This led to new insight in the sound generation of moving bodies and, in particular ..... Ffowcs Williams, J. E. ; and Hawkings, D. J. : Sound Generated by Turbulence and Surfaces in Arbitrary Motion, Phi I . Trans. of the Royal Society. (London) ...
NASA Contractor Report 165732 (HAS A-CR-165732) BOISE A I E PEREOBBAYCE C E V B c P E L L G E s 1 ' b LIGHT A I R C B A F I (tassachusetts CSCL 23A Lust. cf I e c b . ) 26 &
b61-26805
G3/71
NOISE AND PERFORMANCE OF PROPELLERS FOR LIGHT AIRCRAFT
G. P. Succi, D. H. Munro, J. A. Zimner, P. D. Dunbeck, E. E . Larabee, K. U. Ingard, and J. L . Kerrebrock
MASSACHUSETTS INSTITUTE OF TECHNOLOGY Cambridge, tlassachusetts 02139
Contract NAS1-15154 June 1931
Nattonal Aeronauttcs and Space Admintstratlon
Unclas 26724
CONTENTS
1.0
INTRODUCTION
2.0
REPORT ORGANIZAT ION
3.0
THEORY 3.1 3.2 3.3 3.4
4.0
WIND TUNNEL TESTS 4.1 4.2
5.0
Aerodynamic Theory Acoustic Theory Numerical Parametric Studies Sample Calculation
lerodynamic Measurements Acoustic Measurements
FLIGHT TESTS Performance
REFERENCES FIGURES
This r e p o r t s u m r i z e s the r e s u l t o f the p r o j e c t "Noise and Performance o f Propellers f o r L i g h t A i r c r a f t ,"Contract No. NAS1-15154.
The o b j e c t i v e of the
study was t o explore the p o s s i b i l i t y of ncise reduction of general a v i a t i o n propellers w i t h minimal aerodynamic performance penal t i e s .
The p r o j e c t involved
extensive t h e o r e t i c a l and experimental aerodynamic and acoustic study of both mod21 and f u l l - s c a l e propellers.
The t h e o r e t i c a l aerodynamic and acoustic
study concerns 1i g h t l y 1oaded p r o p e i l e r s w i t h subsonic t i p speed.
The e f f e c t
o f v a r i a t i o n o f some parameters such as blade sweep, r a d i a l load d i s t r i b u t i o n and blade number on the noise c h a r a c t e r i s t i c s o f the p r o p e l l e r was studied i n detail.
This l e d t o new i n s i g h t i n the sound generation o f moving bodies and,
i n p a r t i c u l a r , propellers.
A l i n e a r acoustic formula of Succi f o r noise calcu-
l a t i o n s was coded f o r a computer and the predicted noise spectra and signatures compared very well w i t h the measured data f o r model propellers.
On the basis
of t h e o r e t i c a l analysis and model t e s t s , a two-bladed f i x e d p i t c h p r o p e l l e r was designed f o r a single-engine a i r c r a f t . on t h i s f u l l - s c a l e propeller.
Some f l i g h t t e s t s a l s o were performed
The peak sound l e v e l during a f u l l power f l y o v e r
a t 305 in (1 000 f t ) a1 t'tude was 4.8 dBA lower.
This study has derrlonstrated
t h a t the technology i s now a v a i l a b l e f o r the control of the noise o f general a v i a t i o n propellers.
-1.0 ----
INTRODUCTION The p r o j e c t "Noise and Performance o f P r o p e l l e r s f o r L i g h t A i r c r a f t , "
Contract No. NAS1-15154 between NASA Langley and MIT, has now been completed, and the n u i n r e s u l t s obtained are s u m r i z e d i n t h i s r e p o r t . The primary p r ~ c t i c a lo b j e c t i v e o f the ztudy was t o explore the p o s s i b i l i t y o f reducing the noise from a general a i d t i o n type p r o p e i l e r w i t h o u t a l t e r i n g s i g n i f i c a n t l y it s aerodynamic performance o r the engine c h a r a c t e r i s t i c s .
After
an extensive study o f t h i s question, i n v o l v i n g aerodynamic and acoustic theory, design, c o n s t r u c t i o n and wind tunnel t e s t i n g o f m d e l p r o p e l l e r s , design and manufacturing o f f u l l - s c a l e propel l e r s and, f i n a l l j , f l i g h t t e s t s , we a r e pleased t o r e p o r t t h a t f o r one o f the p r o p e l l e r s t e s t e d an o v e r a l l reduction o f 4.8 dBA as measured i n a f l i g h t t e s t was achieved. The theory deals w i t h aerodynamics and acoustics o f l i g h t l y loaded p r o p e l l e r s w i t h subsonic t i p speeds and includes studies o f the e f f e c t s o f sweeping the blades, a l t e r i n g the r a d i a l load d i s t r i b u t i o n , and changing t h e number o f blades. These studies lead t o new i n s i g h t i n t o the general problem o f sound generation from moving bodies.
Of p a r t i c u l a r value a r e the algorithms, which are w e l l
su ited f o r cmpu t e r coding . The wind tunnel t e s t s involved three p r o p e l l e r s , 1/4 scale, i n c l u d i n g a r e p l i c a o f a f i x e d p i t c h p r o p e l l e r used on a airplane.
112 kw (150 hp) s i n g l e engine
The other two p r o p e l l e r s were designed t o have the peak r a d i a l l o a d
d i s t r i b u t i o n s h i f t e d inboard.
The acoustic wind tunnel which was used i n
these t e s t s enabled measurement n o t o n l y of the r a d i a t e d sound f i e l d b u t a l s o the t h r u s t and torque o f the p r o p e l l e r . was determined i n d i r e c t l y from wake surveys.
I n a d d i t i o n , the load d i s t r i b u t i o n
Sound pressure signatures were obtained a t d i f f e r e n t l o c a t i o n s and speeds (up t o a t i p
t l a c ~ l number
o f 0.75) and 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 i n
which o n l y the shape and motion o f the , r o p e l l e r were needed as i n p u t parameters; no empirical adjustments were made.
Agreement t o w i t h i n a few percent u ~ s
obtained throughout except i n the presence o f a transonic "buzz" i n s t a b i l it y which was encountered w i t h i n a narrow speed range. On the basis o f the t h e o r e t i c a l analysis arid i t s v e r i f i c a t i o n i n the model t e s t s , a two-bladed f i x e d p i t c h p r o p e i l e r was designed f o r a 112 kw (150 hp) s i n g l e en$ ;ne airplane.
F l i g h t t e s t s w i t h t h i s p r o p e l l e r i n d i c a t e d about the
same performance as the production p r o p e l l e r f o r t h a t airplane, b u t the maximum sound l e v e l during a f u l l power f l y o v e r a t 305 m (1000 f t ) was found t c be 4.8 dBA lower.
A second p r o p e l l e r , w i t h three blades and fixed p i t c h , was designed a t M I 1 f o r an Ohio State U n i v e r s i t y 134 kw (180 hp) s i n g l e engine a i r p l a n e .
Fiight
t e s t s o f t h i s p r o p e l l e r showed a 6 dBA reduction o f noise i n the near f i e l d w i t h no measureable change i n performance. 2.0
REPORT ORGANIZATION During the course o f t h i s p r o j e c t , i n a d d i t i o n t o the r e g u i a r progress
reports, a number of theses were w r i t t e n and several papers published w i t h d e t a i l e d accounts of the various aspects of the work (see references).
The
reviews of t h e i r content are presented under the headings Theory, Wind Tunnel Tests, and F l i g h t Tests i n Sections 3, 4, and 5. 3.0
THEORY The t h e o r e t i c a l analysis included both the aerodynamics and acoustics o f
the p r o p e l l e r .
I n our formulation the aerodynamic c a l c u l a t i o n i s an e s s e n t i a l
p a r t o f the noise p r e d i c t i o n scheme since i t y i e l d s the load d i s t r i b u t i o n on the blades from a given p r o p e l l e r shape and motion.
Aerodynamic--. Theory. - The aerodynamic theory addresses two questions.
3.1
g what i s the most e f f i c i e n t p r o p e l l e r F i r s t , given a s e t o f o p e r a t i ~ ~parameters, shape? Second, given the p r o p e l l e r shape and motion, what i s i t s load distribution? 1 Larrabee explores the aerodynamic problem using a 1 if t i n g 1i n e theory w i t h induced v e l o c i t i e s supplied by a he1 i c a l l y convoluted t r a i 1 i n g vortex sheet.
The e s s e n t i a l s o f t h i s theory, w i t h regard t o the design o f optimum
propellers, were published i n 1919 by Betr and Prandtl.'
A more accurate
d e s c r i p t i o n o f the c i r c u l a t i o n d i s t r i b u t i o n f o r 1i g h t l y loaded p r o p e l l e r s o f minimum induced loss, which accounts f o r vortex sheet curvature, was given by
old stein.^
We used the Betr-Prandtl approximation which i s adequate f o r
low advance r a t i o s .
The design o f a minimun~induced l o s s p r o p e l l e r i s analogous
t o determining the pianform and t w i s t d i s t r i b u t i o n o f a wing which w i l l develop an e l l i p t i c a l span loading.
I n a d d i t i o n t o the optimum design i t i s a l s o necessary t o d e t ,mine the 1 performance o f a r b i t r a r y propel l e r s . Larrabee deals w i t h a r b i t r a r y propel l e r s by means o f a r a d i a l l y graded nwmentum theory, which, however, requires an
i t e r a t i v e c a l c u l a t i o n t o y i e l d the r a d i a l load d i s t r i b u t i o n .
4 Munro was able
t o r e s t r u c t u r e t.le c a l c u l a t i o n so t h a t a closed form a n a l y t i c expression i s obtained.
3.2 Acoustic -
Theory.-
Given the shape and motion o f the propellers, i t
i s possible t o c a l c u l a t e the sound i t produces by subdividing i t i n t o many
small elements, each a c t i n g as an acoustic source.
Each source consists of
two components, the force and the volume associated w i t h each element. source s p i r a l s forward along a h e l i c a l path.
Each
The e f f e c t o f each i s c a l c u l a t e d
independently and the various c o n t r i b u t i o n s are then summed t o f i n d the sound radiated by the e n t i r e p r o p e l l e r .
The power o f t h i s technique l i e s i n the
f a c t t h a t there i s an exact a n a l y t i c expression f o r the sound emitted from each p o i n t .
This g r e a t l y s i m p l i f i e s the computation.
The d e r i v a t i o n o f t h i s
technique from the Ffowcs Williams-Hawkings equation6 i s given by S u ~ c i . A~ good d e s c r i p t i o n of the r e l a t i o n o f t h i s technique t o o t h e r computation methods may be found i n a review by ~ s r a s s e t and by Farassat and Succi
.8
4 Munro approached the problem by general i z i n g the K i r c h o f f formula f o r the s o l u t i o n o f the wave equation i n terms o f i t s boundat-y values.
The
' f o r the motion of generalization, whicb i s o r i g i n a l l y due t o ~ o r ~ a n , allows the boundary surface.
Munro u l t i m a t e l y reduced t h e formulas t o an array o f The
p o i n t sources 1 ike those above and made several i n t e r e s t i n g observations.
"thickness" term was i d e n t i f i e d as a couplet o f mass sources w i t h a time-1 ike separation r a t h e r than a s p a t i a l separation, and he noted t h a t the indiced drag forces must be treated d i f f e r e n t l y from the p r o f i l e drag source.
These two
2 i s negligible.
types o f source, however, are i d e n t i c a l i n the 1i m i t as M
Addi-
t i o n a l terms due t o the r a d i a l c o n t r a c t i o n o f the shed vortex sheet and the r o l l u p o f the shed vorte, sheet were derived b u t n o t implemented on the computer. These terms cannot be treated u n t i l improvements i n the aerodynamic theory are made so as t o accurately p r e d i c t the time-dependent s t r u c t u r e o f the wake.
For
l i g h t l y loaded subsonic t i p speed p r o p e l l e r s the d i s t i n c t i o n between Munro's and 5 u c c i ' s source models i s n e g l i g i b l e . 3.3
Numerical Pardmetric Studies.- A s e r i e s o f noise reduction schemes
was explored w i t h p a r t i c u l a r a p p l i c a t i o n t o a 112 kw (150 hp) a i r c r a f t , w i t h the o b j e c t i v e o f minimizing the peak dBA l e v e l s recorded by a ground observer
as the a i r c r a f t f l i e s along a l e v e l path a t an a l t i t u d e o f 305 m (1000 f t ) , i n " states "Overfl i g h t must be accordance w i t h FAA advisory c i r c u l a r ~ 3 6 - 1 ~ which performed a t r a t e d maximum continuous power, s t a b i 1 ized speed.. a i r c r a f t i n c r u i s e condition."
. and
w i t h the
I n the numerical studies only the p r o p e l l e r
parameters were varied; we d i d n o t explore noise reductions t h a t are possible by changing engines o r introducing a gear box. Succi
5
presents the r e s u l t s o f the parametric studies i n terms o f the
aerodynamic penal t i e s and acoustic gains.
A t y p i c a l r e s u l t was t h a t if the
p r b p e l l e r r8dius was reduced by 20 percent, the sound l e v e l decreased 4 dB o r 8 dm, and the efficiency dropped by 4-1/2 percent. I n studies o f the r o l e o f r a d i a l load d i s t r i b u t i o n , the idea i s t o s t a r t w i t h an aerodynamically optinlum load p a t t e r n and then p e r t u r b i t . Since the o r i g i n a l d i s t r i b u t i o n i s an aerodynamic extremum, the load perturbation causes o n l y second order changes i n the e f f i c i e n c y .
However, such read changes w i l l
a l t e r ttx acoustic f i e l d t o f i r s t order since the aerodynamic optimum d i f f e r s from the acoustic optimum.
A f a m i l y o f load curves was explored and i t was
found t h a t moving the lodd inboard 20 percent decreased the sound l e v e l by 1.4 dB, 4.2 dBA, but reduced the e f f i c i e n c y by o n l y 1 percent. The r o l e o f the number o f blades das also explored as well as the blade sweep.
I n regard t o sweep our simple aerodynamic model could n o t accurately
p r e d i c t the e f f i c i e n c y changes.
However, we d i d estimate the sound l e v e l
changes and two f a m i l i e s o f swept blades were explored.
I n the f i r s t the r a t e
of sweep per u n i t radius was f i x e d and the maximum sweep angle was increased. I n t h i s scheme i t was possible t o reduce the noise without l i m i t , as, i n the l i m i t i n g case, the p r o p e l l e r occupied the e n t i r e d i s k plane and became a noiseless actuator d i s k .
This strategy, o f course, i s impractical.
A second
f a m i l y o f swept curves was explored. and the r a t e o f sweep was a l t e r e d .
Here the maximum sweep angle was f i x e d I n t h i s instance there was an a c o u s t i c a l l y
optimum d i s t r i b u t i o n . ~ u n r o " performed a d e t a i l e d study o f the aerodynamic, s t r u c t u r a l and acoustic problcns associated w i t h swept blades.
The program r e s u l t e d i n a design
procedure which was the basis f o r the design o f a series o f three swept p r o p e l l e r blades f o r a Cessna 172.
The blades cannot be f a b r i c a t e d from Aluminum, b u t
they are w e l l w i t h i n the range o f carbon-epoxy construction technique.
The
q u i e t e s t o f these blades i s swept forward 5 degrees from the hub t o the 50 percent radius, then swept back 45 degrees from the 50 percent radius t o the tip.
I t offers a noise reduction of approximately 1.3 dB and 3.4 dBA over
a s i m i l a r s t r a i g h t blade. process.
Acoustics p l a y a minor r o l e i n the actual design
This i s due t o the c o n s t r a i n t s d i c t a t e d by the extremely l a r g e
c e n t r i f u g a l forces. 3.4
Sample Calculation.- Before c a r r y i n g o u t the wind tunnel t e s t s , Succi 13
tested the computational scheme i n a comparison w i t h data given i n Magliozzi's study of the influence o f forward f l i g h t on p r o p e l l e r noise.''
The study
involved a 1i g h t twin engine STOL t r a n s p o r t a i r c r a f t w i t h three-bladed p r o p e l l e r s which was operated under a v a r i e t y o f f l i g h t c w i d i t i o n s f o r a range o f p r o p e l l e r t i p speeds and powers.
A boom was i n s t a l l e d on the wing t i p and used t o support
two microphones, one i n the d i s k plane and one slight1,y a f t .
I n f l i g h t , the
tone noise was found t o be thickness and steady loading noise. When comparing these r e s u l t s ;vith our c ~ m p u t a t i o n a l procedure, r a t h e r than t o c a l c u l a t e the exact load d i s t r i b u t i o n we made the approximation t h a t , f o r each f l i g h t condition, the p r o p e i l e r loading minimized the induced losses for t h a t rpm, power, blade number, radius and forward v e l o c i t y .
We also made an
estimate o f the unsteady loading due t o flow blockage by the engine nacelle. Calculations were c a r r i e d out f o r a11 f l i g h t t e s t s where o n l y one p r o p e l l e r i s The Fourier amplitude spectra, which we derived from the pressure time
powered.
signature, were conpared w i t h the observed spectra and good agreement was found, even out t o the twentieth harmonic (reference 5).
The one instance o f poor
agreement occurred f o r f l i g h t t e s t s a t a l t i t u d e s described as "low."
We
assume t h a t the reason f c r the disagreement i s nan-uniform i n f l o w t o the propelSample c a l c u l a t i o n s from reference 5 are provided i n f i g u r e s 1 and 2.
ler.
Besides providing the experimental comparison, we took great care t o document tne i n p u t used for each t e s t case (reference 13).
I t i s hoped t h a t
t h i s t a b u l a t i o n w i l l be o f some use t o those who would w r i t e t h e i r own computer programs and are searching f o r d e t a i l e d compa, isons w i t h an e x i s t i n g program. 4.0
WIND TUNNEL TESTS The purpose of the wind tunnel t e s t s was t o provide a d e t a i l e d t e s t o f
our conputat.ions. of three nacelles.
Three p r o p e l l e r s were constructed and were operated i n f r o n t
A v a r i e t y o f o p e r a i i n g conditions were explored and the
data obtained have been d i g i t i z e d and stored c7 magnetic tape.
The best
summary of our r e s u l t s i s contained i n reference 14 and the d e s c r i p t i o n o f the t e s t f a c i l i t y and instrumentation can be found i n the theses by Munro, ~imr," and Dunbeck.
4
16
The t e s t s were made i n the anechoic wind tunnel a t MIT as indicated i n
f i g u r e 3. thrust
The major piece of equipment constructed f o r the experiment was the
stand, the support apparatus f o r the motor used t o d r i v e the p r o p e l l e r s .
This stand was instrumented t o neasrrre t h r u s t , torque, r o t a t i o n r a t e and propeller position.
The p r o p e l l e r blades were attached t o the hub so as t o
a l l o w the blades t o be r o t a t e d t o d i f f e r e n t p i t c h settings.
The r a d i a l l o a d d i s t r i b u t i o n was explored w i t h a probe moved r a d i a l l y through the s l ipstream by a motorized traverse.
The sound f i e l d was explored
w i t h a microphone mounted i n the a i r s t r e a m so as t o avoid s c a t t e r i n g and r e f r a c t i o n e f f e c t s i n the tunnel j e t shear l a y e r .
The r a c k p o s i t i o n was se.
by a small motor so as t o a1 low continuous angular surveys o f t h e p r o p e l l e r w i t h o u t e n t e r i n g the tunnel.
A1 1 s i g n a l s were d i g i t i z e d u s i n g an Explorer I!I o s c i l l o s c o p e .
Temporary
storage was done on t q e o s c i l l o s c o p e f l o p p y d i s k ; the data were ultimately t r a n s f e r r e d t o magnetic tape on a VAX and IBM 370.
I n our t e s t s i t was import-
a n t t o keep the t i p Mach number s i m i l a r t o f u l l scale and s i n c e the p r o p e l l e r was 1/4 scale, i t s rpm had t o be f-,ur times the f u l l - s c a l e value.
Similarly,
t o cover adequately a 20 kHz f u l l scale frequency range, we had t o make nleasurements up t o 80 kHz and sample a t 160 kHz.
A 118-in B&K microphone s i g n a l
d i g i t i z e d w i t h an Explorer I 1 1 o s c i l l o s c o p e was s u f f i c i e n t t o meet these requirements 4.1
.
Aerodynamic Measurements. - The aerodynamic measurements were made t o
t e s t the v a l i d i t y o f our v e r s i c n o f l i f t i n g 1 i n e theory f o r these general a v i a t i o n propellers.
We were p a - t i c u l a r l y concerned w i t h v e r i f y i n g the pr: fitted load
d i s t r i b u t i o n s as a l t e r a t i o n s i n t h i s d i s t r i b u t i o n can be used as a noise reduct i o n strategy. The f i r s t t e s t s measured the power and t h r u s t c o e f f i c i e n t as a f u n c t i o n o f advance r a t i o ( X = v/L?R).
Since the f l o w i n our tunnel had a maximun~speed o f
30 MIS, advance r a t i o was iacreased by reducing rpm a t f i x e d maximum tunnel velocity.
When the p r o p e l l e r speed was reduced below 7000 rpm t o go t o values
o f h i n excess o f .17, the experimental values o f t h r u s t and pcwer absorption
f e l l below the p r e d i c t i o n s .
This was most l i k e l y due t o a degradation i n
a i r f o i l section c h a r a c t e r i s t i c s w i t h decreasing Reynolds number.
This d i s -
crepancy d i d n o t a f f e c t our acoustic r e s u l t s as the propel1 ?r noise could be measured above the background noise only a t r o t a t i o n r a t e s gi-eater than '000 rpm. The r a d i a l load d i s t r i b u t i o n was examined by measuring t h e wake behind the propeller.
Two probes were used
--
a three-hole pressure probe which gave
adequate response when the r a d i a l component of v e l o c i t y was small
, and a hot
wire velocimeter which was useful under a l l f l o w c m d i t i o n s . Three bodies were used i n conjunction w i t h each p r o p e l l e r :
:-Mi
the smallest f a i r i n g t h a t could f i t around the motor
Symmetric bod$-:
a l a r g e a x i a l l y symnetric body w i t h a crgss area
d i s t r i b u t i o n s i m i l a r t o a 1i g h t airplane. Asymnetric body:
a v a r i a t i o n o f the symnetric body wherein the upper
p o r t i o n o f the body was modified t o be a 2 : l a x i s r a t i o e l l i p s e and then was transformed i n t o a conical "windshield" region which vas f a i r e d i n t o thcl symnetric afterbody. The wake measurements w i t h the minimum body were used as a reference simulating the operation o f the p r o p e l l e r i~ a uniform flow. the wake was d i s t o r t e d only by a small contraction.
I n t h i s instance
I t was possible t o r e l a t e
the momentum i n the wake t o the r a d i a l blade loading by assuming a one-to-one correspondence between percentage r a d i i o f the blade and wake This procedure d i d n ' t work when the wake was d i s t o r t e d by a l a r g e body. The search f o r a proper i n t e r p r e t a t i o n o f the wake measurement made w i t h the large symmetric body r e s u l t e d i n a data reduction procedure wherein measurements were compared d i r e c t l y t o theory.
The procedure was t o c a l c u l a t e the
c i r c u l a t i o n from the measured s w i r l v e l o c i t y and r e l a t e i t t o the appropriate
p r o p e l l e r by mappiqg o u t the l i n e s o f equal mass f l u x . vortex sheet by the nacelle
('
'd
r
~
The d i s t o r t i o n o f the
a1t t e r the conservation o f vortex 1 ines.
A l l vortex l i n e s o r i g i n a t e d i n the bound v o r t i c i t y on the blade. d i s t r i b u t i o n varied, the vortex l i n e s were shed i n t o the flow.
As the load For a l i g h t l y
ladded p r o p e l l e r the shed vortex l i n e s followed the stream surfaces, which were measured by constructing the mass f l u x .
~ i m n e r 'reduced ~ h i s measurements
i n t h i s manner and obtained a consistent d r ... r i p t i o n o f p r o p e l l e r performance. I n f i g u r e 4, a comparison between theory and experiment i s made f o r the production p r o p e l l e r operating i n f r o n t of the minimum body.
I n figure 5 a
s i m i l a r comparison i s presented f o r a p r o p e l l e r w i t h an inboard load peak operating i n f r o n t o f a l a r g e s y m e t r i c bady.
O f the three p r o p e l l e r s studied, two were designed t o have the peak loading m v e d inboard and the wake measurements v e r i f i e d t h i s design o b j e c t i v e . This strengthened our r e l i a n c e on thc aerodynamic theory as a s u f f i c i e n t l y accurate t o o l i n the design of low noise p r o p e l l e r s .
4.2
Acoustic Measurements.- Angular surveys of the sound pressure were
made a t constant p r o p e l l e r rpm as w e l l as a t a constant advance r a t i o and i n each case the various propeller-body combinations were used.
The r e s u l t s o f
these measuraernents f o r the symnetric body w i t h production p r o p e l l e r and the M I T p r o p e l l e r designed t o replace i t are given i n reference 17.
These e a r l y
r e s u l t s showed t h a t the theory was q u i t e accurate. For a d e t a i l e d cornpari son between experimental and t h e o r e t i c a l curves we needed t o improve the experimental pressure signatures so as t o e x t r a c t t h a t p a r t of the trace w i t h a period equal t o the blade passage period. only t h i s p a r t t h a t was obtained from the theory.)
( I t was
70 do t h i s the raw signal
composed o f several pressure pulses, was Fourier t r a n s f o m d and the average "cleaned" signal was produced by inverse transforming the f i r s t 64 harmonics 4 o f the blade passing frequency. This procedure was used by Munro (Chapter 5), who also gave sample r e s u l t s f o r other propeller body combinations. cases the agreement between prediction and theory was good.
In all
A sample c a l c u l a t i o n
f o r the production propeller mounted i n f r o n t o f the symnetric body i s presented i n f i g u r e 6. An unexpected phenomena was uncovered i n measurements made w i t h production propellers over a narrow range o f operating conditions.
A t a t i p Mach number
o f 0.7 extrenlely intense coherent bursts o f high frequency sound were produced
from the region near the t i p s o f the blades where the a i r f o i l chords were roughly 2 cm.
The measured (Doppler s h i f t e d ) frequency w i t h i n the burst was 38
kHz near the disk plane, which indicated a frequency o f 13 kHz i n the frame o f the blade.
The flcw i n s t a b i l i t y responsible f o r the sound has not been
p o s i t i v e l y i d e n t i f i e d , but most l i k e l y i s related t o a transonic shock instab i l it y (see ~ u n r ,4 o Chapter 5).
5.0
FLIGHT TESTS The f l i g h t tests on the low noise propeller for the Cessna 172 are documented
by Succi
.18
During a level flyover a t 305 m (1000 f t ) , the maximum sound
level f o r the M I T propeller was found t o be 4.8 dBA lower than t h a t o f the production propel l e r a t e s s e n t i a l l y the same aerodynamic performance o f the two propellers.
Table I sumnarizes the r e s u l t s o f these tests.
The MIT f l i g h t t e s t propeller was a v a r i a t i o n of the propeller no. 3 used i n the wind t ~ n n e l !I~t was designed t o match the power absorption o f the production propeller, a t the design flyover condition, and have the peak r a d i a l load moved inboard.
Howtver, off-design calculations indicated t h a t t h i s
TABLE I
* Runs
Standard P r o p e l l e r (dBA)
MIT Propel l e r
Level Difference (dBA)
(dBA)
305 m (1000 f t ) Flyover
*
.4
-5.1
77.9 2 .3
73.8 2 .2
-3.5
77.3 t .2
72.1 2 .8
6
77.1
.7
6
-
6
--
18
77.4
72.0
k
-5.2 A
72.6
-4.8
153 m (500 f t ) Flyover
I
6
84.5
.7
78.0 t .6
-6.5
6
83.7 i 1.0
80.8 r .5
-2.9
6
83.1
79.5
18
2
k
83.8
.7
+
.7
-3.6
79.4
p r o p e l l e r absorbed too much power a t low speeds.
-4.4
To m i t i g a t e t h i s e f f e c t the
radius was reduced t o 92.5 percent o f t h a t o f the production p r o p e l l e r .
This
had l i t t l e change on the high speed performance, since the t i p s were already unloaded, b u t s i g n i f i c a n t l y improved the low speed performance. t o consider was the danger o f overspeeding the engine.
Another f a c t o r
To avoid t h i s problem
the p r o ~ el le r !:as designed conservatively so t h a t i t turned 100 rpm slower than the production p r o p e l l e r a t f u l l engine t h r o t t l e . Performance.-
-
Comparisons o f the MIT and production p r o p e l l e r s i n s t a l l e d
on a 150 hp iessna 172 are i n d i c a t e d i n f i g u r e s 7 and 8.
These data are f o r
an average a1 t i tude of 710 m (2000 f t ) MSL, taken under s i m i l a r atmospheric conditions and are n o t corrected t o a standard day.
Figur
\
power absorption i s s i m i l a r except a t the h i g h speed p o i n t .
7 i n d i c a t e s the Figure 8 shows the
r a t e o f climb, which indicates propeller efficiency i f the power input i s identical, i s also similar.
Thus, noise was reduced w i t h a minimal a l t e r a t i a n
o f performance. Each o f the modifications contributed t o the noise veduction.
However,
the basic strategy o f moving the load inboard (when the thickness noise does not dominate) was the most important i n reducing the flyover noise and i t represented a rewarding demonstration of the usefulness of the aeroacous t i c computational procedure which we used. On the basis of the r e s u l t s o f t h i s project, we conclude t h a t the aerodynamic and aeroacoustic technology i s now available t o control the noise o f general aviation propellers.
I t i s hoped t h a t the study reported
be useful f o r future propeller designs.
here w i l l
REFERENCES 1.
Larrabee, E. E. : Practical Design o f Minimum Induced Loss Propellers. Transactions, SAE Business A i r c r a f t fketing, Paper 790585, Apri 1 1979.
2.
Betz, A. : Schraubenpropel l e r m i t geringstem Energieverlust, and Prandtl , L, Appendix f o r the above. Goettinger Nachrichtern, 1919. Reprinted 1927 i n Vier Abhandl ungen zur Hydrodynamik and Aerodynami k , also a t Goettingen.
3.
Goldstein, S.: On the Vortex Theory o f Screw Propellers. o f the Rcyal Society (A) 123, 440, 1929.
Proceedings
4.
Munro, D. H.: The production o f Sound by Moving Objects. MIT Department o f Physics, June 1983.
?;ID Thesis,
5.
Succi , 6. P. : Design o f Q u i e t E f f i c i e n t Propellers. Transactions o f 1979 SAE Business A i r c r a f t Meeting, Paper 790584, A p r i l 1979.
6.
Ffowcs Williams, J. E. ; and Hawkings, D. J. : Sound Generated by Turbulence and Surfaces i n A r b i t r a r y Motion, Phi I . Trans. o f the Royal Society (London) A264, 1969.
7.
Farassat, F. : A Collection o f Formulas f o r Calculation of Rotating Blade Noise -- Compact and Non-Compact Source Results, AIAA 6 t h Aeroacoustics Conference, June 1980. AIAA-80-0996.
8.
Farassat, F.; and Succi, G. P.: Review o f Propeller Discrete Frequency Noise Prediction Technology w i t h Emphasis on Two Current Methods f o r Time Domain Calculations. 3. Sound and Vibration (198 0) 71 (3).
9.
Morgan, W. R.: The Kirchoff F o n u l a Extended t o a Moving Surface. Philosophical Magazine 9 (;930), 141-1 61.
10.
Federal Aviation Administration Noise Sta.#dards, T i t l e 14 Code o f Federal REgulations , Chapter 1 , p a r t 36.
11.
Munro, 0. H. : Sweepback as a Strategy f o r Noise Reauction o f General A i r c r a f t Propellers. NASA CR-165724, June 1979.
12.
Magliozzi , B. : The Influence o f Forward F l i g h t on Propeller Noise. NASA CR-145105, 1977.
13.
Succi , G. P. : Computed and Experimental Spectra for a Wing Mounted Microphone on a L i g h t STOL A i r c r a f t . NASA C8-165725, May 1978.
14.
Succi, 6 . P.; Munro, D. H.; and Zimr, J.: Ex~erimentalV e r i f i c a t i o n o f Propeller Noise Prediction, A I A A 6th Aeroacoustics Conference, AIAA 80-0994, June 1980.
1
Zimner, 3 . A. : Wakes ;.nd Performance o f L i g h t A i r c r a f t Propellers, Volumes I and 11. NASA CR-165726, June 1980.
16.
Dunbeck, P. B. : Performance o f L i g h t A i r c r a f t Fropellers. February 1979.
17.
Succi , G. P. : Computed and Experimental Pressure Signatures from Two 114-Scale General Aviation Propel l e r s . NASA CR-165728, September 1979.
1
Succi, G . P.: Noise and Performance of the M I T and Production Propellers for a 150 hp Single Engine A i r c r a f t . NASA CR-165729, June 1980.
NASA CR-165727,
HARMONICS OF BLADE PASSING FREQUENCY FLIGHT 7
Figure 1.
-
RUN 10
IN-PLANE MICROPHONE
Experimental and computed a f t microphone spectra. rpm 2145, power 262 KW, v e l o c i t y 50 m/s.
0
10 20 30 HARMONICS OF B L A D t PASSING FREQUENCY FLIGHT 7
Figure 2.-
RUN 10
AFT-MICROPHONE
Experimental and computed fore microphone spectra. rpm 2145, power 262 KW, v e l o c i t y 50 m/s.
.
PROPELLER: McCAULLEY BLADE ANGLE: FORGING PITCH DISTRIBUTION NACELLE: MINIMUM rpm: 10,000
TUNNEL SPEED: 29.3 mls (96 fthec) AXIAL LOCAT!ON: 0.41R PROBE: HOT WIRE NO. 2 AIR DENSITY: 1.16 kg/m3
EQUIVALENT RADIUS
Figure 4 . - Medsured and predicted circulation distribution for wind tunnel t e s t s on production propeller.
>
PR0PELLER:MIT PROPELLER BLADE ANGLE: MIT PITCH DISTRIBUTION NACELLE: SYMMETRIC rpm: 7,000
0
0.2
0.4
0.6
0.8
TUNNEL SPEED: 29.3 m/s (96 ft/sec) AXIAL LOCATION: 0.13R PROBE: HOT WIRE N 0 . 2 AIR DENSITY: 1.16 kg/m3
1.O
1.2
1.4
EQUIVALENT R A D I U S
Figure 5.- Measured and predicted c i r c u l a t i o n d i s t r i b u t i o n f o r wind tunnel t e s t s on M I T p r o p e l l e r .
1.6
WJ
0
7 C
n
3
OC
W
5
A
-
-20
-
20"
40
Figure 6.- Cessna Blade: 10 K rpm, 29 m/s, 0".
TIME (ms)
ANGULAR SPEED: 9,998 rpm FORWARD SPEED: 29.6 m/s (97 ftlsec) SOUND SPEED: 352 m/s 11155 ft/sec) MICROPHONE LOCATION: RADIUS: 48.4 cm (1.6 ft) AZIMUTH: 0'
SYMMETRIC AFTER BODY
-.-
r
2800
-n 2600
OBSERVED ENGINE rpm CESSNA MODEL 172 M GROSS WEIGHT 2300 Ibs., ACTUAL WEIGHT AT TAKE OFF: 2000 Ibs. 150 hp ENGINE
.
u a tn
W
W
z 13
2400
z u
0 MIT PROPELLER 0 McCAULLEY PROPELLER
2200
60
Figure 7
80
.- Measured engine propeller
100 VELOCITY (mph)
120
meed o f the M I T and production f o r f l i g h t t e s t conditions.
140
700
-
-.I
0
-
