Progress Report on Propeller Aircraft Flyover Noise Research

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. The blade surface pressure spectra under static conditions were found to contain many peaks at harmonics of propeller shaft speed which showed a general decrease in amplitude with increasing frequency. Levels of blade pressure spectra are higher at the blade tip than at points closer to the hub. Under forward flight conditions, the levels of blade pressure peaks at harmonics of shaft speed drop rapidly at the higher frequencies until a random noise floor is reached at about the 6th harmonic. It is tentatively concluded that propeller noise generation in flight may be dominated by steady loading at blade passage frequency, but at higher frequencies unsteady loading due to interaction with natural atmospheric turbulence may be the dominant mechanism of noise generation. Under static conditions the total noise signature appears to be the result of interaction of the propeller with persistent turbulent eddies passing through the propeller disc.