Propeller Noise Reduction by Means of Unsymmetrical Blade-spacing

Abstract

The noise reduction potential of propellers with circumferentially unsymmetrical blade-spacing is predicted on theoretical grounds and substantiated through both aerodynamic and aeroacoustic full scale wind tunnel experiments. To avoid potential balancing problems such propellers have two (or several) pairs of opposite blades, each such pair constituting a symmetrical two-blade propeller. Spacing angles between these individual blade pairs are optimized towards achieving minimum A-weighted noise radiation in the plane of rotation. The result is then compared with the corresponding noise level from a symmetrical reference propeller with the same total number of geometrically identical blades. The study reveals that the value of the optimum spacing angle depends almost entirely on the operational helical blade-tip Mach number, assuming values of about 40° at a Mach number of 0·5 and decreasing to 15° at a Mach number of 0·8. The noise reduction to be achieved from such unsymmetrical blade-spacing is limited to about 4 dB(A) in the direction of maximum noise radiation since the related acoustic effect is due to interference between the sound pressure signatures of the individual blades. It is found that both the harmonic ("rotational") sound pressure level spectrum and the acoustic directivity pattern is affected. Full scale wind tunnel experiments confirm the results of the theoretical predictions. No degradation in aerodynamic performance was observed for such propeller configurations.