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National Advisory Committee for Aeronautics, Technical Notes - Noise Survey Under Static Conditions of a Turbine-Driven Full-Scale Modified Supersonic Propeller with an Advance Ratio of 3.2

Overall sound-pressure levels and frequency spectra have been
obtained under static conditions from a modified supersonic propeller
designed to operate efficiently at a high forward speed without the high
noise levels associated with the supersonic propeller. The three—blade,
lO-foot—diameter, l,700—rpm.propeller is powered by a turbine engine and
is designed to operate at a Mach number of 0.95 at h0,000 feet.

The results consist of overall sound-pressure levels and frequency
spectra obtained from analyses made of recordings taken during ground
runups of the modified supersonic propeller. These results are compared
with similar results obtained with a conventional subsonic propeller
reported in NAGA Technical Note 5&22 and with a supersonic propeller
reported in NACA Technical Note #059.

The noise output of the modified supersonic propeller displays
approximately the same overall sound-pressure level and frequency-spectrum
characteristics, under static conditions, as the current subsonic
transport propeller reported in NACA Technical Note 5h22. The maximum
overall sound-pressure level produced was 120 decibels at a distance of
100 feet. This overall noise output represents a lowering of the maxi-
mum overall sound—pressure levels by approximateLy 10 decibels at com,
parable engine horsepowers as compared with the output of the supersonic
propeller reported in NACA Technical the #059. In general, it may be
stated that a propeller may be designed to possess good aerodynamic per-
formance at high forward speeds and still provide, under static condi-
tions, an overall noise output not greater than that of propellers cur-
rently being used on transport airplanes, and with a similar frequency
spectrum.

Airplane propellers are known to possess good efficiencies at high
forward Mach numbers. Optimum efficiency is obtained by operating thin
blade sections at supersonic resultant speed. The supersonic speed is
necessary in order to maintain an optimum advance angle (approximately #50)
of the propeller that will result in maximum profile efficiency for the
chosen thickness—ratio distribution. A propeller design of this type
is referred to as a supersonic propeller.