^ ^)ftc/r/^^'t ''^l ^?DH^S NACA RM L5i4-A29a CONFIDENTIAL NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS RESEARCH MEMORANDUM ON SLENDER-BODY THEORY AT TRANSONIC SPEEDS By Keith C. Harder and E. B. Klunker SUMMARY The basic ideas of the slender-body approximation have been applied to the nonlinear transonic- flow equation for the velocity potential in order to obtain some of the essential features of slender-body theory at transonic speeds. The results of the investigation are presented from a unified point of view which demonstrates the similarity of slender-body solutions in the various Mach nvimber ranges. The primary difference between the results in the different flow regimes is repre- sented by a certain function which is dependent upon the body area distribution and the stream Mach niimber. The transonic area rule and some conditions concerning its validity follow from the analysis. INTRODUCTION Slender-body theory originated with Monk's paper (ref. l) in 192^+ in which the forces on slender airships were calculated for low-speed flight. In 1938 Tsien (ref. 2) pointed out that Munk's airship theory also applied to the flow past inclined, pointed bodies at supersonic speeds. The subject gained new importance in 19^6 with the appearance of Jones's paper (ref. 3) in which it was shown that the basic ideas of the slender-body approximation could be used to calculate the forces on slender lifting wings at both subsonic and supersonic speeds provided that proper account was taken of trailing- vortex sheets. Since Jones's paper, the subject has received wide treatment in the literature. In an important paper in 19^9 j Ward (ref. h) developed a general unifying theory for the flow past smooth, slender, pointed bodies at supersonic speeds which contains as special cases the lifting planar wings of Jones and the slender nonlifting bodies treated by Von Karman (ref. 5)' The corresponding problem at subsonic speeds has been examined by Adams and Sears (ref. 6) who also extended the slender-body concepts to shapes which are not so slender. Lighthill (ref. 7) has given a method for calculating the flow past bodies with discontinuities in slope. Keime (ref. 8) has developed solutions for slender wings with thickness and various lifting configurations have been treated by Heaslet, Spreiter, Lomax, Ribner, and others (refs. 9 to li<-). CONFIDENTIAL CONFIDENTIAL NACA RM L5i<-A29a The slender-body theory presented in references 2 to Ik has been based upon the linearized equation for the velocity potential. In the present paper^ the basic ideas of the slender-body approximation are applied to the nonlinear transonic equation for the velocity potential in order to obtain some of the essential features of slender-body theory at transonic speeds. The attempt has been made to present the results from a unified point of view which demonstrates the similarity of the slender-body solutions in the various Mach number ranges. The authors wish to acknowledge the invaluable aid and advice of Dr. Adolf Busemann of the Langley Laboratory during the writing of this paper. SLENDER-BODY APPROXIMATION Slender-body theory deals with that class of shapes whose length is large compared with any lateral dimensi-dn. For such shapes at both subsonic and supersonic speeds, the flow in planes normal to the stream direction can be approximated by solutions of Laplace's equation. The justification is that for very slender wings or bodies the variation of the geometrical properties in the stream direction is small and, consequently, the rate of change of the longitudinal component of the velocity in the stream direction is also small. The various slender- body solutions have all been developed on the basis of the linearized potential equation. However, a similar development can be made on the basis of the nonlinear transonic equation. The simplest differential equation for the disturbance potential which is generally valid at transonic speeds (ref. I5, for example) is E w (7 1)m2$x]<] XX + $ $ rr ^98 = (1) where x, r, and -d Mach number, and 7 and constant volume, coordinates | and less potential by equation becomes are cylindrical coordinates, M is the stream is the ratio of specific heats at constant pressure With the introduction of the dimensionless r\ by X = Z| and r = bi^, and of the dimension- 2 = — 0(^^11;^)^ "the transonic potential- flow l/f (7 DM^d) 0, <^r 0. ee h^-h^-f--^ = ^ (2) CONFIDENTIAL NACA RM L54A29a CONFrDEWTIAL where Z is a characteristic length and b is a characteristic width such as the largest lateral dimension of the configuration. For suffi- ciently small values of the width parameter b/z, it appears that the terms involving derivatives in the stream direction can be neglected to obtain the result that the flow satisfies Laplace's equation <^rr + ^<^r + ^^^^ = (5) in the cross-flow plane. Equation (3) represents the slender-body approximation to equation (l). Some conditions will be determined subsequently which are necessary in order for solutions of equation (3) to be approximate solutions of equation (l). The boundary conditions for the flow about a bod;^'' in a uniform stream are the vanishing of the disturbance velocities at infinity and Sn dx dx on the body where n is in the direction normal to the body contour in the cross-flow plane. For flows satisfying Laplace's equation in the cross-flow plane, the surface boundary condition can be integrated (ref. k, for example) to give where dt is an element length in the direction of the tangent to any contour C in the cross-flow plane enclosing the body, S(x) is the cross-sectional area distribution of the body, and the prime denotes differentiation with respect to the indicated argument. The slender-body solution of equation (l) can be represented by a solution of equation (3) plus a function of integration. Since equa- tion (3) is independent of Mach number, the form of the solution is identical with the known slender-body solutions for subsonic and super- sonic flow. The slender-body analyses of references k and 6 have established that the solution can be represented by a distribution of sources and higher order singularities on the axis; an equivalent form is given by a distribution of sources in the region of the cross-flow- plane interior to the surface boundary. The slender-body solution is then expressed as CONFIDENTIAL CONFIDENTIAL NACA RM L5i|A29a •JLO + H Q. a c\j H Q. tH Ic^ CM ^1 «• 4-> •H C •H Cm 'Xi O ^ O J>5 OJ U • 0) u^ ^■-^ -P CO OJ 0) to m (U u li^ o •rH ai U 0) CQ •H O •H -P .a •H ■P X 'd CQ i>5 QU ^ 0) -P Tj C 0) ■H C •H Ch Ch O (D O •H -p O -d 0) •rH O o w ■p o •H HJ O 0) 0) fl CO -P I W ' o CJ •H H cti c O *jLn to ^1 (U -P 0) bO (D -p O u Cm ^ -d Q. a r-J O 0) Q. Jh ^ -P tJO O •^ W H 11 W C O •H pi . 0) oj ^ ^ ^H ^ W (U a •H Pi O Oj Jh Pi -d a w o o I o o o a* cu (U O p -P oj a o o P" >^ CI ^— U 0) ITN cd pi — -do (U o •M p> cd ft Eh a* cd pi CO cd xj^^ is required to be small compared with any of the other terms in the transonic differential equation. If this condition is to be satisfied in the neighborhood of weak shock waves where g'(x/z) would be required to have a jump proportional to the pressure rise and g"(x/z) would be infinite, the quantities (which were included in the terms denoted as 0(1)) g"(x/z)[l - m2 - (7 + Dm^^oJ (8) and g"(x/l)|^ Jj fip-^,i;^ix/l)lQgd(cos ^ - ^ COS ^ij + (sin ,3 - ^ sin ^1) Pi dp^ diS^ (9) CONFIDENTIAL NACA RM L5l<-A29a CONFIDENTIAL must be bo\inded there. Thus, at shock vaves the coefficients of g"(x/z) in expressions (8) and (9) must vanish. In the first of these expres- sions, which is axisyTnmetric, the coefficient of g (x/z) vanishes for a local Mach number of 1. Moreover, the average value of the local Mach number ahead and behind a weak normal shock wave is 1. With this interpretation of shock waves, then, the coefficient of g (x/z) vanishes and the solution should represent the flow in the neighborhood of weak normal shocks. Also, since g(x/z) is the same function for both a slender configuration and its associated body of revolution, their shock-wave systems would have the same strength and location. In order for expression (9) to be bounded in the neighborhood of a shock wave, the double integral must vanish. Since this double integral gives rise to asymmetric shapes and is zero for chordwise locations where the body is axisymmetric, the additional restriction is obtained that the body cross section must be circular in the vicinity of shock waves. The slender-body solutions in the various Mach number ranges are similar in that they are all represented by equation (6) although the function g(x/z) differs for the various speed ranges. Analytic expressions for g(x/z) have been obtained for supersonic and subsonic flows by considering general solutions of the complete linearized equa- tion satisfying the boundary condition of vanishing disturbance veloc- ities at infinity. These solutions were then expanded in the neighbor- hood of the body to evaluate g(x/z). Ward (ref. k) has determined this function for supersonic flows as g(x/z) 2rt s'(x/z) ■-x/z ^°^(^) - J^ ="(si'-•- .5 '^ s = ^ m c: 0) 0) -a •^ <" ^ « . =^ o1 m -o "S 2 ^ •2^2- CO 0) :n m « " P „ 5 2 m *- M 2 "S S 3 — n ID O rt ti £ t 0) - "> > = ? 2£ 22 ■a e i n <" U < z £ i! S S o s >■ ffl ^ eg 0) — 01 £ o o O T3 > 5 o M.2i 1. 2a£ i Ji £ So c E an rt '-' Q ■^ u 2 5 So in -r" K ■< rt Z z zo < z i e , eg O .2 s " o c u o O. CO a c OS rt •c , CO - o i •:;; ^ CO £ tic j: — " ■ ^:3 tj o 41 S o . 0) a to ■ 0) o " o -g £ > i ^^ S S -2 o o •? .5 c to C I- O c •o U i^ <; K z U d j= CM c o u. 0) "^^ oj c ". -ti Q ; CO (U > o ^ 0) t< B (u o x: to *J •o a 5 p a. — m T3 rn in • Oi m -- W u M s s o — & m ;" J= o ptc - *J (1) CO rt rt "O 'w O cij CO > iS Z (X Z o u w w y ^ ^ S to ^ o « . (J c r- .3 a 2 1/3 o o 01 -^ .-» 5=0 P'^£ to & 3 ' b. to u. Oj <" CO '.t' " in -:i ■^ < 1^ ^ Z ^ CM eo . U •r o 3 CO S Z 0) H tH o < So a s an lU ^ Q £>0 K 15 w <: S J <: rt z z z o < H Z -^ S = ^Z 2«o . CM U "3 •o U !; < xB ^d j:: CM 01 M -i Ox: " is 2 ? ™ * 2 ■a o J3 ii o S :•?£ «« to o — T o< •- ■- .5 = to a, Qj ^ lU 0) a] = ° 2 = "> u w o> •3 " °-o •a . to a to ^ 2 0; 'w c c 3 O • to 0) o,£ £ o E-i x: it; «•- ~ u — . O t« . =• 5 0) »J to X ■CMS' *J Q to >- (U O ££c I o § rt S t- 3 C — — o ^ to C4 3 :;:; t« a am o ^ « i ?i 2 m — to 13 c 3 C o /~i ^ y ^ t. £ D. 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