Some Measured and Calculated Effects of Forward Velocity on Propeller Noise

Abstract

The results of a program investigating the sources of noise in unshrouded propellers under forward flight conditions and a comparison with theory are reported. Tests were conducted using an instrumented three-bladed propeller installed on a turbine-powered, twin-engine, general aviation airplane. Measurements included far-field noise on the ground and on the aircraft wing tip, propeller blade surface pressures, atmospheric turbulence, and aircraft operating conditions. The primary result of the full-scale flight tests was to confirm that foward-flight propeller noise levels are lower than those experienced under static conditions and that the most significant reductions occur at the mid-frequencies which dominate perceived and A-weighted noise levels. Analytical techniques have been used to predict the observed experimental trends and to provide further insight into the noise generating mechanisms. Correlation with experimental data is shown to be good at low frequencies under static conditions and at all frequencies in forward flight. It is tentatively concluded that propeller noise generation in flight may result from steady loads (including blade thickness effects). Under static conditions, the principal noise source appears to be the intersection of the propeller with persistent turbulent eddies passing through the propeller disk.