Novel airborne technique for aircraft noise measurements above the flight path

Abstract

Wing-shielding effects for a TriStar airliner in flight have been deduced from a comparison of the engine noise measured simultaneously above and below the flight path; the specialized technique developed for these experiments is described and evaluated. Noise radiated upwards was monitored by flying the aircraft beneath a large instrumented fiberglass sphere suspended from a hovering helicopter; the sphere incorporated a flush mounted pressure microphone and a battery operated telemetry system. Flyover noise at ground level was recorded conventionally. Predicted acoustic diffraction by the sphere is shown to result in good directivity and frequency-response characteristics with pressure-doubling at the microphone surface over a useful wave-number range so that reliable free-field noise data could be readily derived from the flyunder; the sphere also shielded the installed microphone from unwanted helicopter background noise. Experimental checks on this behavior at both model and full scale are reported, including the results of some diffraction studies on a half-scale sphere in an anechoic room.