A STUDY OF THE ORIGIN AND MEANS OF REDUCING HELICOPTER NOISE
BELL HELICOPTER TEXTRON INC FORT WORTH TX
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Results are presented of an experimental and analytical study of the origin and possible means of reducing helicopter noise. Acoustical and performance data are presented for a single rotor turbine powered helicopter HU-1A with several main rotor configurations. Also, simultaneously recorded rotor blade pressure and acoustical test data are given. Noise criteria are reviewed and established on the basis of over-all sound pressure and loudness level. The latter pertains to the auditory sensation as perceived by an observer and is preferable as a measure of the relative importance of the various noise sources. Based on the loudness level criterion, the most prominent noise components of the test helicopter are identified. For the far field case these are, in their order of prominence main rotor blade slap when it occurs, tail rotor rotational noise, main rotor vortex and rotational noise, and drive system and power plant noise. Blade slap is characterized by high intensity sound pressures of all frequencies and occurs at the blade passage frequency. This noise is shown to be dependent upon flight condition and configuration. It is noted that single rotor helicopters are less susceptible to blade slap than tandem machines.