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A STUDY OF THE NOISE RADIATION F R O M FOUR HELICOPTER ROTOR BLADES A l b e r t Lee Beam Engineering, I n c .
Marianne Mosher NASA A m e s Research Center S U M M A R Y Acoustic measurements were taken of a modern h e l i c o p t e r r o t o r w i t h f o u r b l a d e - t i p shapes i n t h e NASA A m e s 40-by-80-Foot Wind Tunnel. The f o u r t i p shapes a r e : r e c t a n g u l a r , swept, t r a p e z o i d a l , and swept-tapered i n planform.
Acoustic e f f e c t s due t o t i p shape changes w e r e s t u d i e d based on t h e dBA l e v e l , peak n o i s e p r e s s u r e , and s u b j e c t i v e r a t i n g . The swept-tapered b l a d e was found t o be t h e q u i e t e s t above a n advancing t i p Mach number of about 0 . 9 , and t h e swept b l a d e was t h e q u i e t e s t a t low speed. The measured high-speed impulsive n o i s e was compared w i t h t h e o r e t i c a l p r e d i c t i o n s based on t h i c k n e s s e f f e c t s ; good agreement was found.
INTRODUCTION The h e l i c o p t e r r o t o r b l a d e t i p r e g i o n is one of t h e most important s o u r c e s of h e l i c o p t e r n o i s e , and t h e e f f e c t s of shaping t h e t i p on t h e n o i s e g e n e r a t i o n a r e complicated because of t h e many phenomena involved i n t h e t i p aerodynamics. For d i f f e r e n t t i p shapes, t h e b l a d e aerodynamic loading d i s t r i - b u t i o n s and t h e r e s u l t i n g t i p v o r t i c e s a r e d i f f e r e n t . Due t o t h e combined e f f e c t s of t i p v o r t e x changes and d i f f e r e n t aerodynamic response a t t h e t i p r e g i o n , t h e b l a d e l v o r t e x i n t e r a c t i o n n o i s e w i l l b e changed. Because of t h e change i n unsteady b l a d e l o a d i n g , t h e r o t a t i o n a l n o i s e r a d i a t i o n w i l l a l s o be changed. A t h i g h speed, b l a d e t h i c k n e s s can b e a s i g n i f i c a n t n o i s e s o u r c e ( r e f s . 1-4). The t h i c k n e s s n o i s e i s d i r e c t l y r e l a t e d t o t h e t i p planforms and t o t h e i r t h i c k n e s s d i s t r i b u t i o n s . Noise w i l l a l s o b e g e n e r a t e d when t h e t i p r e g i o n e x p e r i e n c e s s t r o n g c o m p r e s s i b i l i t y e f f e c t ( r e f . 5 ) , which is c l o s e l y r e l a t e d t o b l a d e t i p shapes.
Because of t h e complexity of t h e tip-shape e f f e c t s on r o t o r n o i s e g e n e r a t i o n , no complete a n a l y t i c a l method h a s been developed. Lyon, Mark, and P y l e ( r e f . 6 ) conducted a t h e o r e t i c a l s t u d y of t h e r o t o r t i p sound r a d i a t i o n and t r i e d t o s y n t h e s i z e r o t o r t i p s f o r less n o i s e . Lowson, Whatmore, and W h i t f i e l d ( r e f . 7 ) found t h a t c u t t i n g o f f one c o r n e r of r e c t a n g u l a r f a n t i p s c a n s i g n i f i c a n t l y r e d u c e t h e h i g h f r e q u e n c y broadband n o i s e . F a r a s s a t and Brown ( r e f . 2 ) and F a r a s s a t ( r e f . 8) f o u n d , i n a t h e o r e t i c a l s t u d y , t h a t a i r f o i l t h i c k n e s s d i s t r i b u t i o n and planform sweep of t h e b l a d e t i p r e g i o n have s i g n i f i c a n t e f f e c t s on n o i s e r a d i a t i o n . S i n c e t h e o r e t i c a l p r e d i c t i o n s c a n n o t c o m p l e t e l y d e t e r m i n e t h e a c o u s t i c e f f e c t s of t i p s h a p e s , e x p e r i m e n t s have b e e n performed t o e v a l u a t e s e v e r a l t i p s h a p e s , e i t h e r i n a wind t u n n e l o r i n f l i g h t . R e c e n t l y , a f u l l - s c a l e , o g e e - t i p h e l i c o p t e r r o t o r was t e s t e d on a w h i r l tower and i n f l i g h t ( r e f . 9 ) . Favor- a b l e e f f e c t s on a c o u s t i c s , performance, and l o a d s were f o u n d .
It i s e x p e c t e d , t h e r e f o r e , t h a t s u i t a b l e d e s i g n of r o t o r t i p s h a p e s c o u l d r e d u c e n o i s e and improve performance. To i n v e s t i g a t e t h i s p o s s i b i l i t y , a w i n d - t u n n e l e x p e r i m e n t was conducted u s i n g a modern h e l i c o p t e r r o t o r , 13.4-m i n d i a m e t e r , w i t h i n t e r c h a n g e a b l e t i p s . The r o t o r t e s t encompassed a n advance r a t i o r a n g e of 0.2 t o 0.375 and a n advancing t i p Mach number r a n g e of 0 . 7 2 t o 0.97. Four t i p s h a p e s were t e s t e d . The t e s t d a t a w e r e used t o d e t e r m i n e t h e a c o u s t i c e f f e c t s of t h e s p e c i f i c t i p s h a p e s and t o e s t a b l i s h a d a t a b a s e f o r t h e o r e t i c a l modeling and p r e d i c t i o n s of high-speed r o t o r i m p u l s i v e n o i s e .
SYMBOLS l i f t c o e f f i c i e n t t o s o l i d i t y r a t i o 'LR'* M b l a d e r o t a t i o n a l t i p Mach number
1 , o
b l a d e advancing t i p Mach number . M1,90 P a c o u s t i c a l p r e s s u r e , ~ / m V w i n d - t u n n e l s p e e d , m/sec a r o t o r s h a f t a n g l e , deg s QR b l a d e r o t a t i o n a l t i p s p e e d , mlsec EXPERIMENT A 13.4-m-diameter, f o u r - b l a d e d r o t o r w i t h i n t e r c h a n g e a b l e t i p s con- s t r u c t e d by S i k o r s k y A i r c r a f t was t e s t e d i n t h e NASA-Ames 40-by 80-Foot Wind Tunnel. Four d i f f e r e n t t i p s h a p e s were t e s t e d ( f i g . 1 ) . The t i p s h a p e s a r e : r e c t a n g u l a r , swept, t a p e r e d ( t r a p e z o i d a l ) , and s w e p t - t a p e r e d . The r e c t a n g u l a r t i p s e r v e s a s a b a s e l i n e ; t h e o t h e r t h r e e t i p s were used t o s y s t e m a t i c a l l y e v a l u a t e t h e e f f e c t s of t a p e r and sweep. The r o t o r b l a d e s had a c o n s t a n t chord and 9.5% t h i c k n e s s r a t i o a i r f o i l i n b o a r d of 95% r a d i u s . The r e c t a n g u l a r t i p m a i n t a i n e d t h e c o n s t a n t chord and t h i c k n e s s o u t t o 100% r a d i u s . The t r a p e - z o i d a l t i p was t a p e r e d t o 60% of t h e b a s e l i n e c h o r d a t t h e t i p , w i t h a c o n s t a n t t h i c k n e s s r a t i o and a n unswept q u a r t e r chord l i n e . The swept t i p had c o n s t a n t chord and t h i c k n e s s w i t h 20' sweepback. The s w e p t - t a p e r e d t i p had 35" sweep of t h e l e a d i n g edge, a 1 0 " sweep of t h e t r a i l i n g e d g e , and a c o n s t a n t t h i c k - n e s s r a t i o .
The i n v e s t i g a t i o n c o v e r e d a wide r a n g e of o p e r a t i n g c o n d i t i o n s . The r a n g e of advancing t i p Mach number was 0.72 t o 0.96, and t h e a d v a n c e r a t i o s w e r e from 0 . 2 t o 0.375. The r o t o r performance is g i v e n i n r e f e r e n c e 10.
Seven 13-mm (1/2-in.) B & K c o n d e n s e r microphones w i t h c a t h o d e f o l l o w e r s were used f o r t h e a c o u s t i c a l measurements. Each microphone was equipped w i t h a n o s e cone t o r e d u c e t h e wind-induced n o i s e . The microphone l o c a t i o n s a r e g i v e n i n t a b l e 1 and shown i n f i g u r e 2. The microphones were c a l i b r a t e d d a i l y w i t h a B & K p i s t o n p h o n e . C o n v e n t i o n a l a c o u s t i c power s u p p l y and a m p l i f i e r u n i t s w e r e used f o r d a t a c o n d i t i o n i n g . The a c o u s t i c a l s i g n a l s as w e l l a s l / r e v and 2 5 6 / r e v v o l t a g e p u l s e s were r e c o r d e d on a n Ampex 1300A, 1 4 - t r a c k F M t a p e r e c o r d e r . The r e c o r d e r s e t t i n g w a s I R I G wide-band 1 and 19.05 cm/sec ( 7 . 5 i p s ) , w i t h a c e n t e r f r e q u e n c y of 27 kHz, and a bandwidth of 5 kHz. An a c o u s t i c a l p o l a r i t y c a l i b r a t i o n d e v i c e , which g e n e r a t e d a s t r o n g p o s i t i v e p r e s s u r e p u l s e , was u s e d t o c a l i b r a t e t h e p o l a r i t y of t h e a c o u s t i c a l d a t a system.
The A-weighted SPL was o b t a i n e d by u s i n g a B & K a u d i o f r e q u e n c y a n a l y z e r , t y p e 2107. The a c o u s t i c a l waveforms were reduced by a minicomputer-based t i m e s e r i e s a n a l y z e r . The n o i s e s i g n a l was sampled a t a r a t e of 5 1 2 0 / s e c f o r 0.2 s e c , b e g i n n i n g w i t h t h e t r i g g e r of l / r e v p u l s e s . The r e s u l t i n g f r e q u e n c y r e s o l u t i o n was 5 Hz w i t h a N y q u i s t f r e q u e n c y of 2.56 kHz. A 2 kHz a n t i - a l i a s i n g f i l t e r was u s e d . By a v e r a g i n g 50 r e c o r d s i n a s y n c h r o n i z e d f a s h i o n , t h e n o n p e r i o d i c n o i s e w a s s i g n i f i c a n t l y r e d u c e d . A d i s c r e t e F o u r i e r t r a n s - form was t h e n a p p l i e d t o o b t a i n t h e a m p l i t u d e and p h a s e r e l a t i o n s h i p of each f r e q u e n c y component. By z e r o i n g o u t t h e f r e q u e n c y components below 25 Hz and a p p l y i n g i n v e r s e F o u r i e r t r a n s f o r m , a n a v e r a g e d , 25 Hz h i g h - p a s s e d , p h a s e d i s t o r t i o n - f r e e a c o u s t i c a l waveform was o b t a i n e d . These waveforms a r e p a r t i c u l a r l y u s e f u l i n t h e s t u d y of h e l i c o p t e r i m p u l s i v e n o i s e . A c o m p l e t e s e t of n o i s e waveforms i s g i v e n i n r e f e r e n c e 11.
EXPERIMENTAL RESULTS dBA L e v e l s The a c o u s t i c a l d a t a measured i n t h e wind t u n n e l w e r e c o n t a m i n a t e d by t h e background n o i s e and r e v e r b e r a t i o n s . The background n o i s e d a t a w e r e measured a t v a r i o u s wind-tunnel s p e e d s w i t h t h e r o t o r hub t u r n i n g ( w i t h o u t b l a d e s ) .
The A-weighted SPL of background n o i s e is p r o p o r t i o n a l t o t h e 5 . 6 t h power of t h e wind-tunnel v e l o c i t y . The A-weighted SPL of r o t o r n o i s e was c o r r e c t e d f o r t h e background n o i s e . These c o r r e c t e d dBA q u a n t i t i e s s h o u l d n o t b e c o n s i d e r e d t o b e t h e a b s o l u t e v a l u e s b e c a u s e of r e v e r b e r a t i o n s from t h e h a r d wind-tunnel w a l l s . N e v e r t h e l e s s , t h e s e d a t a a r e u s e f u l f o r comparisons of t h e d i f f e r e n t t i p s h a p e s .
F i g u r e 3 shows t h e dBA n o i s e l e v e l s of M i c (microphone) 3 as a func- t i o n of CLR/a, f o r t h e r o t o r o p e r a t i n g at V/G!R = 0.2, M1,O = 0.6, and Cis = -5".
No d a t a f o r t h e t r a p e z o i d a l t i p are a v a i l a b l e a t t h e s e c o n d i t i o n s . The n o i s e of t h e swept b l a d e is about 2 dBA lower t h a n t h a t of t h e r e c t a n g u l a r b l a d e o r t h e range of b l a d e l o a d i n g . The d i f f e r e n c e is swept-tapered b l a d e over most of F i g u r e 4 shows t h e n o i s e l e v e l of M i c 3 a t small a t h i g h b l a d e l o a d i n g .
The dBA l e v e l s of swept b l a d e s V/RR = 0.375, M 1 , o = 0.65 and as = -5".
a r e t h e l o w e s t , w i t h t h e swept-tapered b l a d e s second. The r e c t a n g u l a r b l a d e and t r a p e z o i d a l b l a d e s a r e l o u d e s t . S i m i l a r t r e n d s were observed a t Mic 6.
The advancing t i p Mach number is a n i m p o r t a n t parameter d e f i n i n g t h e r o t o r n o i s e . F i g u r e 5 shows t h e n o i s e l e v e l s of t h e f o u r b l a d e s over a Mach number range. Below about M1,90 = 0.9 t h e swept b l a d e s have t h e lowest dBA.
When t h e advancing t i p Mach number i s above 0.9, t h e swept-tapered b l a d e s have t h e lowest dBA. S i m i l a r t r e n d s were found a t Mic 6.
Waveforms The n o i s e waveforms may b e more u s e f u l i n s t u d y i n g t h e r o t o r n o i s e when impulsive components a r e dominant. The n o i s e waveforms i n t h e d i f f e r e n t s t a g e s of d a t a r e d u c t i o n a r e shown i n f i g u r e 6. The background n o i s e and r o t o r broadband n o i s e a r e reduced o r e l i m i n a t e d by averaging 50 t i m e s , a s s e e n i n f i g u r e 6 ( b ) . The 25 Hz high-pass f i l t e r i n g mainly e l i m i n a t e d t h e f i r s t b l a d e passage harmonic of t h e t h r u s t - and drag-generated r o t a t i o n a l n o i s e .
The averaged and f i l t e r e d waveforms a r e u s e f u l i n t h e s t u d y of r o t o r impulsive n o i s e . Although t h e t u n n e l background n o i s e and r o t o r broadband n o i s e can be averaged o u t , t h e r e f l e c t e d n o i s e from t h e t u n n e l s u r f a c e s a r e s t i l l p r e s e n t i n t h e processed waveforms. However, i f t h e t i m e l a g of r e f l e c t i o n s i s l a r g e r than t h e i n c i d e n t p u l s e w i d t h , t h e r e f l e c t i o n s w i l l n o t mask t h e impulsive n o i s e . For t h e test c o n f i g u r a t i o n considered h e r e , i t was v e r i f i e d e x p e r i - m e n t a l l y t h a t t h e f i r s t r e f l e c t i o n (from t h e wind-tunnel f l o o r ) a r r i v e s about 4 msec a f t e r t h e d i r e c t wave. The sound p r e s s u r e p u l s e width was found t o be much l e s s than 4 msec, p a r t i c u l a r l y a t h i g h speed. A c t u a l l y , t h e r e was l i t t l e evidence of impulsive n o i s e r e f l e c t i o n s i n t h e measured sound p r e s s u r e s i g n a l ( s e e f i g . 6 ) . A probable f a c t o r i n t h e absence of s t r o n g r e f l e c t i o n s is t h e l o c a t i o n of t h e microphone (Mic 3 ) n e a r l y i n t h e r o t o r t i p - p a t h p l a n e , where t h e impulsive n o i s e d i r e c t i v i t y is g r e a t e s t . The p u l s e r e f l e c t e d o f f t h e t u n n e l f l o o r o r c e i l i n g thus h a s much s m a l l e r magnitude than t h e p u l s e t r a v e l i n g d i r e c t l y from t h e r o t o r t o t h e microphone.
F i g u r e 7 shows t h e a c o u s t i c a l waveforms (averaged 50 times) of t h e f o u r t i p s a t V/RR = 0.375, MI-0 = 0.65 (M1,go = 0.90), and as = -5".
The swept- tapered t i p b l a d e s produce t h e lowest impulsive n o i s e . This i s a l s o t r u e f o r t h e advancing Mach number g r e a t e r than 0.90. However, t h e dBA r e s u l t s of f i g u r e 5 show t h e swept t i p b l a d e t o be t h e lowest among f o u r t i p shapes a t M1,90 = 0.90. This is because dBA i s a n o v e r a l l r a t i n g of n o i s e w i t h a n emphasis on t h e high f r e q u e n c i e s (around 3 kHz); r o t o r n o i s e c o n t a i n s many components i n a d d i t i o n t o impulsive n o i s e . A t h i g h advancing t i p Mach number, both t h e dBA and impulse peak i n d i c a t e t h e swept-tapered t i p b l a d e s a r e q u i e t e s t .
S u b j e c t i v e Rating of High Speed Impulsive Noise S u b j e c t i v e r a t i n g of r o t o r high-speed impulsive n o i s e (with i t s compli- c a t e d waveform) cannot b e immediately d i s c e r n e d based on dBA measurements of t h e n o i s e . An o r d e r i n g based on dBA measurements w i l l n o t n e c e s s a r i l y a g r e e w i t h a n o r d e r i n g based on peak p r e s s u r e l e v e l s . To f i n d s u b j e c t i v e r a t i n g s of t h e f o u r b l a d e s e t s of t h i s i n v e s t i g a t i o n , a s u b j e c t i v e e v a l u a t i o n w a s conducted .
Twenty s u b j e c t s were used f o r t h i s t e s t . Each s u b j e c t was p r e s e n t e d w i t h t h e n o i s e from a p a i r of r o t o r s , f i r s t one and t h e n t h e o t h e r , s e p a r a t e d by a 3-sec gap. Ten seconds l a t e r , a n o t h e r p a i r was presented. A l l p o s s i b l e p a i r s were t h u s p r e s e n t e d ( i n scrambled o r d e r ) and t h e s u b j e c t s were asked t o judge which of t h e two r o t o r s of each p a i r w a s l o u d e s t .
For a l l samples, t h e advance r a t i o was 0.375, t h e r o t a t i o n a l Mach number was 0.65 (corresponding t o a n advancing t i p Mach number of 0. g o ) , and CLR/a w a s 0.07. Recordings from Mic 3 were used. The r e c o r d i n g s were played baclc t o t h e s u b j e c t s i n a n anchoic chamber. The r e c o r d i n g s were band-pass f i l t e r e d from 25 Hz t o 2.5 kHz, and played t o t h e s u b j e c t s a t a reduced b u t c o n s t a n t l e v e l . The p h y s i c a l measurements of t h e o r i g i n a l s i g n a l s a r e shown i n t a b l e 2.
A s u b j e c t i v e o r d e r i n g of t h e loudness of t h e d i f f e r e n t r o t o r t i p s , p r e s e n t e d can be d e r i v e d from t h e f r e q u e n c i e s with which t h e v a r i o u s t i p shapes were judged l o u d e s t . The r e c t a n g u l a r t i p was judged l o u d e s t the most o f t e n , followed by t h e swept t i p , then t h e t r a p e z o i d a l t i p . The swept- t a p e r e d t i p was judged l o u d e s t t h e l e a s t of t e n ( s e e t a b l e 2 ) . A s t a n d a r d s t a t i s t i c a l t e s t ( t - t e s t ) showed t h a t t h e d i f f e r e n c e s i n responses between d i f f e r e n t t i p shapes was s i g n i f i c a n t f o r a l l p a i r s except between t h e trape- z o i d a l t i p and swept- tapered t i p .
The o r d e r i n g d e r i v e d from t h e s u b j e c t s ' responses a g r e e s w i t h t h e o r d e r i n g d e r i v e d from t h e peak n e g a t i v e impulses b u t n o t t h e o r d e r i n g from t h e dBA measurements. This i n d i c a t e s t h a t when t h i s impulse is presented i n t h e r o t o r n o i s e , p e r c e p t i o n of loudness c o r r e l a t e d more w i t h impulsive peak l e v e l than w i t h t h e dBA l e v e l of t h e n o i s e . It should n o t , however, be assumed t h a t t h e peak l e v e l of impulsive n o i s e i s t h e only r e l e v a n t f a c t o r determining subjec- t i v e loudness. More e x t e n s i v e t e s t i n g would have t o b e done t o determine p r e c i s e l y what a f f e c t s s u b j e c t i v e loudness.
COMPARISON OF MEASURED IMPULSIVE NOISE WITH THEORY Time h i s t o r i e s of t h e measured impulsive n o i s e a r e shown i n f i g u r e 8 f o r t h e t r a p e z o i d a l t i p s , a t t h r e e advancing t i p Mach numbers. The n e g a t i v e p r e s s u r e p u l s e i n c r e a s e s i n amplitude w i t h Mach number s o t h a t i t dominates t h e sound p r e s s u r e s i g n a l a t high speed. A t very high speeds a p o s i t i v e p r e s s u r e s p i k e c l o s e l y f o l l o w s t h e n e g a t i v e p u l s e . S i m i l a r r e s u l t s were found from f l i g h t measurement of a UH-1 h e l i c o p t e r n o i s e ( r e f . 1 2 ) . Calcula- t i o n s were made based on t h e t h i c k n e s s n o i s e theory which was developed by Johnson ( r e f . 13). C a l c u l a t i o n s based on t h e t h e o r y of F a r a s s a t and Brown ( r e f . 2) o r t h e t h e o r y of Schmitz and Yu ( r e f . 3) c a n r e s u l t i n s i m i l a r p r e d i c t i o n s . There is more t o t h e p e r i o d i c r o t o r n o i s e t h a n j u s t t h e thick- ness n o i s e component, b u t t h e impulse is w e l l accounted f o r by t h e t h i c k n e s s n o i s e theory. More comparisons can b e found i n r e f e r e n c e 4.
F i g u r e 9 compares t h e measured and c a l c u l a t e d peak impulsive n o i s e p r e s s u r e f o r f o u r d i f f e r e n t b l a d e s over t h e r a n g e of advancing t i p Mach number. The advance r a t i o i s 0.375 f o r a l l c a s e s . The o v e r a l l c o r r e l a t i o n is q u i t e good. It c a n b e s e e n t h a t t h e i m p u l s i v e n o i s e can be reduced by t h e c r o s s - s e c t i o n a l a r e a of t h e b l a d e t i p . Sweeping t h e b l a d e t i p w i t h o u t chang- i n g t h e chord o r t h i c k n e s s h a s l i t t l e e f f e c t on t h e t h i c k n e s s n o i s e . F i g u r e 10 shows t h e d i r e c t i v i t y i n t h e e l e v a t i o n p l a n e f o r t h e swept-tapered r o t o r a t a n advancing t i p Mach number of 0.90. A s can be s e e n , t h e impulsive n o i s e is q u i t e d i r e c t i o n a l . Good agreement between t h e e x p e r i m e n t a l d a t a and c a l c u l a t i o n i s found.
CONCLUSIONS The a c o u s t i c d a t a of a 13.4-m r o t o r w i t h f o u r b l a d e - t i p shapes were o b t a i n e d i n a wind-tunnel t e s t . These t i p shapes a r e r e c t a n g u l a r , swept, t r a p e z o i d a l ( t a p e r e d ) , and swept-tapered. Below a n advancing t i p Mach number of a b o u t 0.9, t h e dBA d a t a appear t o i n d i c a t e t h a t t h e swept t i p is t h e q u i e t e s t , t h e swept-tapered t i p second, t h e t r a p e z o i d a l t i p t h i r d , and t h e r e c t a n g u l a r t i p t h e most noisy. Above a n advancing t i p Mach number of about 0.9, a d i s t i n c t n e g a t i v e a c o u s t i c a l p u l s e , which o c c u r s once p e r b l a d e passage, was observed. The amplitudes of t h e s e p u l s e s a r e s t r o n g l y dependent on t h e advancing t i p Mach number. Based on t h e a m p l i t u d e of impulsive n o i s e , t h e d a t a i n d i c a t e t h e swept-tapered t i p is t h e q u i e t e s t , t h e t r a p e z o i d a l t i p second, t h e swept t i p t h i r d , and t h e r e c t a n g u l a r t i p l o u d e s t .
The o v e r a l l comparisons show good agreement between measured impulsive n o i s e and c a l c u l a t e d r e s u l t s based on t h i c k n e s s n o i s e theory. This c o r r e l a - t i o n s u g g e s t s t h a t t h e r o t o r high-speed impulsive n o i s e i s t h i c k n e s s n o i s e dominated. Changing b l a d e chord o r t h i c k n e s s h a s s i g n i f i c a n t e f f e c t s on t h e n o i s e r a d i a t i o n . Simply sweeping a l o n e has l i t t l e e f f e c t - o n high-speed impulsive n o i s e . A complete p r e d i c t i o n of h e l i c o p t e r n o i s e w i l l , of c o u r s e , r e q u i r e a n a c c u r a t e t r e a t m e n t of a l l n o i s e components.
REFERENCES 1. Lowson, M. V . ; Hawkings, D. L.: Noise of High Speed R o t o r s . AIAA P a p e r 75-450, AIAA 2nd A e r o a c o u s t i c s C o n f e r e n c e , 1975.
2. F a r a s s a t , F.; and Brown, T. J.: A N e w C a p a b i l i t y f o r P r e d i c t i n g Helicop- t e r Rotor and P r o p e l l e r Noise I n c l u d i n g t h e E f f e c t of Forward Motion.
NASA TM X-74037, 1977.
3 . Schmitz, F. H . ; and Yu, Y . H.: T h e o r e t i c a l Modeling of High Speed H e l i c o p t e r I m p u l s i v e Noise. European R o t o r c r a f t and Powered L i f t A i r c r a f t Forum, F r a n c e , S e p t . 1977.
4. Johnson, W . ; and Lee, A . : Comparison of Measured and C a l c u l a t e d H e l i - c o p t e r Rotor I m p u l s i v e Noise. NASA TM 78473, 1978.
5. A r n d t , R. E.; and Borgman, D . C . : Noise R a d i a t i o n from H e l i c o p t e r R o t o r s O p e r a t i n g a t High T i p Mach Numbers. 2 9 t h Annual N a t i o n a l Forum of American H e l i c o p t e r S o c . , P r e p r i n t No. 402, J u n e 1970.
6 . Lyon, R. H . ; Mark, W. D . ; and P y l e , R. W . , Jr.: S y n t h e s i s of H e l i c o p t e r R o t o r T i p s f o r L e s s Noise. H e l i c o p t e r Noise Symposium, ARO-Durham, September 1971.
7. Lowson, M. V . ; Whatmore, A . ; and W h i t f i e l d , C. E.: S o u r c e Mechanisms f o r Rotor Noise R a d i a t i o n . TT7202, Department of T r a n s p o r t a t i o n Technology, Loughborough U n i v e r s i t y of Technology, March 1972.
8. F a r a s s a t , F.: Theory of Noise G e n e r a t i o n from Moving Bodies w i t h a n A p p l i c a t i o n t o H e l i c o p t e r R o t o r s . NASA TR R-451, 1973.
9 . Mantay, W . R. ; S h i d l e r , P. A. ; and Campbell, R. L. : Some R e s u l t s of t h e T e s t i n g of a F u l l - S c a l e Ogee T i p H e l i c o p t e r R o t o r ; A c o u s t i c s , Loads, and Performance. AIAA P a p e r 77-1340, A I A A 4 t h A e r o a c o u s t i c s C o n f e r e n c e , 1977.
10. S t r o u b , R. H . : F u l l S c a l e Wind Tunnel T e s t of a Modern H e l i c o p t e r Main Rotor - P a r t A and P a r t B. T h i r t y - f o u r t h Annual N a t i o n a l Forum of American H e l i c o p t e r Soc., P r e p r i n t No. 3 , May 1978.
11. Lee, 8.: A c o u s t i c a l E f f e c t s of Blade Tip Shape Changes on a F u l l S c a l e H e l i c o p t e r R o t o r i n a Wind Tunnel. NASA CR-152082, A p r i l 1978.
12. Schmitz, F. A . ; and Boxwell, D. A.: I n - F l i g h t F a r - F i e l d Measurement of ~ e l i c o ~ t e r N o i s e . J. American H e l i c o p t e r Soc., October 1976.
13. Johnson, W . : H e l i c o p t e r Theory. P r i n c e t o n U n i v e r s i t y P r e s s , P r i n c e t o n , New J e r s e y , 1978.
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ROTOR HUB SIDE VIEW @3 Figure 2.- Microphone l o c a t i o n s .
0 RECTANGULAR A SWEPT
- 0 SWEPT-TAPERED
BACKGROUND NOISE = 97 dBA VInR = 0.2 - M1,gO = 0.72 a ; = -5 deg 102 - I .04 .05 .06 .07 .08 .09 .10 .ll .12 .13 Figure 3 . - dBA n o i s e l e v e l s a s a f u n c t i o n of C L g / o Mic 3; a , = -5"; V/PR = 0.2; = 0.72.
V/nR = 0.375 0 RECTANGULAR = 0.90 A SWEPT a, = -5 deg 0 TAPERED
0 SWEPT-TAPERED
Figure 4.- dBA noise levels as a function of C R / ~ .
Mic 3; a , = -5O; V l Q R - 0.375; MI,go = 0 . 4 . .
RECTANGULAR 0 TAPERED A SWEPT
0 SWEPT-TAPERED
I I 1 I
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.6 .7 .8 .9 1.0 ADVANCING TIP MACH NUMBER, Figure 5.- The e f f e c t of Mach number on dBA level.
Mic 3; CLR/a = 0.07; a, = - 5 ' .
SOUND PRESSURE, P, ~ / m ~ SOUND PRESSURE, P, ~ / m ~ SOUND PRESSURE. P. ~ / m * I I m m b ' o a o z o z g 3 : r t r 2 " I D w 0 3 II P.
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0 100 g 50 -50 -100 -150 0 1 T, rec T. sec Figure 7.- Waveforms of four blades. V / R R = 0.375; M1,O = 0.65; = 0.9; as = -5O.
M1,90 ----- THEORY EXPERIMENT V/RR = 0.375 - W E - -40 n - z -60
a -80 - I - - 0.01 sec
-100 - (a) M , , ~ o = 0.9 fb) = 0.939 (c) M ~ , = 0.966 Figure 8.- Comparison of measured and calculated thickness noise time histories at several advancing tip Mach numbers. Trapezoidal tips; V/QR = 0.375.
I (a) RECTANGULAR
EXPERIMENT
- THEORY
(b) SWEPT (d) SWEPT-TAPERED - - - - - I .75 .8 .85 .90 .95 1 . O ADVANCING TIP MACH NUMBER, ADVANCING TIP MACH NUMBER, MI, w Figure 9.- Comparison of measured and calculated impulsive noise peak pressures.
V/RR = 0.375.
EXPERIMENT
- THEORY
Figure 10.- Impulsive noise peak pressure in the vertical plane forward of the rotor disk. V/QR = 0.375; = 0 . 6 5 ; = 0.90.
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