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National Advisory Committee for Aeronautics, Report - Sound Level Measurements of a Light Airplane Modified to Reduce Noise Reaching the Ground

An army liaison-type airplane, representative of personal
airplanes in the 150 to 200 horsepower class, has been modified
to reduce propeller and engine noise according to known
principles of airplane-noise red-action. Noise-lerel measure—
ments demonstrate that, with reference to an observer on the
ground, a noisy airplane of this class can be made quiet—
perhaps more quiet than necessary. In order to amid extreme
and unnecessary modpications, acceptable noise lerels must be
determined.

An important factor in the problem of increasing the
utility of the personal airplane is the provision of more con-
venient access to airports. For this reason it is desirable
that airports be close to centers of population. Strong
objections to the noise of airplanes are, however, seriously
hindering the proper development and location of airports.
A solution to the problem of airplane-noise reduction is
therefore necessary to the healthy growth of personal and
commercial aviation.

The National Advisory Committee for Aeronautics first
began to investigate airplane noise in about 1930. Emphasis
was placed almost entirely on the study of propeller noise.
Since that time a theory for predicting propeller noise has
been developed and a number of papers which aid in the
design of quiet propellers for personal airplanes have been
issued. These, and other papers relating to the noise
problem, are listed as references 1 to 10. Increased
emphasis has recently been placed on this work because
of the expanding personal—airplane market.

In addition to the theoretical and ground test work, a
typical light airplane has been modified for flight tests to
determine the applicability of the published data. This
modified, or quiet, airplane was first flown and demonstrated
at the Sixteenth Annual Inspection at the Langley Labora-
tory in May 1947. Since that time, this airplane has been
tested and the test results compared with those for the
unmodified airplane. The results of these tests, showing the
sound-pressure levels of both airplanes as measured from the
ground, are presented in this report.