Results of the initial phase of a research project to study the design constraints on helicopter noise are presented. These include the calculation of nonimpulsive rotor harmonic and broadband hover noise spectra, over a wide range of rotor design variables and the sensitivity of perceived noise level (PNL) to changes in rotor design parameters. The prediction methodology used correlated well with measured whirl tower data. Application of the predictions to variations in rotor design showed tip speed and thrust as having the most effect on changing PNL

Schaeffer, E. G.

Sternfeld, H., Jr.

English

NASA Technical Reports Server

30 
DESIGN OF HELICOPTER ROTORS TO NOISE CONSTMINTS 
Edward G. Schaef fer  and Harry S t e r n f e l d ,  Jr. 
Boeing Ver to l  Company 
SUMMARY 
Resul ts  from the i n i t i a l  phase of a r e sea rch  c o n t r a c t ,  "Study of Design Con- 
s t r a i n t s  on Hel icopter  Noise," NAS1-15226, sponsored by t h e  NASA Langley 
Research Center are presented .  
cluded. 
no ise  s p e c t r a ,  over a wide range of r o t o r  des ign  v a r i a b l e s  w e r e  accomplished; 
and t h e  s e n s i t i v i t y  of PNL t o  changes i n  r o t o r  desigfi parameters  are presented.  
Measured r o t o r  n o i s e  d a t a  w e r e  used t o  c o r r e l a t e  t h e  c a l c u l a t i o n s  i n  v e r i f y i n g  
t h e  p r e d i c t i o n  methodology. 
A d e s c r i p t i o n  of t h e  o v e r a l l  program i s  in- 
Basic c a l c u l a t i o n s  of nonimpulsive r o t o r  harmonic and broadband hover 
INTRODUCTION 
Increased emphasis on reducing t h e  n o i s e  generated by h e l i c o p t e r  r o t o r s  t o  
minimize a u r a l  d e t e c t i o n  t i m e s  i n  m i l i t a r y  a p p l i c a t i o n s ,  and i n c r e a s e  community 
acceptance dur ing  commercial ope ra t ions ,  now r e q u i r e  t h e  h e l i c o p t e r  manufac- 
t u r e r  t o  cons ider  n o i s e  c o n s t r a i n t s  of h i s  product  e a r l y  i n  t h e  des ign  phase. 
Impending n o i s e  r e g u l a t i o n s ,  such as t h e  FAA/ICAO p o s s i b l e  no i se  l i m i t s  f o r  
c e r t i f i c a t i o n  of h e l i c o p t e r s ,  are f o r c i n g  des igne r s  t o  implement n o i s e  c o n t r o l  
measures dur ing  pre l iminary  des ign  performance s t u d i e s  when t h e  s i z i n g  of 
r o t o r s  is  being dytermined. 
Basic r o t o r  des ign  parameters  such as t o t a l  t h r u s t ,  b l ade  t i p  speed, d i s k  
loading ,  number of b lades  p e r  r o t o r ,  and r o t o r  s o l i d i t y ,  which i n v a r i a b l y  
a f f e c t  t h e  n o i s e  produced by t h e  r o t o r ,  have g e n e r a l l y  been decided long be fo re  
no i se  r e s t r i c t i o n s  are considered.  One reason  i s  t h a t  most pre l iminary  
des igners  do n o t  have s i m p l i f i e d  g u i d e l i n e s  f o r  p r e d i c t i n g  r o t o r  n o i s e  which 
can  be meaningful dur ing  e a r l y  r o t o r  des ign  dec i s ions  s t ages .  Consequently, 
most des igns3are  semi-f inal ized,  be fo re  n o i s e  estimates of t h e  conf igu ra t ion  
can be made. Subsequent changes t h a t  may be  r equ i r ed  i n  reducing t h e  no i se  t o  
comply w i t h  c e r t a i n  r egu la t ions  f i n d  themselves i n  c o n f l i c t  wi th  des igns  t h a t  
have a l r eady  been set. 
This  s tudy,  when completed, w i l l  r e s u l t  i n  a gene ra l  method, and sets of design 
c h a r t s ,  which w i l l  permi t  eva lua t ions  of  t h e  n o i s e  and performance t r a d e o f f s  
of s i n g l e  r o t o r s  dur ing  t h e  e a r l y  des ign  s t a g e .  
w i l l  be  t h e  percentage of a v a i l a b l e  r o t o r  t h r u s t  which must be  expended i n  
l i f t i n g  t h e  d r i v e  system ( r o t o r  b l ades ,  hub, and r o t o r  t ransmiss ion) .  
The measure of performance 
551 
https://ntrs.nasa.gov/search.jsp?R=19790002683 2020-03-22T02:08:40+00:00Z
Given a desired thrust and noise limit, the charts can be used to define the 
corresponding radius, chord, and tip speed for 2, 3, 4 ,  5 or 6 bladed rotors. 
The rotor which requires the lowest drive system weight is the optimum design. 
Conversely, given a completely defined rotor the charts can be used to predict 
the noise. 
Results from the completed initial phase of the study, which includes the cal- 
culation of both rotor harmonic and broadband, nonimpulsive hover noise and 
the relative importance of various rotor design parameters that influence 
changes in Perceived Noise Level (PNL) are diseussed in this paper. j 
SYMBOLS AND ABBREVIATIONS 
Values are given in both SI and U.S. Customary Units. 
made in U.S. Customary Units. 
The calculations were 
T 
VT 
fP 
Ab 
(J 
S j 
CL 
r 
SPL 
PNL 
dBA 
dBC 
BB 
PNLT 
NOY 
thrust, N (lb) 
blade tip speed, m/sec (ft/sec) 
peak frequency, Hz 
blade area, m2 (ft2) 
angle between centerline of rotor shaft 
and line to observer, deg 
one-third octave frequency band correction 
lift coefficient 
distance to observer, m (ft) 
Sound Pressure Level, dB (re 2 x 
Perceived Noise Level, in PNdB 
A-weighted network 
C-weighted network 
2 N/m 
Broadband noise 
Tone-corrected Perceived Noise Level 
Unit used in the calculation of Perceived Noise Level. It is the 
noisiness of a noise for which the Perceived Noise Level is 40 PNdB. 
The noisiness of a noise that is judged by a subject to be n times 
that of a 1-NOY noise is n NOYS. 
552 
PROGRAM 
he ob jec t ive  of t he  program is  t o  provide a "handbook" f o r  he l i cop te r  designers 
nd conf igura t ion  managers t o  eva lua te  t h e  noise  of r o t o r s  during t h e  prelim- 
nary design phase, and t o  estimate the  e f f e c t  on r o t o r  payload. 
n order t o  produce an e f f e c t i v e  des igner ' s  t o o l  t h a t  can be  used during noise  
.nd performance tradeoff eva lua t ions ,  t he  t o t a l  r o t o r  no ise  s igna tu re  has  t o  
)e represented accura te ly .  A l l  major sources of r o t o r  no i se  are included i n  
.eveloping t h e  design c h a r t s  f o r  t h e  handbook. Figure 1 shows an example of 
:hese sources and t h e i r  cont r ibu t ion  t o  t h e  o v e r a l l  no ise  s igna ture .  The sub- 
jective weighting of these  noise  sources; harmonic, broadband (nonharmonic), 
i d  impulsive, which i s  t h e  prelude t o  determining t h e  PNL, are shown i n  
'igure 2 as t o t a l  NOY values per octave band. 
w t ion  is  shown f o r  completeness only. Examination of t h i s  f i g u r e  ind ica t e s  
:hat i n  terms of annoyance, r o t o r  impulse i s  the  major f a c t o r ;  bu t  i f  t h e  
:otor d id  not have an impulsive c h a r a c t e r i s t i c  then broadband noise  predomi- 
iates the  Perceived Noise Level (Figure 2) t o  a much g r e a t e r  ex ten t  than the  
Sound Pressure Level Spectrum (Figure 1). 
The engine noise  minor cont r i -  
Che o v e r a l l  study c o n s i s t s  of the  following phases: 
L .  Calculating the  nonimpulsive r o t o r  harmonic and broadband noise  spec t r a  
using es tab l i shed  p red ic t ion  procedures recognized and used by indus t ry  
and found i n  open l i t e r a t u r e .  The range of r o t o r  physical  parameters 
included i n  t h e  ca l cu la t ions  are: t h r u s t ,  44 t o  356 kN 10 000 t o  
80 000 l b ) ;  d i sk  loading, 287 t o  575 N/m2 (6 t o  1 2  l b / f t  ); s o l i d i t y ,  
0.04 eo 0.12; number of blades,  2 t o  6; and t i p  speed, 152 t o  244 m/sec 
(500 t o  800 f t / s e c ) .  
150 meters from r o t o r  and a height of 150 meters (which corresponds t o  
the measurement loca t ions  being considered i n  t h e  regula t ions) .  
bining t h e  noise  s igna tures  i n t o  one-third octave frequency bands 
ca l cu la t ing  PNL, dBA and dBC. 
;i 
Calculations are f o r  a s i d e l i n e  d is tance  of 
Com- 
2. Appwqng impulsive co r rec t ions  developed by t h e  Boeing Vertol Co. and 
sub jee t ive  adjustments from Reference 1 to  a d j u s t  dBA, dBC and PNL values 
t o  a sub jec t ive ly  equivalent broadband' l e v e l .  
Preparing a set of design c h a r t s  t o  permit d i r e c t  determination of values 
of dBA, dBC and PNdB f o r  range of r o t o r  physical  parameters. 
of a poss ib le  design cha r t  format i s  shown i n  Figure 3 f o r  determining t h e  
PNdB i n  hover and, providing a r a t i o n a l e  showing the  e f f e c t s  o f  r o t o r  
configuration on forward f l i g h t  noise. 
3.  
An example 
4. Evaluating t h e  performance penal ty  f o r  each main ro to r , conf igu ra t ion  and 
t i p  speed combination. The r a t i o  o f  dr ive  system weight t o  r o t o r  t h r u s t  
s h a l l  be  used as an  index bf t he  design e f f i c i ency .  
553 
RESULTS AND DISCUSSION 
Prediction of Nonimpulsive Rotor Hover Noise 
The harmonic rotation noise calculation was based on the method developed in 
Reference 2. This widely accepted rotor noise calculation includes the design 
variables of thrust, disk loading, tip speed, and number of rotor blades. The 
only change made to the equations of Reference 1 was that an airloads harmonic 
decay exponent of 1.3 was used instead of 2.0, as specified by the original 
authors. 
of 15 dB per octave which has been measured by other researchers and provides 
better agreement with measured data in the higher harmonic range. 
This modification reflects a more realistic airload harmonic decay 
The broadband, or nonharmonic, rotor noise calculation used was from the 
unpublished semiempirical prediction made by Robert J. Pegg of the NASA 
Langley Research Center. The equation from this prediction, 
fp 
SPL 
= -240 log T + .746 VT + 786 
= 10 log Ab 4- 60 log vT 10 log (COS20 + .I) 
+ Sj -20 log r + f (CL) - 53.29 
- 
cL for E < .48 f(CL) = 10 log .4 - 
f(CL) = .9 + 80 log for .48 
.48 
has as its design variables, thrust (T), tip speed (V,), blade area (Ab) 
and lift coefficient (CL). 
A computer program was written to include all of the design variables and to 
provide an automatic calculation of both the harmonic and broadband noise, 
then combine them into one-third octave frequency bands and print-out the 
resultant dBA, dBC and PNL. 
Nine hundred sixty computer cases were run during the initial phase of 
the program to provide adequate definition of the design variables for 
preparation of the "handbook" charts. 
Figure 4 shows a sample of this output. 
Prediction-Data Correlation 
Measured noise data, shown in Figures 5 and 6 ,  from a nonimpulsive and moder- 
ately impulsive rotor were directly compared to the calculated one-third octave 
SPL using the developed computer program. The agreement between predictions 
and measurement for the inonimpulsive case (fig. 5) are generally quite good, 
the discrepancy in the 500 Hz octave band is probably due to destructive inter- 
ference between the direct and first ground reflected waves which calculates 
to occur at 556 Hz. In the case of the impulsive rotor (fig. 6 )  good agree- 
ment is attained in the first two harmonics and higher frequency broadband 
noise since the harmonic noise prediction method does not account for the 
increase in mid-harmonic loading which typifies impulsive rotor noise. 
554 
Perceived Noise Level S e n s i t i v i t y  t o  Rotor Design 
To provide an  i n d i c a t i o n  of t he  s e n s i t i v i t y  of PNL t o  changes i n  design vari- 
a t ions ,  f i v e  base l ine  r o t o r  designs represent ing  d i f f e r e n t  classes of he l i -  
copters  w e r e  inves t iga ted .  
of t h r u s t ,  d i s c  loading, t i p  speed and number of blades w e r e  var ied  one a t  a 
t i m e  (at cons tan t  l i f t  c o e f f i c i e n t )  and the  r e s u l t a n t  PNdB ca lcu la ted .  
For each base l ine  conf igura t ion  t h e  r o t o r  parameters 
Figure 7 shows an  example of t h e  ca lcu la ted  nonimpulsive hover SPL f o r  one 
p a r t i c u l a r  case (-3-bladed, 89-kN (20 000-lb) t h r u s t  r o t o r ) .  Taking t h i s  con- 
f igu ra t ion  as a base l ine  design and varying each of t h e  parameters one a t  a 
time r e s u l t s  i n  the  PNL s e n s i t i v i t y  cha r t  shown i n  f i g u r e  8. Similar s t u d i e s  
have been done f o r  four  o ther  base l ine  designs which cover a wide range of values 
and the  r e s u l t a n t  summary ( t a b l e  I) i n d i c a t e s  some rough guide l ines  which can 
be used pending release of t he  f i n a l  design cha r t s  which w i l l  r e s u l t  from 
t h i s  study. 
CONCLUDING REMARKS 
The ca l cu la t ion  of t he  nonimpulsive harmonic and broadband hover noise  f o r  a 
wide range of r o t o r  design v a r i a t i o n s  w a s  accomplished. The p red ic t ion  method- 
ology used co r re l a t ed  w e l l  with measured whi r l  tower da ta .  
t h e  pred ic t ions  t o  va r i a t ions  i n  r o t o r  design ( t h r u s t ,  t i p  speed, d i s c  loading, 
and number of blades per  r o t o r )  has shown t i p  speed and t h r u s t  as having t h e  
most e f f e c t  on changing the PNL. 
Application of 
, 
REFERENCES 
1. Sternfe ld ,  Harry, Jr.;  and Doyle, Linda Bukowski: Evaluation of the 
Annoyance Due t o  Helicopeter Rotor Noise. NASA CR-3001, 1978. 
2. Lowson, M.V.; and Ollerhead, J.B.: Studies of Helicopter Rotor Noise. 
USAAVLABS Tech; Rep. 68-60, U.S. Army, Jan. 1969. 
955 
TABLE I.- INTERIM RESULTS SUMMARY O F  SENSITIVITY OF PNL 
TO DESIGN PARAMETER VARIATION 
Parameter Range 
Tip speed 137 to 290 m/sec 
(450 to 950 ft/sec) 
11 121 to 358876 N 
(2 500 to 80000 lb) 
96.1 to  574.6 N/m2 
(2 to 12 lb/ft2) 
2 to 6 
Thrust 
Disk loading 
Number of blades 
per rotor 
Sensitivity * 
2 to 5 PNdB per 30.5 m/sec 
(100 ft/sec) 
2 PNdB per doubling of 
0.5 PNdB per 96.1 N/m2 
thrust 
(2 lb/ft2) 
<0.5 PNdB per blade addition 
*Based on varying parameter under study while holding all others constant. 
ALTITUDE 
GROSS WEIGHT 
TIP SPEED 
AIRSPEED 
1 2 0  m (394 ft) 
(40,000 Ib) 
234 m/sec 
(769 ft/sec) 
11  1 km/hr 
(69 mi/hr) 
i a  140 kg 
SOUND PRESSURE 
2 X 1 OJ N/m* 
LEVEL - dB RE 
OCTAVE BAND CENTER FREQUENCY - HZ 
Figure 1.- Helicopter noise source contribution during 6-degree approach. 
556 
ALTITUDE 120 m (394 ft) 6' 
GROSS WEIGHT 16 140 kg 
(40,000 Ib) 
TIP SPEED 234 m/sec 
(769 ft/sec) 
50 
AIRSPEED 111 km/hr 4c 
(69 mi/hr) 
noys 
PNL - 107 PNdB 
3c 
OCTAVE BAND CENTER FREQUENCY - HZ 
Figure 2.- Subjec t ive  weighting of he l i cop te r  no i se  during 6-degree approach. 
PNdB 
DISK LOAD RANGE 
96.1 - 575N/m' 
(2-1 2 Ib/ft*) 
TIP SPEED (VT) 
RANGE 152-244 m/sec 
(500-800 ft/sec) 
SOLIDITY RANGE 
0.04-0.1 2 
vTtvlIN { 7 
V 
L I I I I 
THRUST 
RANGE 44 484  - 355 876 N (10,000-80,000 Ib) 
Figure 3.- Poss ib l e  design c h a r t  format f o r  2-, 3-, 4-, 5-, and 6-bladed ro to r s .  
557 
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Figure 4.- Rotor noise calculation - computer program sample output. 
558 
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Figure 4 . -  Concluded. 
559 
ROTOR THRUST 66.727 N 
ROTOR TIP SPEED 
ROTOR SOLIDITY 
DISK LOAD 
BLADE RADIUS 
1 / 3  OCTAVE BAND CENTER FREQUENCY - HZ 
Figure 5.- Corre la t ion  of ca l cu la t ions  with wh i r l  tower nonimpulsive r o t o r  
no ise  a t  152-m (500-ft) distance.  
ROTOR THRUST 266,907 N 
(60,000 Ib) 9 0  
ROTOR TIP SPEED 229 m/sec 
(750 ft/sec) 
80 ROTOR SOLIDITY 0 09 
DISK LOAD 430 N/mz 
(9 Ib/ft') 
70 
BLADE RADIUS 14m (46 ft) 
SOUND PRESSURE 
LEVEL - dBRE 6o 
2 X 10 -sN/mz 
50 
40 
1/3OCTAVEBANDCENTERFREQUENCY- Hz 
Figure 6 . -  Corre la t ion  of ca l cu la t ions  wi th  wh i r l  tower impulsive r o t o r  no ise  
a t  152-m (500-ft) distance.  
560 
BASE POINT VALUES 
THRUST 88,969 N 
(20,000 Ib) 
TIP SPEED 
-6- 
-8 
DISK LOADING 
/ 
/' 
/ 
- 
I I I I 1 
SOLIDITY 
LIFT COEFF 
Figure 7.- Calculated - nonimpulsive hover noise 3-bladed rotor at 
152-m (500-ft) distance. 
I I I I I 
1Bs 188 Pg 259 260 
(m) (SO1 (7601 laso) (SWl 
TIP WEED. m / m  (fthd 
I I I 
1 2 3 4 6 
I I 
BLADES PER ROTOR 
Figure 8.- Relative change in PNdB with 
design parameter variation. 
561 


Design of helicopter rotors to noise constraints

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