Multiple Pure Tone Noise from Pitch-Offset-Induced Load Imbalance in High-Speed Propellers

This study employs experimental and numerical simulations to investigate the generation mechanism of multiple pure tone (MPT) noise from propellers with a tip Mach number of approximately 0.82, focusing on the impact of load imbalance caused by blade pitch offset on noise characteristics. The aerodynamic and acoustic performance of a six-bladed swept propeller and its pitch offset configurations are evaluated by introducing parameters previously regarded as uncertain errors. The results show that even minor offsets significantly alter the noise spectrum, inducing discrete subpeaks between blade pass frequencies (BPFs) and reducing the overall sound pressure level by over 2 dB. Formula derivation reveals that the number of subpeaks between BPFs is equal to the blade count minus one, with their amplitude relationship predictable via the weighted Fourier coefficients of blade thrust deviation. Sound source localization based on Farassat formulation 1A and dynamic modal decomposition reveals that MPT noise primarily originates in the 0.7[Formula: see text]–0.95[Formula: see text] region of the blade. This region radially coincides with the load variation region and is modulated by both propagation efficiency and the Doppler effect at high Mach numbers. Consequently, this study provides an integrated experimental, numerical, and analytical framework for controlling the overall sound pressure level and MPT noise via blade pitch offset adjustment.