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NASA PROGRESS IN AIRCRAFT NOISE PREDICTION J. P . Raney, S . L. Padula and W. E . Zorumski Langley Research Center SUMMARY For several years NASA has maintained an a i r c r a f t noise prediction a c t i v i t y a t the Langley Research Center w i t h the goal o f developing methodology f o r predicting the e f f e c t i v e perceived noise level (EPNL) produced by j e t - powered CTOL a i r c r a f t t o an accuracy of ? 1.5 dB. Another goal i s t o e s t a b l i s h , in terms of fundamental acoustic theory, the relationship of noise t o the .
design and operation of a i r c r a f t and t o demonstrate the f e a s i b i l i t y of incor- porating a i r c r a f t noise constraints i n t o the preliminary design process.
Much progress has been made toward these goals. The Aircraft Noise Prediction Program (ANOPP) contains a complete s e t of prediction methods f o r CTOL a i r c r a f t which i ncl udes propul si on system noi s e sources , aerodynami c o r airframe noise sources, foward speed e f f e c t s , a layered atmospheric model w i t h molecular absorption, ground impedance e f f e c t s including excess g r o u n d attenuation ( E G A ) , and a received-noise contouring capabi 1 i ty. A method for calculating noise-constrained o r noise-minimized a i r c r a f t operations i s presently in the validation phase. Comparisons of ANOPP calculations with measured a i r c r a f t noise levels a r e encouraging and highlight areas where further improvements a r e required.
INTRODUCTION In 1973, a focused a i r c r a f t systems noise prediction a c t i v i t y was established a t the Langley Research Center. The mission was t o develop a state-of-the-art computer system f o r calculating a i r c r a f t noise ( r e f s . 1 and 2 ) . The commitment t o develop the Aircraft Noise Prediction Program ( A N O P P ) stemmed from the need f o r a credible means of quantifying the expected benefits from NASA's noise reduction research programs. I t was a l s o anticipated t h a t t h i s program could from time t o time support the prediction needs of other government agencies concerned with a i r c r a f t noise and could be useful t o NASA contractors.
One of the f i r s t major applications of ANOPP was t o support the Supersonic Cruise Research (SCR) project a t Langley; ANOPP continues t o be applied t o SCR research a t this time. The next application was in conjunction with the FAA i n an International Civil Aviation Organization (ICAO) study t o determine economically reasonable and technologically f e a s i b l e noise 1 imits f o r f u t u r e supersonic transports ( r e f . 3 ) .
7 21 The ANOPP development group has a continuing commitment to assess and improve NASA's noise prediction capability. This i s done by comparing predictions t o measured data from b o t h laboratory models and full-scale Recent prediction assessment, or validation studies, f l i g h t measurements.
have included comparisons of prediction w i t h flyover noise from the McDonnell- Douglas DC-9 and DC-10, the Boeing 747, and the Lockheed L-1011 a i r c r a f t .
Protocol established i n conjunction w i t h the SCR project has been improved and methodology f o r incorporating noise as a design constraint i s being developed. An engine modeling capability which will allow investigation of the effects of variations i n the relationships of engine control variables i s planned, and a method f o r cal cul a t i ng noi se-constrai ned takeoff procedures has recently been incorporated in ANOPP ( r e f , 4 ) .
Several research projects which address c r i t i c a l weaknesses in noise prediction have been identified as a r e s u l t of the focus provided by the ANOPP development and application a c t i v i t i e s . These include shock cell noise generation, ground effects on propagation, forward f l i g h t effects on j e t noise, coaxial and inverted coaxial j e t noise prediction, and jet-on-jet shielding effects.
The purpose of t h i s paper i s t o describe ANOPP i n i t s present s t a t e , t o assess i t s accuracy and applicability t o the preliminary a i r c r a f t design process, and t o indicate where further theoretical and experimental research on noise prediction i s required. The elements of the noise prediction problem which are incorporated in ANOPP will f i r s t be described. Next, the results of comparisons of ANOPP calculations w i t h measured noise levels will be presented. Progress toward treating noise as a design constraint i n a i r c r a f t system studies will then be discussed. The paper will conclude w i t h a summary of noise-prediction-related research a c t i v i t i e s which have been i n i t i a t e d as a r e s u l t of the need t o improve a i r c r a f t noise prediction accuracy.
SYMBOLS source noise prediction parameters ai A atmospheric propagation effects factor ambient speed of sound, m/sec 'a D overall source d i r e c t i v i t y factor DI d i r e c t i v i t y index f frequency , Hz G ground effects factor H a1 ti tude, m 7 22 I source intensity, watt/m M a i r c r a f t Mach number n number of frequency bands P acoustic pressure, N/m reference pressure, N / m 'ref power setting , percent PS R a i r c r a f t position vector w.r.t. earth-fixed axes r noise propagation vector w.r.t. body axes R relative spectrum factor RL r e l a t i v e spectrum level ( = l o l o g R ) S power spectrum factor SL power spectrum level ( = l o l o g S ) t time, sec W weighting factor Cartesian coordinate system ( X 3 Y J ) angle o f attack, deg source elevation angle, deg source d i r e c t i v i t y angle, deg acoustic power, w a t t 7r = 3.1415926 ambient density, kg/m pa 0 atmospheric attenuati on source azimuth angle, deg (b cylindrical polar coordinate system
(5, r l Y z 1
Subscripts f f i nal i i n d e x max maximum m i n m i n i mum 0 o b s e r v e r r e f r e f e r e n c e S source AND SPECIAL SYMBOLS ABBREVIATIONS ANOPP A i r c r a f t Noise P r e d i c t i o n Program c o n v e n t i o n a l t a k e o f f and l a n d i n g CTOL l i f t - d r a g r a t i o (CL/CD) EGA excess ground a t t e n u a t i o n EPNL e f f e c t i v e p e r c e i v e d no se l e v e l I CAO I n t e r n a t i o n a l C i v i l Av a t i o n O r g a n i z a t i o n OASPL o v e r a l l sound p r e s s u r e l e v e l PNLT t o n e - c o r r e c t e d p e r c e i v e d n o i s e l e v e l <P > mean-squared p r e s s u r e SAE S o c i e t y o f Automotive Engineers SCR Supersonic C r u i s e Research S N E CMA S o c i e t 6 N a t i o n a l e D'Etude e t de C o n s t r u c t i o n de Moteurs D ' A v i a t i on SPL sound p r e s s u r e 1 eve1 Supersonic T r a n s p o r t n o r m a l i z e d s p e c i f i c t h r u s t t h r u s t - w e i g h t r a t i o ANOPP NOISE PREDICTION METHODOLOGY The purpose o f ANOPP i s t o p r e d i c t n o i s e from an a i r c r a f t by accounting f o r t h e e f f e c t s of i t s engines, i t s operations, t h e atmosphere i n c l u d i n g ground e f f e c t s , and o t h e r c h a r a c t e r i s t i c s which may i n f l u e n c e t h e n o i s e i t generates. The approach t o t h i s problem has been placed on a fundamental basis, as d e p i c t e d i n f i g u r e 1 ( r e f . 1 ) . The a i r c r a f t f o l l o w s an a r b i t r a r y f l i g h t p a t h i n t h e presence o f an observer on t h e ground. During t h i s o p e r a t i o n , n o i s e sources on t h e a i r c r a f t e m i t r a d i a t i o n w i t h d e f i n e d power, d i r e c t i o n a l i t y , and s p e c t r a l d i s t r i b u t i o n c h a r a c t e r i s t i c s , a l l o f which may depend on time. This source n o i s e propagates through t h e atmosphere ( b e i n g attenuated) t o t h e v i c i n i t y o f t h e observer. The observer r e c e i v e s the n o i s e s i g n a l f r o m t h e d i r e c t r a y p l u s a s i g n a l from a r a y r e f l e c t e d by t h e l o c a l ground surface.
o f t h e a i r c r a f t n o i s e p r e d i c t i o n problem which The e s s e n t i a l i n g r e d i e n t s a r e embodied i n ANOPP a r e as f o l l o w s : ( 1 ) t h e source i n t e n s i t y I, ( 2 ) t h e ( 3 ) t h e a i r c r a f t o r i e n t a t i o n given by a i r c r a f t p o s i t i o n giverl by v e c t o r R ( t ) , 8 and C p , ( 4 ) t h e atmospheric and ground-impedance c h a r a c t e r i s t i c s g i v e n by A and G, and ( 5 ) the l o c a t i o n o f t h e observer g i v e n by t h e v e c t o r r ( t ) .
A number o f approaches a r e a v a i l a b l e f o r t h i s general p r e d i c t i o n problem.
These approaches a r e d i v i d e d i n ANOPP i n t o f o u r c a t e g o r i e s , c a l l e d f u n c t i o n a l l e v e l s , which a r e d e p i c t e d by t h e schematic i n f i g u r e 2. The f u n c t i o n a l l e v e l s a r e d e f i n e d by t h e amount o f data which i s processed and by t h e degree o f approximation i n t h e p r e d i c t i o n methods ( r e f . 5 ) . Level I p r e d i c t s an e f f e c t i v e measure o f n o i s e which depends on t h e observer l o c a t i o n and assumes uniform f l i g h t c o n d i t i o n s . Level I 1 p r e d i c t s a n o i s e l e v e l which depends on t h e observer and time, b u t assumes standard atmospheric c o n d i t i o n s . I n Level 111, frequency e f f e c t s a r e p r e d i c t e d i n a d d i t i o n t o t h e e f f e c t s o f observer and time. Both nonstandard atmospheric e f f e c t s and d e t a i l e d f l i g h t procedures can be handled i n Level 111. I n Levels I 1 and 111, t h e n o i s e measured may be subdivided as t o t h e n o i s e source which generates them. Level I V p r e d i c t s t h e same i n f o r m a t i o n as Level 111, b u t w i t h more d e t a i l i n t h e s p e c t r a l data.
The p r e s e n t paper d e a l s p r i m a r i l y w i t h Level I11 n o i s e p r e d i c t i o n .
An ANOPP Level I11 n o i s e p r e d i c t i o n i s c h a r a c t e r i z e d by t h e p r e d i c t i o n o f 1/3-octave band noise. The band c e n t e r s a r e based on observer frequencies and a r e independent o f time. A l l o t h e r i n p u t s t o t h e p r e d i c t i o n modules a r e t i m e dependent. The v e c t o r s from t h e source t o t h e observer a r e n a t u r a l l y dependent on t h e observer and time so t h a t t h e o u t p u t from a source i s a f u n c t i o n o f frequency, time, and observer.
The p r e d i c t i o n o f 1/3-octave band n o i s e i s a l i m i t a t i o n which should Some o f t h e more i m p o r t a n t n o i s e sources a r e n o t be passed over l i g h t l y .
a c t u a l l y tones, f o r example, from t h e fan r o t o r o f a bypass-type engine.
I n t h e p r e d i c t i o n module, these tones a r e assigned t o a 1/3-octave band and subsequently t r e a t e d as broadband noise. T h i s w i l l cause subsequent e r r o r s i n t h e p r e d i c t i o n o f atmospheric a t t e n u a t i o n , ground e f f e c t s and even n o i s e levels. Nevertheless, the added complexity o f carrying a separate procedure for tones suggests that t h i s is not an appropriate task for ANOPP Level I11 and t h i s type of analysis has been assigned t o Level IV.
Source Noise Prediction ANOPP source modules use standard forms f o r the prediction equations.
The s t a n d a r d equation o f Level 111 prediction modules i s shown in equation ( 1 ) .
where S ( f , e ; a i ) D(B,f,;ai) R(f,e;a.) 1 = . T S ( f ; a i = D(O;ai) The basic noise variable i s mean-squared pressure, <P >. W i t h i n ANOPP, a dimensionless g r o u p i s used, with paca2 being the reference pressure. The equation i s shown in dimensional form i n equation ( 1 ) so t h a t i t will be more familiar t o the reader. The use o f mean-squared pressure allows noise from different sources t o be added d i r e c t l y , t h u s avoiding the time consuming logarithmic and exponentiation operations required to add sound pressure level , SPL.
This power, Each noise source i s characterized by an acoustic power I I .
divided by the area of a sphere with r a d i u s rs and multiplied by the charac- terist-ic impedance of the atmosphere, paca, gives the average overall mean-squared pressure for virtual observers a t distances rs from the source.
The power i s a function of source parameters a i , which have been previously evaluated by analysis of the engine, and the a i r c r a f t flight- The average overall mean-squared pressure i s not adequate f o r most predictions. I t must be known how the sound i s directed and how the acoustic energy i s distributed i n different frequency bands. T h i s information i s contained in three factors: the overall d i r e c t i v i t y factor D , the power spectr factor S , and the relative spectrum factor R.
The overall d i r e c t i v i t y and power spectrum factors are defined in figure The d i r e c t i v i t y factor i s the r a t i o of the overall mean-squared pressure a t angle 8 t o the average overall mean-square pressure on the virtual observer sphere of radius rs. The equation shown in figure 3 i s for an axisymmetric source, however, ANOPP provides the d i r e c t i v i t y effects i n the azimuthal
direction as well as in the polar angle e shown here. The d i r e c t i v i t y factor
i s usually plotted as a d i r e c t i v i t y index, DI, which is simply ten-log of the d i r e c t i v i t y factor against e , the polar d i r e c t i v i t y angle.
7 26 The power spectrum factor, S ( f ) , i s the r a t i o of the acoustic power i n a band to the overall acoustic power. T h i s factor may a l s o be expressed in terms Again, the of integrals of the mean-squared pressure as shown in figure 3 .
equation shown is for an axisymmetric source. The integrals are used in computing S ( f ) from experimental data. The power spectrum factor i s usually plotted in logarithmic form against frequency or Strouhal number. Since the factor must be less t h a n one, i t s l o g a r i t h m i s negative and usually has a peak value a t a b o u t -10 dB.
The overall d i r e c t i v i t y and the power spectrum give some information about how the mean-squared pressure i s directed over angles and distributed over frequency bands, b u t t h i s information i s n o t complete. What is needed i s e i t h e r the spectrum factor for the mean-squared pressure a t each angle or the d i r e c t i v i t y a t each frequency band of the acoustic power. Either of these variables can be expressed i n terms of the r e l a t i v e spectrum factor as shown i n figure 4. In logarithmic form, the r e l a t i v e spectrum level i s the difference between the mean-squared pressure spectrum level and the power spectrum level. I t can be shown t h a t t h i s is identical t o the difference between the d i r e c t i v i t y index of the frequency band and the overall direc- t i v i t y index. The reader may observe t h a t many empirical prediction formulas assume a r e l a t i v e spectrum level o f zero dB.
Forward f l i g h t effects on noise sources are not easily expressible i n a standard form. This i s a current research area and there i s a tendency t o use specialized procedures f o r each source. There are two definite relations, however, which distinguish the Level IV ANOPP system from the Level I11 and lower versions. These are shown i n equations ( 2 a ) and ( Z b ) , where the subscripts o and s denote quantities a t the observers and a t the source, respectively.
f o ( M y @ ) = f s ( 1 - M COS e)-’ where a i = a ( M ) i i s the relationship f o r Level IV moving source system and fs(Fl,B) = fo(l-M COS e ) - l a i - - a i (M,e) where is the relationship f o r Level 111 fixed source system. In Level IVY the frequencies are fixed a t the source and the Doppler factor adjusts the observer frequency as a function of Mach number and d i r e c t i v i t y angle. In Level 111, a l l sources a r e treated l i k e broadband noise so that the observer frequency i s fixed and the noise frequency i s accordingly shifted by the Doppler factor.
The noise source parameters i n Level I11 may accordingly be a function of Mach number and d i r e c t i v i t y angle in some f l i g h t e f f e c t schemes.
7 27 Some of the ANOPP modules which are presently used f o r CTOL subsonic cruise a i r c r a f t and SST noise prediction are shown i n Table I . Since the noise source modules f i t within the standard form equation described previously, there i s no need t o go into further detail here. All ANOPP methods are f u l l y referenceable and the reader may refer t o the documents l i s t e d i n Table I for f u l l d e t a i l s on a particular method.
Propagation and Noise Effects Having discussed source noise computations, the next t a s k i s t o account for propagation effects as the sound travels t h r o u g h a real atmosphere t o an observer on the g r o u n d . I t i s necessary t o understand how the source noise information i s organized and stored, how propagation effects are included and how the resulting noise i s measured and reported. The final portion of t h i s section will compare three different noise contouring methods available i n ANOPP.
Level 111 propagation effects. - All of the source noise prediction methods covered above calculate mean-squared pressure a t a given distance rs from the center of the source. The geometry f o r any of the engine sources i s shown i n figure 5. Since these sources are axisymmetric, i t i s sufficient t o define acoustic pressures on a half c i r c l e centered a t the center of the Usually, the predicted pressures are tabulated a t eighteen values j e t nozzle.
of d i r e c t i v i t y angle, 8, s t a r t i n g a t the engine i n l e t axis a n d ending a t the j e t nozzle axis. Pressures are also tabulated a t each 1/3-octave band center frequency from 50 Hz t o 10,000 Hz as indicated in figure 6.
Level I11 propagation effects are represented schematically i n figure 6 as correction factors which modify the near-field curve t o become the far-fie11 Equation (3) contains a more detailed representation showing t h a t curve.
mean-squared pressure a t the observer equals mean-squared pressure a t radius rc mu1 t i pl i ed by correction terms for impedance di fferences , spheri cal spreading ,- atmospheric attenuation, and ground effects.
where i s an average atmospheric attenuation measure and G i s a ground effects factor. Notice t h a t propagation effects m u s t be recomputed a t each time step along the trajectory because the distance from source t o observer, and the elevation angle between source and observer, 8 , are changing rO 2 <P > s may not rapidly with time. The mean-squared pressure a t the source need t o be r e c a l c u l a t e d a t e v e r y t i m e s t e p s i n c e e n g i n e parameters v a r y s l o w l y w i t h t i m e and s i n c e t h e v a r i a t i o n o f <P*>s w i t h 8 can be accounted f o r b y i n t e r p o l a t i o n o v e r a s e t o f v i r t u a l observers.
N o i s e r e c e i v e d b y each o b s e r v e r i s measured i n terms o f sound p r e s s u r e E q u a t i o n ( 4 ) g i v e s a g e n e r a l e x p r e s s i o n f o r SPL and i n d i c a t e s l e v e l (SPL).
two o f t h e most common w e i g h t i n g f u n c t i o n s .
n r r e f where A c t u a l l y , PL i s t h e l o g a r i t h m o f a r a t i o o f t h e a r e a under a we,ghted mean- squared p r e s s u r e spectrum and t h e square o f t h e r e f e r e n c e p r e s s u r e . L e v e l I11 ANOPP approximates t h e i n t e g r a l o v e r a l l f r e q u e n c i e s b y a summation o f i n t e g r a l s o v e r each t h i r d o c t a v e band. The w e i g h t s , wi, a r e chosen f r o m many p o s s i b l e w e i g h t i n g f u n c t i o n s used t o e v a l u a t e t h e e f f e c t o f sound on humans. O v e r a l l sound p r e s s u r e l e v e l , OASPL, i s a f l a t w e i g h t i n g f u n c t i o n importance t o each frequency band. P e r c e i v e d n o i s e l e v e l , which g i v e s equal annoyance curves.
PNL, i s a compl i c a t e d w e i g h t i ng f u n c t i o n based on e m p i r i c a l The e m p i r i c a l d a t a i n d i c a t e t h a t b o t h t h e frequency c o n t e n t and t h e loudness o f a sound c o n t r i b u t e t o i t s n o i s i n e s s . The measure PNLT uses t h e same w e i g h t s as PNL b u t i n c l u d e s c o r r e c t i o n s f o r d i s c r e t e tones i n t h e sound spectrum As t h e a i r c r a f t f l i e s b y an o b s e r v e r l o c a t i o n , t h e p e r c e i v e d n o i s e l e v e l s w i l l r e a c h a peak and t h e n s u b s i d e as i n d i c a t e d i n f i g u r e 7. Psychoacoustic r e s e a r c h suggests t h a t t h e o b s e r v e r r e a c t s t o t h e peak n o i s e l e v e l and t o t h e d u r a t i o n o f t h e almost-peak n o i s e l e v e l s . E f f e c t i v e p e r c e i v e d n o i s e l e v e l (EPNL) i n c l u d e s t h i s d u r a t i o n e f f e c t b y measur'ing t h e a r e a o f t h e shaded r e g i o n i n f i g u r e 7. The p r e s c r i b e d method o f c a l c u l a t i n g EPNL i s t o approximate t h e i n t e g r a l o f PNLT o v e r t i m e b y a p p l y i n g t h e t r a p e z o i d r u l e a t h a l f - s e c o n d t i m e i n t e r v a l s.
C o n t o u r i n g methods. - E f f e c t i v e p e r c e i v e d n o i s e c o n t o u r s a r e u s e f u l v i s u a l a i d s f o r r e p r e s e n t i n g t h e n o i s e l e v e l r e c e i v e d b y a l a r g e number o f observers. ANOPP p r o v i d e s two p o s s i b l e avenues toward p r o d u c i n g c o n t o u r p l o t s i n a reasonable amount o f computer t i m e . The u s e r may e i t h e r use L e v e l I a p p r o x i m a t i o n s t o c a l c u l a t e a l a r g e number o f EPNL v a l u e s o r he may use t h e ANOPP c o n t o u r enhancement methods t o produce smooth c o n t o u r s f r o m a l i m i t e d number o f a c c u r a t e EPNL v a l u e s . Both approaches w i l l be d i s c u s s e d below.
The s i m p l e s t c o n t o u r i n g method uses Level I a p p r o x i m a t i o n s which a r e based on l e v e l f l y o v e r d a t a c o r r e c t e d t o s t a n d a r d day c o n d i t i o n s .
EPNL can 7 29 be t a b u l a t e d as a f u n c t i o n of minimum approach distance, ro, and engine power s e t t i n g as p i c t u r e d i n f i g u r e 8. I n a d d i t i o n t o t h e EPNL table,the user must supply o r compute a i r c r a f t p o s i t i o n and power s e t t i n g a t each t i m e s t e p i n t h e f l i g h t and must s p e c i f y t h e d i r e c t i v i t y angle e a t which t h e maximum n o i s e occurs. P l o t t i n g a g i v e n contour i n v o l v e s i n t e r p o l a t i n g i n t o t h e EPNL t a b l e f o r t h e value o f ro a t which t h a t n o i s e l e v e l occurs. The values o f ro, 8 , and a i r c r a f t p o s i t i o n then d e f i n e an observer l o c a t i o n as shown i n f i g u r e 9. T h i s process i s repeated a t each t i m e s t e p and t h e contour i s drawn by a graphics s u b r o u t i n e which connects t h e observer l o c a t i o n s .
T h i s simple c o n t o u r i n g method has been t h e accepted p r a c t i c e f o r a number o f years. It i s c l e a r , however, t h a t t h i s method can be no more accurate t h a n t h e Level I p r e d i c t i o n s on which i t i s based. Using t h i s method t o draw n o i s e contours f o r a maneuvering a i r c r a f t o r f o r r e a l i s t i c t a k e o f f and l a n d i n g o p e r a t i o n s i s n o t recommended.
A much more powerful and v e r s a t i l e method i s i l l u s t r a t e d i n f i g u r e 10.
Noise l e v e l s a r e p r e d i c t e d f o r an evenly spaced g r i d o f observer l o c a t i o n s u s i n g e i t h e r Level I 1 o r Level I11 p r e d i c t i o n methods. A standard c o n t o u r i n g computer package can draw t h e n o i s e f o o t p r i n t from these d a t a which a r e a p p r o p r i a t e f o r any nonuniform a i r c r a f t o p e r a t i o n . The major drawback o f t h i s b a s i c c o n t o u r i n g method i s t h e computing c o s t s i n c e a dense g r i d o f observer l o c a t i o n s i s needed t o produce smooth contours. A secondary problem i s t h e q u a l i t y o f t h e contours produced. The standard c o n t o u r i n g s o f t w a r e i s f o r general purpose a p p l i c a t i o n and u t i l i z e s no knowledge o f t h e b a s i c shapes o f t h e n o i s e contours. These shapes a r e r o u g h l y c o n c e n t r i c e l l i p s e s which a r e symmetric about t h e runway c e n t e r l i n e .
Thus t h e n o i s e f o o t p r i n t s produced r a r e l y conform t o t h e u s e r ' s expectations.
The advanced ANOPP c o n t o u r i n g c a p a b i l i t y overcomes t h e d i f f i c u l t i e s mentioned above i n two ways. F i r s t , i t uses a more r e D r e s e n t a t i v e c o o r d i n a t e system, and second, it enhances t h e data b e f o r e c o n t o u r i n g . The method employs t h e conversion from C a r t e s i a n coordinates ( x , y, z ) t o c y l i n d r i c a l p o l a r c o o r d i n a t e s ( E , q , z ) . It i s o f t e n advantageous t o use s t r e t c h e d p o l a r coordinates, achieved by d i v i d i n g y by a c o n s t a n t b e f o r e conversion t o p o l a r coordinates. By u s i n q t h i s more n a t u r a l r e p r e s e n t a t i o n , i t i s p o s s i b l e t o produce reasonable contours w i t h as few as s i x t e e n observer l o c a t i o n s . The ANOPP enhancement program f i t s a c u b i c s u r f a c e through these s i x t e e n p o i n t s and i n t e m o l a t e s t o form a dense g r i d b e f o r e contouring. T y p i c a l r e s u l t s a r e shown i n f i g u r e 11, i n which countours produced from t h e enhancement of s i x t e e n c a l c u l a t e d p o i n t s compare f a v o r a b l y w i t h contours produced from a v e r y dense g r i d o f c a l c u l a t e d p o i n t s .
ANOPP VALIDATION AND EVALUATION The I C A O Study I n 1977 t h e I n t e r n a t i o n a l C i v i l A v i a t i o n O r g a n i z a t i o n ( I C A O ) requested through i t s C i v i l A i r c r a f t Noise (CAN) committee a recommendation f o r n o i s e standards a p p l i c a b l e t o f u t u r e SST's. P a r t i c i p a t i n g c o u n t r i e s i n c l u d e d t h e U n i t e d S t a t e s , t h e U n i t e d Kingdom, France, and t h e USSR. P a r t i c i p a t i n g o r g a n i z a t i o n s i n c l u d e d Boeing A i r c r a f t Company, McDonnell-Douglas, Lockheed, B r i t i s h Aerospace, General E l e c t r i c , P r a t t & Whitney A i r c r a f t , Rolls-Royce, SNECMA, and NASA Langley.
A p r e d i c t i o n subcommittee was e s t a b l i s h e d and g i v e n t h e t a s k o f choosing a "Reference P r e d i c t i o n Procedure" which would s e r v e as a common denominator f o r t h e p a r a m e t r i c s t u d i e s and n o i s e c a l c u l a t i o n s s u p p o r t i n g each p a r t i c i p a n t ' s recommendations.
I n o r d e r t o p r o v i d e a b a s i s f o r s e l e c t i o n o f t h e Reference P r e d i c t i o n Procedure i t was decided t o r e q u e s t p a r t i c i p a n t s t o c a l c u l a t e component and t o t a l n o i s e l e v e l s f o r a h y p o t h e t i c a l v e r y l o w bypass r a t i o SST engine s p e c i f i e d by SNECMA. Noise d a t a f o r s e v e r a l a i r c r a f t / e n g i n e combinations were a l s o made a v a i l a b l e t o any who wished t o compare p r e d i c t e d n o i s e l e v e l s a g a i n s t measured d a t a .
H y p o t h e t i c a l SST ___L engine. - The r e s u l t s o f t h e h y p o t h e t i c a l SST engine n o i s e c a l c u l a t i o n s a r e summarized i n T a b l e 11. C a l c u l a t i o n s were made f o r each o f t h r e e power s e t t i n g s r e p r e s e n t i n g t a k e o f f , c r u i s e , and l a n d i n g approach T o t a l f l y o v e r n o i s e i s p r e s e n t e d i n terms o f e f f e c t i v e p e r c e i v e d n o i s e l e v e l (EPNL) and t h e component l e v e l s p r e s e n t e d i n terms o f peak p e r c e i v e d n o i s e l e v e l (PNL) f o r j e t , shock c e l l , and combustion n o i s e . The h i g h e s t and l o w e s t l e v e l s c a l c u l a t e d a r e shown t o i n d i c a t e t h e range o f t h e r e s u l t s . The l e v e l s c a l c u l a t e d u s i n g ANOPP a r e a l s o i n d i c a t e d .
Two c o n c l u s i o n s were drawn f r o m t h e r e s u l t s o f t h e paper SST e n g i n e n o i s e c a l c u l a t i o n s . The f i r s t i s t h a t t h e r e were l a r g e d i f f e r e n c e s i n t h e n o i s e l e v e l s p r e d i c t e d b y d i f f e r e n t methods. The second i s t h a t ANOPP produced r e s u l t s which compared v e r y f a v o r a b l y w i t h t h e average o f t h o s e c a l c u l a t e d b y o t h e r o r g a n i z a t i o n s .
One a d d i t i o n a l o b s e r v a t i o n s h o u l d be recorded. The r e s u l t s f o r f a n and t u r b i n e n o i s e were d i s a p p o i n t i n g and i n c o n c l u s i v e . The range f r o m h i g h t o low values exceeded 20 dB w i t h no a p p a r e n t concensus as t o t h e b e s t method.
The SST e n g i n e p r e d i c t i o n e x e r c i s e , t h e r e f o r e , c l e a r l y i d e n t i f i e d t h e need f o r g r e a t l y improved t u r b o machinery p r e d i c t i o n methodology e s p e c i a l l y f o r o t h e r t h a n j e t - n o i s e - d o m i n a t e d a i r c r a f t .
Comparisons w i t h measured a i r c r a f t n o i s e d a t a . - N o i s e l e v e l s f o r f i v e a i r c r a f t i n c l u d i n g Concorde and f o r t h e A e r o t r a i n were a l s o c a l c u l a t e d f o r comparison w i t h measured data. The procedure f o l l o w e d f o r t h i s p o r t i o n o f t h e I C A O s t u d y was f i r s t t o c a l c u l a t e n o i s e l e v e l s based on i n p u t d a t a w h i c h was p r o v i d e d t h r o u g h t h e chairman o f t h e p r e d i c t i o n subcommittee. L a t e r , t h e p r e d i c t e d and measured p e r c e i v e d n o i s e l e v e l s (PNL) were transposed t o t h e same p l o t f o r comparison and e v a l u a t i o n o f t h e accuracy o f t h e p r e d i c t i o n methods.
The d i f f e r e n c e s between measured and ANOPP-predicted v a l u e s o f EPNL f o r a l l o f t h e a i r c r a f t i n t h e I C A O s t u d y a r e summarized i n f i g u r e 12. On average the ANOPP predictions were approximately 2 dB below measured levels. The dashed curve f o r the Concorde indicates underprediction of from 1 t o 4 EPNdB dependi ng on the j e t velocity.
I n summary, the I C A O study provided an early opportunity t o compare The ANOPP with other prediction methods and w i t h measured a i r c r a f t data.
ICAO study also provided a basis for identifying future improvements, particularly in the turbomachinery area, in ANOPP methods. The results of the study were encouraging since the reference procedure selected by the noise prediction subcommittee i n 1978 consisted mostly of ANOPP methodology. 1 DC-9 Following the I C A O study, ANOPP noise predictions were made f o r a DC-9-32 powered by JT8D-9 so-call ed hardwall engines.
McDonnell -Doug1 as Noise levels, f l i g h t p a t h , and a i r c r a f t data for actual t e s t conditions were supplied by the manufacturer. Engine data were made available by P r a t t 81 Whitney. Four f l i g h t s of i n t e r e s t were drawn from a large s e t of t e s t s done Tone corrected by McDonnell-Douglas a t the Yuma t e s t s i t e ( r e f . 6 ) .
perceived noise level predictions were made by summing j e t , core, and fan noise components. There were no shocks present. The fan noise was calculated in two s t a es using a modified Heidmann method, as per the ICAO recommended procedure.! Ground effects and atmospheric attenuation were included i n the prediction scheme since these were present i n the measured data. Finally, effective perceived noise levels were calculated.
The results of the DC-9 exercise are summarized i n Table I11 and i n figure 13. A s seen in the table, the effective perceived noise levels predicted by ANOPP compare very well w i t h the values supplied by the manufacturer. The 1 t o 2 dB underprediction of EPNL value by ANOPP results primarily from an underprediction of peak perceived noise levels. The two graphs presented in figure 13 are representative. The f i r s t graph compares measured and predicted PNLT as a function of radiation angle. The two curves agree very well except i n the region between 100" t o 130". The second g r a p h compares measured and predicted sound pressure level spectra for one angle in t h i s peak noise region. The measured and predicted curves agree in general shape; however, the predicted levels average a b o u t 3 t o 5 dB lower t h a n the measured d a t a .
1The final report of the Subcommittee on SST Noise Prediction was given by the chairman, M . J . T. Smith, t o a meeting of I C A O noise prediction s p e c i a l i s t s a t the Department of State, Washington DC, June 15, 1978.
Tone Corrected Perceived Noise 'LTV/HTC memorandum, 1-25-79 , Subject: Level and Sound Pressure Level Comparisons of McDonnell-Douglas DC-9 Flight Data and NASA/ANOPP Predictions.
jSee Footnote 1 .
DC-10 In the f i r s t of three ANOPP validation studies f o r U.S. wide body aircraft, McDonnell-Douglas submitted comparisons of predicted t o measured noise levels f o r s i x level flyovers of a DC-10 a t power settings ranging from approach t o f u l l takeoff power ( r e f . 7 ) . Inputs of noise c r i t i c a l engine data were prepared by the Douglas propulsion group while airplane tracking and noise data were taken from f i l e s of the f l i g h t t e s t group. Remote computer terminal access t o the Langley computer was arranged so t h a t Douglas could r u n ANOPP a t Langley from t h e i r Long Beach plant.
Comparisons were made on the basis of PNLT vs. angle from the i n l e t axis and on the basis of 1/3-octave band spectra a t selected angles as shown in figure 14. Ground effects are apparent i n b o t h the predicted and measured noise spectra. Predictions EPNL comparisons were also made f o r each f l i g h t .
included j e t , fan, combustion, turbine a n d airframe component noise. Since the JT9D engine was installed in an acoustically treated nacelle, the e f f e c t of,duct treatment was estimated. I t was assumed t h a t the duct treatment eliminated the fan tones b u t did n o t reduce the broadband noise. Even with ANOPP tended t o overpredict the high-frequency fan noise. On t h i s assumption, the other hand, the lower-frequency j e t noise was consistently underpredicted.
These effects are apparent in the frequency spectrum a t 8 = 120". The graph of PNLT versus radiation angle in figure 14 also shows overprediction in both and rear arcs which is caused by the high predicted values of fan the forward noise. On an EPNL basis, ANOPP overpredicted from 0.4 t o 3.1 EPNdB w i t h an average overprediction of 1.3 EPNdB for the s i x flyovers. For the example shown i n figure 14, the overprediction was 1.6 EPNdE, which i s a representative case.
T h e DC-10 was the f i r s t a i r c r a f t f o r which ANOPP had overpredicted the This overprediction could probably be removed by a more accurate noise.
I t i s estimate o f the attenuation of fan noise provided by duct treatment.
also possible that beneficial forward f l i g h t effects on fan noise are responsible f o r these differences.
L-1 01 1 The Lockheed-California Company participated in the second wide-body ANOPP validation study under contract to Langley Research Center. Under t h i s contract, Lockheed selected an a i r c r a f t noise data base consisting of s i x flyovers a t engine power settings from 60 percent t o 100 percent of corrected fan speed. The noise data for these flyovers were accompanied by tracking data and engine performance information on the Rolls Royce RB-211 engines.
Lockheed was linked t o the Langley computer complex via a remote terminal so that the ANOPP noise prediction could be made by Lockheed's engineers.
The results of the L-1011 validation study as published in reference 8 are disappointing. While agreement between measured and predicted data a t the low power settings is quite good, the noise produced a t takeoff power settings i s grossly overpredicted. The difference between measured and predicted noise levels i s as much as 20 PNdB for the f u l l power takeoff case. The agreement i s particularly bad i n the forward quadrant, t h a t i s , for radiation angles between 20 and 80 degrees.
A study of the predicted levels of the component noise sources suggests t h a t the overpredictions a r e due t o high levels for the fan combination tones which are generated by supersonic t i p speed fans. T h i s explains why the low power cases, where fan t i p speed i s subsonic, are not overpredicted.
If the Lockheed engineers had eliminated fan tones, as was done-by Douglas, the results would have been greatly improved.
Figure 15 and 16 are representative of the L-1011 validation study results. Each g r a p h contains measured data, the original predicted noise levels obtained by Lockheed and the revised predicted levels obtained by eliminating the fan combination tone or buzz-saw noise. Figure 15 contains a perceived noise level plot and a spectra plot f o r the f u l l power takeoff case. Even w i t h the revision t o the fan noise prediction, the takeoff noise i s overestimated in the forward quadrant. Figure 16 i s included t o show t h a t for reduced power s e t t i n g s , ANOPP can predict L-1011 flyover noise quite well.
This figure compares the measured and predicted noise spectrums a t a radiation angle of 60" and a power setting of 90% fan speed. Notice t h a t once the the measured and predicted curves look buzz saw noise component i s suppressed, very similar. Even the reinforcements and cancellations caused by ground reflection are correctly predicted. This figure is typical of a l l the reduced power results included i n the validation study.
Boeing 747 The Boeing Aircraft Company has recently completed the third wide body The validation study, which compared ANOPP predictions t o 747 flyover data.
flyovers, depicted in figure 1 7 , were made a t constant 1 2 2 meter a l t i t u d e Noise was measured by flush- (400 f t ) with several engine power settings.
mounted microphones on the airport runway. The predicted total noise was assumed t o be the sum of j e t , fan, core, turbine, and airframe noise components.
The j e t and fan noise components dominated the predicted levels in most cases.
Comparisons of predicted and measured tone-corrected perceived noise levels are shown in figure 1 7 . A t approach power, the predictions were l e s s than the measured data a t a l l d i r e c t i v i t y angles. The approach power predictior for EPNL was 5 dB below the measured data. A t takeoff power, the perceived noise levels were overpredicted i n the forward quadrant and underpredicted i n the a f t quadrant causing a 1 dB difference i n effective perceived noise levels.
N o attempt t o analyze the source of these discrepancies has been made except to note t h a t buzz-saw noise was included i n the ANOPP calculation by the Boeing engineers.
Discussion The three wide body validation studies a l l indicate a need for improved fan noise prediction methods. The fan noise overprediction which often reaches 10 t o 15 dB isthought t o r e s u l t from extrapolating s t a t i c t e s t stand data t o f l i g h t conditions. Acceptable results f o r the DC-10 were obtained because of the Douglas engineers' decision t o "model" fan noise by neglecting the buzz-saw component. For the Lockheed L-1011, results were shown with and without the buzz-saw component demonstrating significant improvement when the buzz-saw component was omitted. The Boeing 747 takeoff power noise levels were apparently overpredicted in the forward arc because of the buzz-saw term.
Improvement i n j e t noise prediction also appears necessary. J e t noise The wide body validation prediction methods are based on scale model d a t a .
studies indicate that significant underpredictions of j e t noise may result from extrapolating these model d a t a t o full-scale engines. Flight effects on j e t noise appear t o be another source of prediction error.
The results of the three wide body validation studies will be documented as NASA Contractor Reports and will be available f o r detailed analysis by the prediction community. The intent in conducting these studies was to provide a component-by-component comparison of ANOPP prediction methods with measured noise levels of current technology a i r c r a f t . The results are encouraging.
Deficiencies in fan and j e t noise prediction methods have been pinpointed which will provide the focus of future prediction research.
SYSTEMS STUDIES The application of ANOPP t o preliminary design systems studies or parametric analyses i s i l l u s t r a t e d in figure 18. A few of the key dimensionless variables are the thrust-weight r a t i o , (T/W) ). which sizes the propulsion system; the lift-drag r a t i o , ( C / C ), which represents the a i r c r a f t ' s aerodynamic characteristics; and the normalkzea specific t h r u s t , (Tihc,), which i s an indicator of source noise. The interrelationships among these and other dimensionless variables must be carefully studied before the ultimate compromise between noise a t the FAA c e r t i f i c a t i o n points, performance, and economics can be reached. The value of ANOPP f o r design studies and, consequently, for quantifying the benefits of proposed noise reduction technology has been established t h r o u g h the NASA SCR project interface and the I C A O / S C R studies.
NASA i s committed t o continued cooperative development and improvement of'ANOPP f o r application t o future parametric and preliminary design studies of advanced a i r c r a f t system concepts.
An example of application of ANOPP t o a systems study involving noise- constrained takeoff procedures i s discussed in the next section.
Optimized Takeoff Procedures The Aircraft Noise Prediction Program has f a c i l i t a t e d a s e t o f systematic noise reducing trajectory studies which i s unique i n a number o f ways. First, a standard optimization program i s used to adjust continuous control functions Second, multiple noise constraints and produce r e a l i s t i c takeoff solutions.
s u r r o u n d i n g the runway tend t o reduce noise i n every direction, n o t j u s t a t a single point. T h i r d , detailed mathematical descriptions of f l i g h t path, engine operations, and noise t a i l o r the solution t o a specific a i r c r a f t . Both the completeness of the studies and the approach t o the problem are unique.
The general optimization problem i s i l l u s t r a t e d in figure 19. The object i s t o f i n d that takeoff trajectory which minimizes noise a t each selected observer location. The range of physically possible and acceptable trajec- i s represented by the shaded region i n figure 19. The lower limit tories represents a minimal adherence to accepted safety practices and the upper Between these extremes l i e s the limit represents the maximum power takeoff.
trajectory which produces minimum noise a t the observers.
A key t o the solution of t h i s general class of optimal control problems I n other words, i s t o realize t h a t the inverse problem i s easier t o solve.
rather t h a n minimizing noise a t mu1 t i p l e observer locations with the constraint t h a t final a l t i t u d e , H exceeds some minimum safe a l t i t u d e , i t i s more natural t o maximize H f w i t h f’multiple noise constraints as summarized below.
Payoff : Maximum a1 t i tude Controls : Constraints: EPNLi < EPNLmax i = 1 , 2 . . .
Side Constraints: < a < a ami n max The optimization problem is t o adjust the f l i g h t controls, angle of attack M , and power s e t t i n g ps, i n order t o maximize final a l t i t u d e while The acceptable r e s t r i c t i n g the noise a t each observer t o some acceptable limit.
noise limit can then be lowered until no feasible solution exists. The side constraints on a and ps establish a range of possible angle-of-attack values and a range of physically attainable engine settings. These constraints a r e equivalent to defining minimum and maximum possible t r a j e c t o r i e s bounding i n figure 19.
the shaded region ANOPP i s especially handy f o r solving optimization problems of t h i s type.
I t contains a module t o calculate the f l i g h t trajectory and (See figure 20.)
a t each observer.
one o r more modules t o evaluate Level I1 noise predictions I t also contains executive control statements which perform i n i t i a l i z a t i o n and decision logic. The optimization code, while n o t s t r i c t l y a part of ANOPP, can be introduced t o the executive system and used as any other functional module. The optimizer i n use was developed a t NASA Ames Research Center by Gary Vanderplaats and i s described i n reference 9.
The optimization approach has already been applied t o advanced design supersonic transport takeoffs. The d e t a i l s of this research, including a description of the f l i g h t dynamics module, are contained i n reference 4. One issue l e f t unresolved i n that work i s the applicability of optimized procedures t o present commercial a i r c r a f t . A study of the L-1011 takeoff procedures has since c l a r i f i e d t h i s point.
Optimized L-1011 Takeoff The Lockheed L-1011 T r i s t a r i s selected for an optimized takeoff study a number of reasons.
f o r The primary reason i s t h a t detailed engineperformance and noise data are published in references 8 and 10. Moreover, the wide-body L-1011 with three high by-passratio RB-211 engines provides a sharp contrast t o the supersonic transport concept studied previously. Finally, the L-1011 has a wide range of operating capabilities which make optimized procedures attractive. Even fully loaded, the L-1011 has a considerable amount of excess power capability so t h a t the a i r c r a f t can maintain a climb in the event of an engine f a i l u r e .
The t e s t problem designed for the L-1011 i s based on FAA c e r t i f i c a t i o n procedures for large commercial a i r c r a f t . Two observer locations are situated along the FAR-36 sideline a t 5500 m and 6000 m from brake release and a third observer location i s on the runway centerline and 6486 meters from brake release. Noise levels a t each observer are restricted t o 96 EPNdB which proved to be the lowest feasible noise goal. (Here buzz-saw noise is omitted). Side constraints on the control functions are s e t very loosely a t - = 16", psmin = 70%, psmax = 100%. a - 4", amax m i n The results of the L-1011 study are presented in figures 21-23. The i n i t i a l condi tjons are based on a representative (constant power/constant velocity) takeoff procedure found i n reference 10. The ANOPP f l ight dynamics routine can approximate t h i s takeoff based- on the i n i t i a l a i r c r a f t position and the angle-of-attack and power setting schedules given i n the reference.
The optimization routine then adjusts the control functions in order t o maximize final a l t i t u d e and t o conform t o the noise constraint. I n i t i a l and optimal values of angle-of-attack, power s e t t i n g , a l t i t u d e and velocity are given in figure 21 and 22. Notice t h a t the optimum thrust schedule i s a gradual cutback such t h a t minimum thrust occurs s l i g h t l y before the a i r c r a f t f l i e s over the centerline microphone. The thrust schedule plus the modified angle-of-attack schedule results in a slower r a t e of climb than in the i n i t i a l takeoff. However, the optimal solution conforms t o FAA safety standards in t h a t the climb gradient remains above 4 percent and in that the thrust cutback occurs a f t e r the a i r c r a f t has reached 213 m (700 f t ) altitude.
Use of the optimal f l i g h t procedure results i n reduced effective the sideline and a t the flyover monitor, perceived noise levels everywhere along (See figure 23.) This t e s t problem demonstrates the use o f optimization t o reduce noise levels f o r c e r t i f i c a t i o n purposes.
T h e same technique could be applied to community noise abatement studies by positioning the observer locations i n areas of h i g h population density or i n areas where citizen complaints are frequent.
NOISE PREDICTION RESEARCH Several areas requiring further research have been identified as a r e s u l t of systems studies using ANOPP t o provide predicted noise levels.
One of these noise-constrained o r optimum takeoff procedures has been discussed i n the previous section.
Three others are indicated i n figure 24.
Shock Cell Noise Shock cell noise was identified as a c r i t i c a l research area d u r i n g the Supersonic Cruise Research studies. Shock cell noise has a nearly omni- directional radiation pattern which causes i t t o dominate the forward arc during takeoff. This forward radiated noise limits the benefits of power cutback as a noise reducing operational procedure. Consequently, the elimination of shock cell noise i s c r i t i c a l to the success of a supersonic vehicle. NASA has a s t r o n g in-house program underway which i s aimed a t developing the a b i l i t y t o understand and control shock c e l l noise. The i n i t i a l portion of t h i s study has been described by Seiner and Norum ( r e f . 1 1 ) .
A new theory of shock cell noise has been developed and i s presently i n the validation process. A s indicated i n figure 24 the essential feature of t h i s new model of shock c e l l noise i s i t s more forgiving nature when the exhaust nozzle i s operated in off design condition.
Lateral Attenuation Research I t became apparent d u r i n g the Supersonic Cruise Research and ICAO studies that more information was needed on ground effects on a i r c r a f t noise.
Quantifying the low angle of incidence phenomenon of excess ( f i g . 25) ground attenuation ( E G A ) was of particular i n t e r e s t . The only large data base available t o check the theoretical predictions were the ground-to-ground propagation d a t a taken by Parkin and Scholes i n the mid f i f t i e s ( r e f . 1 2 ) .
There were almost no air-to-ground d a t a available.
NASA conducted a series of f l i g h t t e s t s a t Wallops Island in 1979 in order t o o b t a i n this needed air-to-ground EGA data ( r e f . 13). Figure 25 shows a smmary of the results of these t e s t s i n terms of a plot of the EPNdB attenuation as a function of elevation angle and distance to the observer. Similar curves a r e available f o r the attenuation as a function of frequency. These curves agree f a i r l y well w i t h theory, however, there i s a tendency t o measure s l i g h t l y more attenuation than i s predicted. Also, the actual data points from the experiment show a sizable amount of s c a t t e r . The timely acquisition and interpretation of this data s e t has supported the development of a credible method of calculating l a t e r a l attenuation which has SAE (A-21 Aircraft Noise Committee) i n an aerospace been documented by the i nformati on report ( r e f . 14).
S t a t i c t e s t s were made u s i n g a source mounted on a tower a t the same Wallops Island s i t e t o provide a further comparison between prediction and experiment. I t i s hoped that these t e s t s will exhibit reduced data s c a t t e r and explain any remaining difference between theory and experiment.
J e t Shielding Research Lateral attenuation measurement on multi-engine a i r c r a f t often show greater attenuations t h a n predicted by ground e f f e c t theory or than measured i n the T-38 t e s t s .
The T-38 t e s t s were made with only the engine nearest the microphones operating a t f u l l power so t h a t there would be no j e t shielding effect.
NASA has a program underway t o determine the shielding e f f e c t of one j e t on another as indicated i n figure 24.
An analytical study i s being conducted t o t r y t o compute t h i s effect.
An in-house study i s being conducted t o measure the shielding of a point source.
A contract study t o provide experimental d a t a o f the shielding of a j e t by a j e t i s also planned.
C O N C L U D I N G REMARKS This paper has attempted t o describe some of the essential features of the ANOPP system f o r a i r c r a f t noise prediction and t o provide a basis for evaluating i t s present capabilities and future potential. In just a few years ANOPP has progressed from a turbojet prediction capability t o i t s present capability of predicting the noise from high-bypass-ratio engines w i t h coaxial flow. B y virtue of participation i n SCR and I C A O systems studies, procedures f o r incorporating noise as a constraint a t the preliminary design stage have been established. A takeoff noise optimizing procedure has been developed and installed in ANOPP which calculates a m i n i m u m noise takeoff procedure subject to multiple s i t e noise constraints.
ANOPP provides the framework i n which more sophisticated source prediction theories may be evaluated when, and i f , these theories show the possibility of representing experimental data over a reasonable range of t e s t conditions. I t also provides the basis f o r evaluating new noise reduction concepts such as inverted flow vs. conventional j e t s by interchanging modules s o that the user immediately sees the effect on flyover noise o r on a takeoff noise contour of the inverted j e t as compared t o the conventional j e t .
T h e program is also useful i n comparing the predictions of different theories t o full-scale f l i g h t data. The ANOPP data base contains flyover spectra from three wide-body a i r c r a f t . N e w fan modules may be installed in ANOPP t o have t h e i r predictions compared t o these data. In this way, the more promising theories may be evaluated and selected f o r use. This procedure f o r the objective evaluation of noise prediction methods is an important contribution to noise research and futher suggests the use of ANOPP as a means of evaluating proposed noise reduction designs and techniques.
Future a c t i v i t i e s t o improve prediction accuracy include the refinement of present empirical procedures and the development of f i r s t principles prediction methodology.
REFERENCES 1. Raney, John P . : Development of a N e w Computer System f o r A i r c r a f t Noise A I A A Paper 75-536, Mar. 1975.
P r e d i c t i o n .
2. Raney, John P.: Noise P r e d i c t i o n Technology f o r CTOL A i r c r a f t . N A S A TM-78700, 1978.
3. S t a f f o f the Langley Research Center: Preliminary Noise Tradeoff Study o f a Mach 2.7 Cruise Aircraft. N A S A TM-78732, 1979.
4. Padula, S. L . : P r e d i c t i o n of Noise Constrained O p t i m u m Takeoff Procedures.
AIAA-80-1055, June 1980, 5. Zorumski, William E . : Aircraft Flyover Noise P r e d i c t i o n . NOISE-CON 77 Proceedings, George C. Maling, Jr., e d . , Noise Control Found., c.1977, pp. 205-222.
6. Hosier, Robert N . : A Comparison of Two Independent Measurements and Analyses o f J e t A i r c r a f t Flyover Noise. N A S A TN D-8379, 1977.
7 . Kapper, C . Y . : V a l i d a t i o n o f A i r c r a f t Noise P r e d i c t i o n Program. N A S A CR-159047, 1979.
8. Godby, Larry: ANOPP V a l i d a t i o n Study - Lockheed L-1011. N A S A CR-159138, 1979.
9. Vanderplaats, G a r r e t N . : CONMIN - A Fortran Program f o r Constrained Func-
t i o n Minimization: User's Manual. N A S A T M X-62,282, 1973.
10. S h a p i r o , Nathan; e t a l . : Commercial Aircraft Noise D e f i n i t i o n - L-1011 T r i s t a r . Volumes I-IV. FAA-EQ-73-6, S e p t . 1974. ( A v a i l a b l e from DTIC a s AD A012 371 t o A012 375.)
11. S e i n e r , 3 . M . ; and Norum, T. D . : Aerodynamic Aspects of Shock Containing J e t Plumes. AIAA-80-0965, June 1980.
12. Parkin, P , H . ; and S c h o l e s , W . E . : The Horizontal Propagation of Sound From a J e t Engine Close t o the Ground, a t R a d l e t t . J. Sound. Vib., vol. 1, no. 1, Jan. 1964, p p . 1-13, 13. Parkin, P . H . ; and S c h o l e s , W. E . : T h e Horizontal Propagation of Sound From a J e t Engine Close t o the Ground, a t H a t f i e l d . J . Sound V i b , , vol . 2 , no. 4 , Oct. 1965, pp. 353-374, 14. SAE Committee A-21: P r e d i c t i o n Method f o r L a t e r a l Attenuation o f Airplane Noise During Takeoff and Landing. SAE Aerospace Information Report 1751.
(To be published Apr. 1981.)
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Figure 1.- ANOPP prediction methodology.
A I RCRAFT I E Figure 2 . - ANOPP functional level computation flow diagram.
10 LOG D(BJ 10 LOG S (f) e f ( a ) OVERALL D IRECT IV ITY ( b ) POWER SPECTRUM LEVEL Figure 3.- Overall d i r e c t i v i t y and power spectrum l e v e l s .
D I ( f, 8) 1 0 L O G S
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Figure 4 .- Relative spectrum l e v e l s .
Figure 5.- Source n o i s e p r e d i c t i o n geometry.
50 10,000 FREQUENCY, Hz Figure 6.- Near-field and f a r - f i e l d n o i s e spectra.
P NLT, dB TIME, sec Figure 7 . - Effective perceived noise level computation.
- Figure 8 . - Noise level/slant range curves.
Figure 9 . - Level I contouring procedure.
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Figure 10.- Level I1 and 111 grid contouring method.
7 49
- 3000 1 L l O O d B Y
I I I I I I I 1 0 3000 6000 9000 12 000 DOWNRANGE D I STANCE, m F i g u r e 11.- Comparison of ANOPP enhanced EPNL c o n t o u r s w i t h v e r y a c c u r a t e nonenhanced c o n t o u r s .
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dB 2 -2 -4 CONCORDE MATCH -6 UNDERPRED I CTED I I I I 1 I 300 400 500 600 700 800 mlsec 1000 1500 2000 2500 JET EXHAUST VELOCITY, ft/ sec F i g u r e 12.- Comparison of AJYOPP p r e d i c t i o n s w i t h measured data i n t h e 1977 ICAO exercise.
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0 63 250 1K2K4K8K16K 20 60 1M) 140 180 FREQUENCY, Hz 8, deg Figure 13.- Comparison of DC-9 noise prediction with measured data.
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108 - 0 DATA 0 DATA PERCEIVED NOISE 100- LEVEL, dB ANOPP - DATA = 1.6 EPNdB 40 - 84 I I ' I I 1 ' I I I I I I l l l I l 0 20 60 100 140 180 63 250 1K 2K A K 8K e, deg FREQUENCY, Hz Figure 14.- Comparison of DC-10 noise prediction with measured data.
-MEASURED Q---o WITHOUT B U Z Z SAW SPL, d B PNL, d B *--*WITH B U Z Z SAW WER SETTING = 100 POWER SETTING = lM)% - L - U 40 '0 20 40 60 80 100 120 140 1 6 0 180 o loo 1000 10 000 R A D I A T I O N ANGLE, deg FREQUENCY, H z Figure 15.- Comparison of L-1011 noise prediction with measured data.
-MEASURED D-U W ITHOUT BUZZ-SAW *--* WITH BUZZ-SAW SPL, dB 10 000 FREQUENCY, Hz Figure 16.- L-1 0 1 1 noise spectra for a reduced power f lyover .
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MEASURED 0 APPROACH POWER TAKEOFF POWER P NLT, dB ANOPP-DATA = -5 EPN dB ANOPP-DATA = -1 EPN dB t , , , , , t , , , , , 30 60 90 120 150 30 60 90 120 150 DIRECTIVITY ANGLE 8, deg DIRECTIVITY ANGLE 6, deg Figure 17.- Comparison of 747 noise prediction with measured data.
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FLIGHT TRAJECTORY LEVEL II OPT IM IZER
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Figure 20.- Flow of the optimization computations.
- I N I T I A L
- - --
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% .25 0 0 4000 5000 6OOo 7000 8000 DISTANCE, m Figure 21.- Comparison of i n i t i a l and optimal control functions.
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I N I T I A L SIDELINE OPTIMAL SIDELINE I N I T I A L FLYOVER OPTIMAL FLYOVER NOISE CONSTRAINT 5000 5500 6000 6500 7000 DISTANCE FROM BRAKE RELEASE. m Figure 23.- Noise reduction obtained from optimized flight procedures.
A SHOCK CELL NOISE A NO SHOCK1 I SE ’ ~ ~ ’ / / NEW THEORY \ /
t r O U N D EFFECTS
JET O N JET SHIELDING ANGLE OF INCIDENCE Figure 24.- New research areas identified by ANOPP.
-
SLANT RANGE
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75 7