, 178. Sines and Cosines, 28. Skidding, 178. Sopwith, 5, 13, 15. Solid Wire table 141. Spad aeroplane, 3. Span, definition, 58. Spars, stresses in, 116. Spar, sections, 117. Spar weakening, 121. Specific ('.ra\ity, 35. Speed and Scale effect, 74. Speed, High and Low, 180. Speed Range, 99. Spheres, Resistance of, 48, 51. Spiral Instability, Spruce, 137. Square normal plates. 48, 49. Stability, definition, 151. Stabilizer, 85. Stagger, def., 81, 82. Stagger adjustment, 127. Stall, opp. p. 174. Staggering, effect of, 82. Stalling, def., 174. Steel, general, 134. Steel, tables, 140, 141. Steps on hydros, 148. Stick control, opp. p. 89. Strain, def., 33. Stream lines, 14, 45, 48, 69. Stream line wire 142. Stream photos, 44, 69. Stress, def., 33. Stress Analysis, 108. Stresses, general, 109, 112. Stresses, maximum. 111. Stresses, on body joints, 122. Structural Air Resistance, 100. Struts, 54, 55. Strut Formula, 115. Stunting, 173. Suction on top aerofoils, 69. Tail high attitude, 155. Tail interference, 86. Tail planes, discussion, 86, 87. Tail Skid, opp. p. 134. Taube, descr., 169. Tandem system, convergent and diver- gent, 158. Tandem seating, 4. Tanks, formulae for, 142. Taxi-ing, 174. Thrust, center of, 88, 160. Torque, 90, 162. Tractor, def. and disc, 12, 13. Trailing edge, 58, 75. Triangles, solution of, 27, 28. Triplane, 12, 13. Trueing up wings, 126, 128. Tubing, steel, table, 141. Turning, 177. U Ultimate Resistance, 35. Uniform load, 117. Unpacking of aeroplanes, 124. Upkeep, 198. "\'" bottom on pontoons or hulls, 149. Variable camber wing, 79. Velocity, mechanics of, 29. Visualizing air, its importance, 44, 45. Volumes of various shapes, for tankage, 142. W Warping, 12, 72. Washout, 89, 90. Weathercock stability, 166. Wheels, resistance of, 55. Wind tunnel, 43, 44. Wing construction, opp. p. 134. Wing, covering, 138. Wing, loading, 114. Wing stresses, 113. Wires, 48, 50. Wire adjustment 126, 127. Wire tightening, 121. Wire ends for solid wire, 136. Woods, kinds of, 137. \\'ork, definition, 36. Wright, Aeroplanes, 16. Yawing, definition, 11. Yawing, stability, 166. Zeppelin dirigible balloon, 8. 10. 202