Effect of non-uniform rotation on acoustics and aerodynamics of propellers

This numerical study investigates the influence of non-uniform rotation on the aerodynamics and acoustics of multi-blade propellers.Non-uniform rotational motion is inherent to piston engine driven propellers. the effect of rotational speed non-uniformity is the generation of excess harmonic noise due to unsteady aerodynamic blade-loading. In case of mismatch between periodicity of non-uniformity and the basic blade passing frequency, additional harmonics are generated due to the complex blade kinematics. For a periodicity coincidence the effects are masked due to overlapping of the frequencies. The level of such extra harmonics may be high enough to dominate the overall A-weighted noise level. Propeller noise radiation for non-uniform rotation is no longer omnidirectional along azimuth and exhibits also a different characteristic in the polar direction.