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Airline deregulation and the high cost of fuel have caused a renewed interest in propeller-driven aircraft as a replacement for existing turbofan aircraft. Since passengers on existing turbofan aircraft have become accustomed to lower interior noise than exists in current propeller aircraft, there has been a renewed interest in interior noise control by reduction of propeller source noise, by design of lightweight fuselage soundproofing and other noise reduction concepts. This paper discusses the noise control problem from a source noise and aircraft design standpoint. The existing state-of-the-art is reviewed and the promising strategies for reducing noise in propeller aircraft are discussed.

The initial results of a program to investigate the sources of noise in unshrouded propellers under forward flight conditions are reported. Tests were conducted using a three-blade, full-scale instrumented propeller mounted on a twin-engine aircraft. Measurements included 1) farfield noise at fixed ground stations and at two aircraft wing tip locations, 2) blade surface pressures at seven locations on one of the propeller blades, 3) atmospheric turbulence encountered by the aircraft in flight, and 4) aircraft operating conditions. The results confirm that significantly lower levels of propeller noise are produced in forward flight than at static conditions. The most significant reductions occurred at mid-frequencies which dominate Perceived and A-Weighted Noise Levels. Blade surface pressure data showed the presence of disturbances in the propeller inflow under static conditions which were seen to disappear as the aircraft started its takeoff roll.