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National Advisory Committee for Aeronautics, Report - Sound from a Two Blade Propeller at Supersonic Tip Speeds

Sound measurements at static conditions hare been made for
a twosblade 47-inch-diameter propeller in the tip Jfach number
range 0.7’5 to 1.80. For comparison, spectrums hare been
obtained at both subsonic and supersonic tip speeds. In addi-
tion, the measured data are compared with. calculations by the
theory of Gutin which has preciously been found adequate for
predicting the sound at subsonic tip speeds.

At supersonic tip speeds, the sound pressures are lancer than
an extrapolation of the subsonic data would indicate. For a
constant power, the o-cer-all measured sound pressures are es-
sentially independent of tip speed for the supersonic tip-speed
range of the tests. The spectrums have a high harmonic content
with some of the higher harmonics being more intense than the
fundamental frequency. For the supersonic tip-speed range of
the tests, the measured intensities were a maximum in the plane
of rotation. The theory of Gutin is found to be adeguate for
predicting intensities of the lower harmonics for the tip Mach
number range of the tests but ocerestimates the intensities of the
higher ones at supersonic tip speeds.

Curres are presented from which the maximum orer-all noise
lerels in free space may be estimated if the power, tip Mach
number, and distance are known.

Propeller noise has been a problem even at subsonic tip
speeds, hence the proposed use of propellers operating at
supcrsonic tip speeds has caused some concern as to the
severity of the associated noise problem. Very little infor-
mation has been published which would allow the prediction
of noise levels associated with the operation of these propel-
lers. Since airplane design and airplane operation may be
affected by noise considerations, there is much interest in
data of this type.

Much information exists on the subsonic noise problem
and the work of Gutin has allowed its unification. Gutin,
in reference 1. gives a theoretical expression for the sound
produced by a propeller in static operation as a function of
tip speed, number of blades, thrust and torque, and the
dimensions of the propeller. Because of simplifying as-
sumptions, these relations are valid only at distances large
compared with the propeller diameter