Characterizing the noise patterns of overlapping propellers in forward flight

The development of distributed electric propulsion systems aims to advance sustainable aviation; however, increased noise levels, particularly with unconventional rotor placements, pose new challenges. This study experimentally investigates the acoustic performance of overlapping propellers, focusing on the effects of lateral and axial separations under varying inflow conditions and rotational speeds. Experiments were conducted in an aeroacoustic wind tunnel using a twin side-by-side configuration with five-bladed propellers. Noise analysis reveals that tonal noise is the primary contributor to the increased overall sound pressure level, exhibiting significant sensitivity to the lateral distance across all advance ratios and directivity angles. At a lateral distance of approximately 40% of the propeller diameter, tonal noise levels are found to be significantly elevated-by approximately 14-18 dB-compared to the non-overlapping case. Results indicate that lateral separation distance is the dominant factor influencing acoustic behaviour due to blade-vortex and blade-wake interactions, while axial separation distance plays a secondary role.