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National Advisory Committee for Aeronautics, Technical Notes - Effects of Compressibility on the Performance of Two Full Scale Helicopter Rotors

An investigation_has been conducted on the Langley helicopter test
tower to determine experimentally the effects of compressibility on the
performance and blade pitching moments of two full-scale helicopter
rotors. Two sets of rotor blades were tested which differed only in
that the blades of one set incorpérated -8° of linear twist, whereas
the blades of the other set were untwisted. The tests covered a range
of tip speeds from_350 to 770 feet per second and a range of pitch angles
from 0° to the limit imposed by extreme vibration.

The results show that the primary effect of compressibility was a
rapid increase in the profile-drag torque coefficient once the critical
combination of tip speed and tip angle of attack was exceeded. The
results also show that the onset of compressibility losses can be pre-
dicted by using two-dimensional section data and that negative blade
twist (washout) is effective in delaying the onset of compressibility
losses and in reducing these losses once they are developed.

The significance of the effect of compressibility on the performance
and blade pitching moments of a helicopter rotor has been emphasized by
design studies of high-speed helicopters, helicopters with high disk
loadings, and convertible aircraft, all of which require higher tip speeds
than are now in general use. Although some information is available on
the decrease in performance of airplane propellers due to compressibility
losses, when the resultant tip speeds approach the velocity of sound,
the information is not altogether applicable in that airfoils suitable
for propellers are not usually employed in helicopter rotor blades.

Consequently, very little experimental data are available on the charac-
teristics of a practical-construction full-scale helicopter rotor operating
in the region in which compressibility effects occur. Compressibility
losses are expected to occur both in hovering and forward flight and may
lead to rotor vibration and loss of control in addition to the expected
decrease in performance. A knowledge of thepcompressibility losses in
hovering would therefore provide a limited basis for prediction of the
compressibility losses to be expected in forward flight.

It is necessary to determine whether the onset of compressibility
losses significantly reduces rotor efficiency before the operating
limitation due to vibration or loss of control is reached. It is also
of interest to check for the hovering condition the onset of blade
stalling losses, as was done for the forward- flight condition reported
in reference 1. In order to differentiate the blade stall losses from
the compressibility effects, the stall losses should be determined at
low tip speeds.