The Effect of Compressibility on Eight Full-Scale Propellers Operating in the Take-Off and Climbing Range Page: 1 of 29
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REPORT No. 639
THE EFFECT OF COMPRESSIBILITY ON EIGHT FULL-SCALE PROPELLERS
OPERATING IN THE TAKE-OFF AND CLIMBING RANGE
Br DAVID BIERMANN and EDwnr P. HART&ANSUIMMIARY
Tests were made of eight full-scale propellers of different
shape at various tip speeds up to about 1,000 feet per
second. The range of blade-angle settings investigated was
from 100 to 800 at the 0.75 radius.
The results indicate that a loss in propulsive efcienc?
occurred at tip speeds from 0.6 to 0.7 the velocity of sound
for the take-off and climbing conditions. As the tip speed
increased beyond these critical values, the loss rapidly
increased and amounted, in some instances, to more than
20 percent of the thrust power for tip-speed values of 0.8
the speed of sound. In general, as the blade-angle setting
was increased, the loss started to occur at lower tip speeds.
The maximum loss for a given tip speed occurred at a
blade-angle setting of about 200 for the take-off and 50 for
the climbing condition.
Although the loss at the take-off condition due to com-
pressibility was greater for fthe R. A. F. 6 section than for
the Clark Y, greater for blades of standard width than for
extremely wide ones, and greater for a thick propeller than
for a thin one, the actual efficiencies at high tip speeds were
found to be about the same because, in each case, the pro-
peller that had the greatest losses from increasing the tip
speed had the highest efciency at low tip speeds.
The compressibility loss at the take-off for controllable
propellers was considerably reduced because of decreased
blade-angle operation necessitated by increased power
coeficients, but the reverse was true for fixed-pitch pro-
pellers inasmuch as the higher power coefficients resulted
in reduced engine speeds.
A simplified method for correcting propellers for the
effect of compressibility is given in an appendix.
INTRODUCTION
The first effects of the compressibility of air to influ-
ence the flight of airplanes are felt by the tips of pro-
peller blades, which usually operate at speeds approach-
ing that of sound. The results of experience and re-
search agree in showing that at sonic tip speeds the
effects of compressibility are very unfavorable. The
flying speeds of airplanes have only recently reached
values where the effects of compressibility on parts of
the airplane other than the propeller are of such magni-
tude as to warrant more than passing attention. The
serious effects of high tip speeds on the performance of
propellers have, however, been of great practical inter-est for many years and considerable research has been
directed toward a better understanding of the phenom-
ena of compressibility as affecting propeller operation.
The principal methods of attacking the problem may be
classified as: (a) airfoil tests, (b) model-propeller tests,
and (c) full-scale-propeller tests.
Airfoil tests are particularly valuable in the study of
compressibility because many of the variables present
in propeller tests do not enter into airfoil tests and the
important compressibility effects are therefore more
easily isolated and revealed. Without them the com-
pressibility phenomena detected in propeller tests would
be difficult to understand or to explain.
An examination of references 1, 2, and 3 reveals a
marked change in airfoil characteristics with increasing
air speed. There appears to be a general tendency for
the slope of the lift curves and of the profile drag in the
usual propeller operating range to increase up to some
critical value of V/VC (ratio of air speed to the speed of
sound) corresponding to that at which the compressi-
bility burble occurs and at which the lift drops sharply
and the drag increases rapidly. The value of the speed
at which the compressibility burble occurs is dependent
on the angle of attack and the thickness of the airfoil;
increasing either of these quantities causes the com-
pressibility burble to occur at lower speeds, sometimes
as low as 0.4 or 0.5 the speed of sound. The com-
pressibility burble is attributed to the formation of a
shock wave caused when the flow over the surface
exceeds the local velocity of sound. (See reference 4.)
A large part of the kinetic energy in the flow is converted
into heat when the particles of air pass through the
shock region, which results in an increased drag of the
airfoil. Also, the reduction in velocity and the con-
sequent increase in pressure behind the shock wave
result in reduced lift.
The influence of compressibility on the character-
istics of model propellers has been observed in many
British tests (reference 5). The results of propeller
tests agreed qualitatively with the results of airfoil
tests in that the power and the thrust increased with tip
speed up to a critical value beyond which the thrust
and the efficiency dropped.
There is some reason to believe that the propeller
characteristics should depend on Reynolds Number as
well as on tip speed; however, tests of propellers of
517
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Biermann, David & Hartman, Edwin P. The Effect of Compressibility on Eight Full-Scale Propellers Operating in the Take-Off and Climbing Range, report, May 18, 1938; (https://digital.library.unt.edu/ark:/67531/metadc66297/m1/1/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.