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Source of Acqulsltion CASI Acquired U XAT IOITAL ADVISORY C OMNITTEE FOR 8 2 B O N A U T I C S
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ADVANCE COlTFI3EiYT I A L REPOZT A I R F O I L 0 3 ' 5-FOOT CEOBD AT HIGE SPXXD BY M n n l e y J . Hood and J o s e p h L. A n d e r s o n A m e s A e r o n a u t i c n l Laboratory Moffett F i e l d , Calif.
Unclassified - Notice remarked 4/17/09 September 1942 NATIOBAL ADVISORY COMMITTEE FOR AEXOATAU';?ICS ADVANCE C OYJFIDENTIAL REPORT TESTS OF AX NACA 66,2-420 AIRFOIL 03' 5-FOOT CEORD A T EIGli SPEED .
By Planley J . Bood and J o s e p h L. Anderson T h i s r e p o r t c o v e r s t e s t s of a 5 - f o o t model of t h e NACA 66,2-420 l o w - d r a g a i r f o i l a t h i g h s p e e d s i n c l u d i n g t h e c r i t i c a l c o n p r e s s i b i l i t y s p s e d . S e c t i o n c o e f f i c i e n t s o f l i f t , d r a g , and p i t c h i n g moment, and e x t e n s i v e p r e s - s u r e - d i s t r i b u t i o n d a t a a r e p r e s e n t e d .
The s e c t i o n d r a g c o e f f i c i e n t a t t h e d e s i g n l i f t c o e f f i c i e n t of 0.4 i n c r e a s e d from 0.0042 a t low s p e e d s t o 0 , 0 0 5 2 a t a Mach number of 0.56 (390 mph a t 2 5 , 0 0 0 f t a l t i t u d e ) . The c r i t i c a l Mach number was a b o u t 0.60.
The r e s u l t s c o v e r a R e y n o l d s number r a n g e from 4 m i l l i o n s t o 1 7 m i l l i o n s .
T e s t s h a v e S e e n madc o f a n 3ACB %6,2-420 n i r f o i l i n o r d e r t o p r o v i d e d a t a on t h e c h a r a c t e r i s t i c s of a t h i c k l o w - d r a g a i r f o i l at h i g h s p e e d s . Thc SAGA 66,2-420 a i r - f o i l was c h o s e n a s b e i n g r c p r a s e n t s t i v c of low-drag a i r - . f o i l s of r a t h e r h i g h cam3cr (0.40 d e s i g n l i f t c o e f f i c i e n t ) and t h i c k n e s s (20 p e r c o n t of t h a c h o r d ) . The a i r f o i l . was t e s t e d a t s p e e d s . u g t o and s l i g h t l y a b o v e t h e c r i t i c a l s p e e d .
APPARATUS AND METHO3 The t e s t s were c o n d u c t e d i n t h e NACA 1 6 - f o o t wind t u n n e l a t t h e Ames A e r o n a u t i c a l L a b o r a t o r y , M o f f e t t F i e l d , C a l i f o r n i a . T h i s wind t u n n e l h a s a c l o s e d c i r c u l a , r t e s t s e c t i o n 1 6 f e e t i n d i a m e t e r .
The a i r f o i l was of t h e iTACA 66,2-420, a = 1 s e c t i o n .
I n t h e u s u a l manner t h i s a i r f o i l d e s i g n a t i o n s i g n i f i e s t h a t t h e a i r f o i l i s a low-drag a i r f o i l of t h e 6 f a m P l y , t h a t f a l l i n g p r e & s u r e s e x t e n d f r o m t h e l e a d i n g e d g e t o a p o i n t 60 p e r c e n t of t h e c h o r d from t h e l e a d i n g edge o v e r a w o r k i n g c t r a n g e of a p p r o x i m a t e l y 2 0 . 2 from t h e d e s i g n l i f t c o e f f i c i e n t , t h a t t h e d e s i g n l i f t c o e f - f i c i e n t i s 0 . 4 , t h a t t h e a i r f o i l t h i c k n e s s i s 20 p z r c e n t of t h e c h o r d , and t h a t t h e l i f t a t t h e d e s i g n l i f t c o e f - f i c i e n t i s u n i f o r m l y d i s t r i b u t e d a c r o s s t h e c h o r d . The c o o r d i n a t e s ox t h e a i r f o i l s s c t i o n a r e shown i n f i g u r o I . , : T h e s e c o o r d i n a t e s wcre d e r i v e d from t h e c o o r d i n a t e s of t h e I NACA 6 6 , 2 - 0 1 8 a i r f o i l a s g i v e n i n r e f e r e n c e 1, by t h e " method e x p l a i n e d t h e r e . The a i r f o i l had a , c o n s t a n t c h o r d of 5 f e e t and c o m p l e t e l y s p a n n e d t h e t a s t s e c t i o n a s shown i n f i g u r e 2. Between t h e ends of t h e a 2 r f o i l and t h e t u n - n e l walls t h e c l e a r a n c e y a s a p p r o x i m a t e l y 3 / 1 6 i n c h r The a i r f o i l s p a r e x t e n d a d t h r o u g h o p e n i n g s i n t h e t u n n e l w a l l s t o t r u n n i o n s which s u p p o r t e d t h e a i r f o i l and p e r m i t t e d c h a n g i n g t h e a n g l e of a t t a c k , E x c e p t f o r t h a s t e e l s p a r , t h e a i r f o i l was of wood. The s u r f a c e was p a i n t e d , sand- p a p e r e d s m o o t h , an& p o l i s h e d . A s m o o t h , a c c u r a t e s u r f a c e . * w a s m a i n t a i n a d t h r o u g h o u t t h e t e s t s . The a n g l e of a t t a c k was checked d u r i n g t h e t e s t s by means of a t o r q u e t u b e e x t e n d i n g t h r o u g h t h e a i r f o i l t o t h e c e n t e r of t h e s p a n w h e r e i t was a n c h o r e d . By t h i s means t h e t r u e a n g l e a t t h e c e n t e r of t h e s p a n w a s a l w a y s d e t e r m i n e d a n d e r r o r s due t o e l a s t i c t w i s t i n g of t h e a i r f o i l were e l i m i n a t e d .
A 1 1 d r a g c o e f f i c i e n t s w e r e computed f r o m measurements of t h e momentum l o s s i n t h e wake of t h e a i r f o i l , The meas- u r e m e n t s w e r e made 3 f e e t b e h i n d t h e t r a i l i n g edge a t t h e c e n t e r of t h e s p a n by means of a r a k e of t o t a l - p r e s s u s e a n d s t a t i c - p r e s s u r e t u b c s c o n n e c t e d t o a m u l t i p l e manometer, The p r e s s u - r e s w e r e r e c o r d e d by p h o t o g r a p h i n g t h e manometer.
The d r a g c o e f f i c i e n t s w e r e computed by J o n e s ' method mod- .
i f i e d t o i n c l u d e c o m p r e s s i b i l i t y e f f e c t s .
The s t a t i c - p r e s s u r e d i s t r i b u t i o n was measured w i t h f l ' u s h o r i f i c e s b u i l t i n t o t h e a i r f o i l 2.5 f e e t t o t h e r i g h t of t h e c e n t e r of t h e s p a n . T h e - p r e s s u r z s w e r e r e c o r d e d by p h o t o g r a p h i n g a m u l t i p l e manometer t o which t h e o r i f i c z s wcre c o n n e c t e d .
B The l i f t c o e f f i c i e n t s w e r e d e r i v e d by i n t e g r a t i n g k p l o t s of t h e p r e s s u r e d i s t r i b u t i o r t o o b t a i n t h e c o e f f i c i e n t of normal f o r c e , and t h e n r e s a l v i n g t h e normal f o r c e c o e f - A f i c i e n t and t h e d r a g c o e f f i c i e n t i n a c c o r d n n c c w i t h t h e a n g l e of a t t a c k t o o b t a i n t h e l i f t c o e f f i c i e n t . C o e f f i - c i e n t s of p i t c h i n g moment w e r e computed from i n t e g r a t e d moments of chordvrise and s p a n w i s e p r e s s u r e - d i s t r i b u t i o n p l o t s .
F o r n few of t h e t e s t s t h e t r n n s i t i q n p o i n t was a r b i t r a r i l y f i x e d a t 1 0 o r 60 p e r c e n t c h o r d w i s e p s s i t i o n s on b o t h u p p e r and l o w e r s u r f a c e s by s p n n w i s e b a n d s 2f carborundum g r a i n s . The b a n d s o f carborundum were a p p r o x - i m a t e l y 112. i n c h v i d e and t h e g r a i n s i z e was n u n b e r 60.
RESULTS A%D DISCUSSIOW 8 x 1 r e s u l t s a r e shown a s s e c t i o n p r o p e r t i e s of t h e a i r f o i l i n t e r n s of t h e u s u a l n o n - d i m e n s i o n a l c o e f f i c i e n t s , No c o r r e c t i o n s have b e e n made f o r t h e c o n s t r i c t i n g e f f e c t of tile w i n d - t u n n e l walls. I t i s e s t i m a t e d t h a t t h e e r r o r s d u e t o t h e o m i s s i o n of t h e s e c o r r e c t i o n s do n o t e x c e e d 2 p e r c e n t a t low s p e e d s and 4 p e r c e n t ' a t t h e h i g h e s t s p e e d s , These e r r o r s t e n d $0 i n d i c a t e t h e c o e f f i c i e n t s a s t o o l a r g e n u n e r i c a l l y , t h u s g i v i n g c o n s e r v a . t i ~ e v a l u e s f o r most u s e s .
~ h e * c h a r a c t e r i s t i c s o f t h e a i r f o i l at 0 d e g r e e a n g l e of a t t a c k a x e shown i n f i g u r e s 3 and 4 2 s t h e y v a r y w i t h Mach numbar a n d Re;rnclds number, r e s p e c t i v e l y . The s e c t i o n d r a g c o e f f i c i e n t f o ? t h e smooth a i r f o i l was o n l y 0.0042 a t low s p e e d s a n d i n c r e p - s e d g r a d u a l l y t o 0.0052 a s t h e Mach number and R e y n o l d s number w e r e i i l c ? - ~ a s e d t o 0 , 5 6 a n d 1 6 , 5 0 0 , 0 0 0 , r e s p e c t i v e l y . The minimum p r e s s u r e c o e f f i c i e n t s p l o t t e d i n f i g u r e 5 i n d i c a t e t h a t t h e c r i t - i c a l Mach number was 0 . 6 , p,nd t h i s v a l u e i s c o n f i r m e d by t h e b r e a k i n t h e c u r v e showing l i f t c o e f f i c i e n t on f i g u r e 3. The d r a g s t a r t e d i n c r e a s i n g r a p i d l y a t Mz.ch numbers a b o v e 0 . 5 6 , h o w e v c r , and a t a @iach number of 0.60 t h e d r a g c o e f f i c i e n t was d o u b l e i t s low-speed v a l u e .
A t a Mach number o f 0 . 6 3 3 t h e d r a g coefficient had i n - c r c a . s e d t o s i x t i m e s i t s low-spoed v ~ l u e . Tho m a j o r p o r - t i o n o f t h i s l a r g e d r a g i n c r a a s e i s b a l i e v o d t o b e t h e r e s u l t of c o m p r e s s i b i l i t y shock. A t t h e h i g h Mrch numbers t h c a d v e r s e p r e s s u r e g r a d i e n t s o v e r t h e e f t e r 40 p e r c e n t of t h e c h o r d become v e r y l a r g e and map h9,ve c a u s e d s e p - a r a t i o n which would c o n t r i b u t e t o t h e i n c r e a s e i n d r a g , A s comp-red t o t h e smogth w i n g , r o n g h n a s s r t t h s 6 0 - p e r c n n t c h o r d s t n t i o n c'-used an ~ l m o s t c o n s t a n t i n - c r e a s e i n t h e d r a g c o e f f i c i e n t , a b o u t 0 , 0 0 0 9 , a t a l l Nach n u n b a r s u p t o 0.56. T h i s c o m p a r i s o n i n d i c a t ~ s t1ic7,t t h e n a t u r a l t r ~ n s i t i o n p ~ i n t ?:?as a f t of t h e 6 0 - p a r c a n t s t a t i o n ( t h e minimum p r e s s u r e p o i n t ) a t l e a s t u n t i l t h e c r i t i c a l Mach number w a s a p p r o a c h e d . The r c s u l t s do n o t show how f a r t h e R e y n g l d s number can b c i n c r e a s e d b e f o r e t h e t r a n s i t i o n p o i n t moves f o r w a r d ~ , n d t h e dr?,g i n c r e a s e s d u e t ? s c a i c e f f e c t . T c s t s o f a l n r g c r c h o r d a i r f o i l 2 f t h e same s e c t i o n a r e t o b e made i n o r d o r t o c v a l u a t e t h e c z p a b i l i t i e s of t h i s a i r f ? i l n t h i g h e r B a y n o l d s numbers.
With t h e smooth a i r f o i l ~t 0 d e g r e c a n g l e of a t t a c k t h e pitching-moment c o e f f i c i e n t r e m a i n e d w i t h i n 0.02 of t h e t h e o r e t i c a l v a l u e of -0.10 t h r o u g h o u t t h e r a n g e of t h e t e s t s .
Only a s m a l l c h a n g e i n o c s u r r e d a t t h e c r i t i c a l speed.
% / 4 The l i f t c o e f f i c i e n t , t h e o r e t i c a l l y 0.40, i n c r e a s e d from ap- p r o x i m a t e l y 0.38 a t low s p e e d s t o 0.48 et t h e c r i t i c a l s p e e d a n d t h e n d e c r e a s e d r a p i d l y a b o v e t h e c r i t i c a l s p e e d . Com- p a r e d t o t h e e x p e r i m e n t a l i n c r e a s e from 0.38 t o 0.48, t h e t h e o r y of P r s n d t l and G l a u e r t t h a t t h e l i f t i n c r e a s e s w i t h Wach number a s l / = would p r e d i c t a n i n c r e a s e t o 0.465.
F i x i n g t h a t r a n s i t i o n p o i n t ~t 6 0 - p z r c e n t c h o r d hnd u n i m p o r t a n t e f f e c t s on t h o l i f t and p i t c h i n g moment. F i x - i n g t h e t r a n s i t i o n p o i n t a t 1 0 - p e r c e n t c h o r d i n c r e a s e d t h e minimum d r a g c o a f f i c i e n t t o 0 , 0 1 0 6 , d e c r e a s e d t h e l i f t Coef- f i c i e n t b y 0.1 o r more, %ad i n c r e a s e d t h e p i t c h i n g moment c o e f f i c i e n t by 0.025 o r more. T h e s e c h z n g e s a r e i n d i c a t i v e of what m i g h t o c c u r i f t h e t r n n s i t i o n t e k e s p l a c e prema- t u r e l y due t o f s u l t y wing c o n s t r u c t i o n , d e t e r i o r a t i o n of t h e wing c o n t o u r o r s m o o t h n e s s i n s c r v i c e , i n t e r f e r e n c e , o r o t h e r c a u s e s .
The c h a r a c t e r i s t i c s of t h e smooth a i r f o i l a t v a r i o u s a n g l e s of a t t a c k a r e shown i n f i g u r e 6 i n t e r m s of Wach number and i n f i g u r e 7 i n t e r m s of B e y n o l d s number. F i g - u r e 8 p r e s e n t s c r o s s - p l o t s t a k e n from f i g u r e 6 a n d shows, f o r a Hach number of 0 . 4 , t h e v a r i a t i o n of l i f t w i t h a n g l e of a t t a c k a n d t h e v a r i a t i o n of d r a g a n d p i t c h i n g moment w i t h L i f t . These f i g u r e s show t h a t t h e d r a g r e m a i n s low f o r l i f t c o e f f i c i e n t s from a p p r o x i m a t e l y 0.35 t o 0 . 5 5 and i n c r e a s e s t o h i g h e r v a l u e s f o r l i f t c o e f f i c i z n t s o u t s i d a of t h i s r a n g e . F o r t h e c o n d i t i o b s of t h c s e t e s t s t h e a i r - f o i l d i d n o t r e t a i n i t s l o w - d r a g c h a r a c t e r i s t i c s o v e r q u i t e a s w i d e a r a n g e of l i f t c o e f f i c i e n t s a s i s i n d i c a t e d by t h e a i r f o i l d e s i g n a t i o n (k0.2) I n p r a c t i c e t h e r a n g e of l i f t c o e f f i c i e n t s o v e r w h i c h t h e low-drag p e r s i s t s c a n be e x t e n d e d by t h e u s c of f l a p s w i t h s m a l l d e f l o c t i o n .
From f i g u r e s 6 e n d 9 i t i s e v i d e n t t h a t t h c c r i t i c a l Xach number i n c r e a s e s s l i g h t l y a s t h e a n g l e of a t t a c k i s de- c r e a s e d t o -2O, and d e c r e a s e s a s t h e a n g l e of a t t a c k i s i n c r e a s e d a b o v e +2O, The r e a s o n f o r t h e c h a n g e i n c r i t i c a l s p e e d w i t h a n g l e of a t t a c k i s e v i a e n t from t h e p l o t s of p r e s s u r e d i s t r i b u t i o n , The v a r i a t i o n of t h e minimum p r e s s u r e c o e f f i c i e n t S w i t h Mach number f o r d i f f e r e n t a i r f ? i l c o n d i t i o n s i s sh3wn i n f i g u r e s 5 and 9, $ T h e r e & r e a l s o shown c u r v e s c~f c r i t i c c l p r e s s u r e coefficients Scr t h a v a l u e s at w h i c h t h e s p e e d of s o u n d - i s r e a c h e d l o c a l l y , The i n t e r - s e c t i o n s of t h e p r e s s u r s - c o e f f i c i e n t c u r v e s w i t h t h e c r i t i c a l c u r v e i n d i c a t e t h e c r i t i c a l M ~ c h numbers a t w h i c h s o n i c v e l o c i t y i s r e a c h e d l o c a l l y . I n f i g u r e 5 t h e r e a r e a l s o shown t w 3 c u r v e s of van ~ k r m A n ' s t h e o - \ r e t i c a l e q u a t i o n f o r t h e r a t e of i n c r e a s e of p r e s s u r e c o c f f i c i c n t w i t h Mach numbar ( r e f e r e n c e 2 ) . With t h e smooth a i r f o i l t h e p r e s s u r e s i n c r e a s e d more r a p i d l y w i t h Mach number t h a n would h ~ v e been p r e d i c t e d from t h e t h e o r y ; c o n s e q u e n t l y , p r e d i c t i o n s b a s e d upon t h e r e s u l t s of l o v - s p e e d t e s t s would o v e r e s t i m a t e t h e c r i t i c a l s p e e d , With t h e t r a n s i t i o n f i x e d by r o u g h n e s s a t 1 0 - p e r c a n t c h o r d , t h e p r e s s u r a s w e r e l e s s t h r o u g h o u t t h e s p e e d r a n g e and i n c r e a s e d l e s s r a p i d l y a s t h e s p e e d was i n c r e a s e d . T h e s e d i f f e r e n c e s p r o b a b l y r e s u l t e d from c h a n g a s i n t h e e f f e c t i v e s h a p e of t h e a i r f o i l c a u s c d by t h e t h i c k a r b o u n d a r y l e y e r w i t h t h e f o r w a r d t r a n s i t i o n , F i g u r e 1 0 shows t h e s e c t i o n n o r m a l f o r c e a n d p i t c h i n g ., m2ment c o e f f i c i e n t s f o r a n g l z s o f a t t a c k from below t h e n e g a t i v e s t a l l t o a b o v e t h e p o s i t i v e s t a l l f o r t h e smooth a i r f o i l a n d a l s o w i t h r o u g h n z s s f i x i n g t h c t r a n s i t i o n a t t h e 1 0 - p e r c e n t c h o r d p o s i t i o n , The m2ximum p o s i t i v e and n e g a t i v e n o r m a l f o r c e c o e f f i c i e n t s were + 1 , 5 0 and -1.35, r e s p e c t i v e l y , and wart u n a f f e c t e d bg t h e r o u g h n s s s .
F i g u r e s 11 t o 17, i n c l u s i v e , p r e s e n t p r e s s u r e - d i s - t r i b u t i o n d a t a f o r var,ious Nach n u m b e r s , a n g l e s of attcxck, and t r a n s i t i o n l o c a t i o n s . T h e s e d a t a a r e u s o f u l f o r com- p u t i n g s t r u c t u r a l l o a d s , and f o r e s t i m a t i n g i n d u c b d v e l o c - i t i e s , p r a s s u r c g r a d i e n t s , a n d c r i t i c a l s p e e d s when t h c a i r f o i l i s combined w i t h o t h e r b o d i s s .
CONCLUSIONS The s e c t i o n d r a g c o e f f i c i e n t of t h e a i r f o i l a t t h e d a s i g n l i f t c o e f f i c i e n t of 0.4 i n c r e a s e d f r c ~ 0.0042 a t low speeds to 0.0052 at 3, Mach number of 0.56, a b o v e which thc drag increased rapidly.
Ames Aeronautical Laboratory, National Advisory Committee for Aeronautics, Noffett Field, Calif.
1 . Jacobs, Eastman N., Abbott, Ira H., and Davidson, Miltoa: ,Preliminary Low-Drag-Airfoil and Flap Data from Tests at Large Reynolds Xumbers and Low Turbulence. A . C.B., NACA, Karch 1942.
2 . von g&rm&n, Th.: CompressiSility Effects in Aerodynamics. Journal of the Aeronautical Sciences, vol, 8, no. -9, - July 1941, pp. 337-56.
M A C H NWBER, M Figure 5 . - Effect of compressibility and eurface condition on the minimum pressure coefficients. a = 0 ' Figure 3 . - Variation of the section coefficients with Mach number for various surface conditions. a = 00 Figure 4.- Variation of the section coefficients with Reynolds number for various surface oonditione. a n o0 Figure 9 . - Variation of section coefficients with Mach number and angle of attack.
Figure 7.- Variation of section coefficients with Reynolds number and angle of attack.
Figure U.- Effect of compressibility and angle of attack on 'the minimum pressure coefiiolent of the upper surface.
IAOP Pig. l a Figure 1 2 . - Ress e distribution on the airfoil with roughmaeas at 10-pescent *or&. c 4 = 0 T NACA Fig. 13 0 0. / 0.2 0. 3 0.4 0.5 Q6 0 7 X/c Figure 1 3 . - Pressme distribution Q E I the s-th a i r f o i l . a = -2'.
/'
0.8 a9 o 0 . 1 a2 0.3 0.4 0.5 0.6 0.7
v c
Figure 1 5 . - Pressure distribution on the smooth airfoil.. a = +bO.
J
Variation of the pressure coefficient for the upper surface with angle Figure 1 6 . - of attack. Mach number = 0.185.
Fig. 17 UACA Figure 17.- Variation of the pressure coefficient for the lower surface with angle of attack. Mach number .: 0.185.