An extension of Hanson's theory of harmonic noise of rotors

The recent advent of EVTOLs has brought rotorcraft noise back into focus within aeroacoustics. Unlike traditional rotorcraft, EVTOLs feature multiple propellers operating simultaneously, complicating the noise they produce and necessitating higher fidelity aeroacoustic models. The present work builds on Hanson's theory for harmonic noise of propellers in the frequency domain, utilizing more sophisticated aerodynamic models. Hanson's theory addresses point, line, and planar acoustic compactness of loading source distributions on the blade.

We developed advanced aerodynamic models to predict the loading distribution for Hanson's harmonic noise model, thereby improving noise prediction accuracy. For point and line load distributions, we used BET to determine the unsteady loads experienced by a propeller at arbitrary pitch inclinations. For planar load distributions, we employed an airfoil panel method to obtain the unsteady chord-wise loads. A discrete Fourier transform is applied to these unsteady loads to derive the loading modes for Hanson's equations. The results are compared to traditional methods using Hanson's standard parabolic functions for loading and thickness distributions, as well as to the time-domain solution of the FW-H equation.

Future work involves developing a harmonic noise model in the frequency domain based on Hanson's model (which only considers propeller pitch) that includes forward flight for propellers inclined at an arbitrary roll, pitch, and yaw.