Baker, John E

Paulnock, Russell S

English

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E F I L E  
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
TECHNICAL NOTE 1966 
EXPERIMENTAL INVESTIGATION O F  FLUTTER O F  A PROPELLER 
WITH CLARK P SECTION OPERATING A T  ZERO FORWARD VELOCITY 
A T  POSITIVE AND NEGATIVE BLADE -ANGLE SETTINGS 
By John E. Baker and Russell S. Faulnock 
Langley Aeronautical Laboratory 
Langley A i r  Force Base, Va. 
Washington 
December 1 94 9 
FILE COPY 
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NATIONAL ADVISO3Y COMMITTEE FOR AERONAUTICS 
TECHNICAL NOTE 1966 
EXPERIMENTAL INVESTIGATION OF FLUTTER OF A PROPELLER 
WITH CLARK Y SECTION OPERATING AT ZERO FORWARD VELOCITY 
AT POSITIVE .4ND NEGATIVE BLADE-ANGLE SETTINGS 
By John E.  Baker and Russell S. Paulnock 
SUMMARY 
A n  investigation was made of the f l u t t e r  of a small one-blade 
wooden propeller with a Clark Y section. 
frequency were obtained with the propeller a t  zero forward velocity a t  
blade-angle set t ings,  measured a t  0.8 radius, from about 2 5 O  t o  - 2 5 O  
and a t  pressures from 0.321 t o  1.0 atmosphere. 
The f l u t t e r  speed and f l u t t e r  
The highest f l u t t e r  speeds were found i n  the v ic in i ty  of the angle 
of zero aerodynamic moment and the f l u t t e r  speed increased considerably 
i n  t h i s  region with decreasing pressure. Over the remainder of the pi tch 
range the f l u t t e r  speeds were much lower and varied l i t t l e  with pressure. 
The maximum experimental f l u t t e r  speed w a s  63 percent of the approximate 
theoret ical  c lass ica l  f l u t t e r  speed, which f a c t  indicates that even under 
ideal  conditions an appreciable factor of safety must be observed by 
deeigners. The f l u t t e r  frequency was found t o  approach the natural  
tors ional  frequency of the propeller a t  both large posit ive and 
negative angles of attack. 
INTRODUCTION 
Propellers and wind-tunnel fans have been known t o  f a i l .  I n  many 
cases these fa i lures  were believed t o  have been caused by s t a l l  f l u t t e r  
of the blades while operating a t  high posit ive and negative angles of 
attack. A n  investigation was therefore made t o  determine the f l u t t e r  
character is t ics  of a propeller w i t h  a Clark Y section a t  both posit ive 
and negative angles of attack. 
from 0.321 t o  1.0 atmosphere, and over a blade-angle range, measured 
a t  0.8 radius, from 2 5 O  t o  - 2 5 O .  
The t e s t s  were made i n  air a t  pressures 
2 NACA TN 1966 , 
0 
SYMBOLS * 
'0.8R 
C 
b 
t 
R 
r 
K 
U 
C l  
X 
a 
rU 
% 
M 
v 
V f  
The 
blade-angle se t t ing  a t  0.8 radius, degree8 
chord of propeller section, feet 
semichord, fee t  
thickness of propeller section, f e e t  
radius to  t i p  of blade, f e e t  
radius t o  propeller section, f e e t  
r a t i o  of mass of a cylinder of air  of diameter equal 
t o  a i r f o i l  chord t o  mass of a i r f o i l  
angle of attack, degrees 
section l i f t  coefficient 
distance of center of gravity measured from leading 
edge, f ract ions of chord 
coordinate of tors ional  s t i f fnes s  axis i n  terms of semichord 
as measured from midchord posit ion 
nondimensional radius of gyration of a i r f o i l  section i n  
terms of semichord referred t o  a 
torsional frequency, radians per second 
Mach number 
velocity a t  0.8 radius, f e e t  per second 
f l u t t e r  speed, f e e t  per second 
APPARATUS AND TEST METHODS 
Langley supersonic sphere used fo r  these t e a t s  consists of a 
s t e e l  tank enclosing a 500-horsepower e l ec t r i c  motor which ro ta tes  the 
model assemblies. 
u r i t i l  the pressures fo r  the desired tes t ing  conditions a re  reached. 
Provisions w e  made t o  pump out the a i r  i n  the sphere 
NACA TIV 1966 3 
F lu t te r  character is t ics  were obtained on a small one-blade 
wooden propeller ( f ig .  1) of laminated S i t k a  spruce construction and 
with a Clark Y a i r f o i l  section. 
bution a re  shown i n  figure 2. 
f l u t t e r ,  and these gages together with the tachometer were wired t o  a 
recording oscillograph which recorded the f l u t t e r  speed and f l u t t e r  
frequency. 
of 0.321, 0.470, 0.686, and 1.0 atmosphere, and a t  
F lu t te r  was not observed, however, during a l l  t e s t s .  
f l u t t e r  speed was higher than the safe operating speed range of the 
propeller, and i n  others the amplitude of the f l u t t e r  was very small. 
The model dimensions and p i tch  d i s t r i -  
Strain gages mounted on the model indicated 
F lu t te r  runs were made a t  zero forward veloci ty  a t  pressure3 
In  some cases, the 
eo,8R fron 250 t o  -25'. 
During each f l u t t e r  run, the rotat ional  speed was increased grad- 
No attempt was made u l l y  u n t i l  large-amplitude f l u t t e r  was observed. 
t o  increase the rotat ional  speed above the point a t  which f l u t t e r  was 
observed because it was quite violent i n  most cases. 
RESULTS AND DISCUSSION 
The theoret ical  variation of l i f t  coefficient cz with blade angle 
80. &, with o n l y  the inflow angle taken in to  account ( re fer -  Sett ing 
ence l), i s  shown i n  figure 3. 
i n  f igures  4 t o  7 as functions of the blade-angle se t t ing  
angle 80,8R i s  the pi tch set t ing when the twisting of the blade, such 
a s  observed i n  the t e s t s  of reference 2, i s  neglected. 
The experimental r e su l t s  a r e  presented 
The eo&. 
The experimental f l u t t e r  coefficients Vf/% a r e  shown i n  figure 4 
as a function of eo,&. 
aerodynamic moment the f l u t t e r  speed shows a marked increase, which 
occurs a s  cz approaches about 0.50. The same tendency was shown by the 
d a t a  of reference 2. The type of f l u t t e r  encountered under these 
conditions i s  believed t o  be very similar t o  the c lass ica l  f l u t t e r  of 
wings. A t  large posit ive and negative blade-angle se t t ings  the f l u t t e r  
speeds a r e  much lower than  the classical  f l u t t e r  speeds, and l i t t l e  
var ia t ion of f l u t t e r  speed with pressure change occurs a t  these angles. 
This f l u t t e r  i s  associated with s t a l l  and i s  commonly cal led stall 
f l u t t e r .  Its character i s  different from the c lass ica l  case i n  that the 
f l u t t e r  i s  almost en t i re ly  torsional.  Conservatively, it appears that 
a propeller of the type used i n  t h i s  investigation w i t h  l e s s  
than 1.0 should not encounter s t a l l  f l u t t e r  i n  a i r  under any of the 
conditions of pi tch se t t ing  and of pressure included i n  these t e s t s .  
These data indicate that a t  the angle of zero 
V/bk  
The r a t i o s  of t i p  speeds a t  f l u t t e r  t o  the theoret ical  c lass ica l  
f l u t t e r  speeds, a s  calculated by the approximate two-dimensional f l u t t e r  
formula with no account taken of centrifugal forces (see table  I ) ,  for  the 
d i f fe ren t  pressures used i n  these t e s t s  a re  presented i n  figure 5 .  This  
method of calculating the theoretical  f l u t t e r  speed was used fo r  com- 
p r i s o n s  because no adequate propeller f l u t t e r  theory i s  available.  Since 
4 NACA TN 1966 
the m a x i m  experimental f l u t t e r  speed was 63 percent of the theoret ical  
f i u t t e r  speed, an appreciable factor of safety must be employed by the 
desi-er even under ides1 conditions. The mlnimi-~m flutter-speed r a t i o  
a t  posit ive angles of a t tack was 0.28, whereas a t  negative angles the 
min imum flutter-speed r a t i o  was 0.18. The flutter-speed r a t i o  decreases 
as  
of attack. 
1/tt i s  increased, par t icular ly  a t  large posit ive and negative angles 
Although no significance i s  attached a t  t h i s  time t o  the t i p  Mach 
numbers a t  f l u t t e r ,  they are  shown i n  figure 6. 
The flutter-frequency values i n  figure 7 tend t o  show an inverse 
The f l u t t e r  frequency a t  both large posit ive 
re la t ion  t o  the f l u t t e r  speed and have the lowest values near the angle 
of zero aerodynamic moment. 
and negative values of e0,8R approaches the natural  tors ional  
frequency of the propeller. 
CONCLUDING REMARKS 
The experimental f l u t t e r  speed i n  these t e s t s  was always below the 
theoret ical  c lass ica l  f l u t t e r  speed. Even under ideal  conditions, the 
designer must apply aa appreciable factor  of safety to  insure against 
f l u t t e r  i f  calculations a re  made by using the approximate f l u t t e r  
formula. The data indicate that a propeller of the type tes ted  i n  t h i s  
investigation having the nondimensional f lutter-speed coeff ic ient  l e s s  
than one, and operating under the same conditions of pi tch se t t ing  and 
of pressure included i n  these t e s t s ,  should not encounter s t a l l  f l u t t e r .  
The r a t i o  of experimental t o  theoret ical  c lass ica l  f l u t t e r  speeds 
decreases as the a i r  density within the sphere i s  decreased, par t icu lar ly  
a t  large positive and negative angles of attack. The f l u t t e r  speeds a t  
negative angles of a t tack  a r e  lower than those for  the corresponding 
posit ive angles of attack. 
The f l u t t e r  frequency approachec the natural  tors ional  frequency 
of the propeller a t  both large posit ive and negative angles of attack. 
Langley Aeronautical Laboratory 
National Advisory Committee for  Aeronautics 
Langley A i r  Force Base, Va., August 30, 1949 
NACA TN 1966 5 
REFERENCES 
1. Glauert, H . :  The Elements of Aerofoil and Airscrew Theory. 
Cambridge Univ. Press, reprint  of 1937, pp. 208-221. 
2. Theodorsen, Theodore, and Regier, Arthur A.: Effect  of the L i f t  
Coefficient on Propeller Flutter.  NACA ACR L5F30, 1945. 
6 NACA TN 1966 
TABLE I 
CAICULATION OF THEOFWITCAL CLASSICAL FLUTTER SPEEDS AT 0.8 RADIUS 
F y  method of reference 4 
Pressure 
(atmospheres) 
1.000 
.686 
.470 
.321 
K 
-A 
Flutter speed 
a t  0.8 radius 
( f t / s ec )  
Sample calculation f o r  atmospheric conditione: 
x = 0.44 
ru2 = 0.24 
a 
838 
1015 
1227 
1490 
1 
" = G  
= 0.182 foot  
R = 2.58 feet  
uu = (390)(24 
C b = - = 0.091 f o o t  
2 
1 x - -  
4 
838 fee t  per second 
NACA TN 1966 7 
Leads t o  
b 
Motor 
s1 
a8 
Counter weight 
Figure 1.- Schematic diagram of propeller assembly. 
8 
3 
.12 
.11 
.lo 
C 
'i 
-0 9 
.o c5 
-0 7 
.06 
MACA 'I" 1966 
ii 
Figure 2.- Propeller blade parameters. 
-7  
t6 
L5 
L4 
t 
C 
13 
12 
11 
,lo 
.o 
Y 
C 
NACA !I" 1966 9 
eo. 8R 
Figure 3.- Theoretical var ia t ion of cz with bladeengle  
se t t i ng  a t  0.8 radius a t  zero forward velocity. 
10 NACA TN 1966 
'0.8R 
Figure 4.- Variation of reduced frequency Vf/bu, w i t h  80,8p 
b 
NACA TN 1966 11 
*7 
.6 
*5 
04 
* 3  
.2 
.1 
atm K l  I 
0 1 000 
k r - l  0- -0 .686  
I I 
0 10 20 30 
0 
'0.8R 
Figure 5.- Ratio of tip speed at flutter to the theoretical classical 
flutter speed as affected by the variation of 
NACA TN 1966 
* 
'0.8R 
Figure 6.- The variation of tip Mach number at flutter with e o . 8 ~  
b 
NACA TN 1966 
00.8R 
Figure 7.- Flutter frequency as a function of 00.8R. 
NACA-Langley - 12-21-49 - 950 
0 


Experimental investigation of flutter of a propeller with Clark Y section operating at zero forward velocity at positive and negative blade- angle settings

Experimental investigation of flutter of a propeller with Clark Y section operating at zero forward velocity at positive and negative blade- angle settings

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