The Impact of Asynchronous Propeller Speeds on Aeroacoustics and Annoyance

This study examines the aerodynamic, aeroacoustic, and psychoacoustic behaviour of a side-by-side twin-propeller UAV configuration under static and forward-flight conditions, focusing on the effects of asynchronous rotational speeds. Experiments were conducted using two identical five-bladed propellers (D = 9”, P/D = 1), with rotational speed differences ranging from ΔN = 0 to 300 rpm. The results show that rotational speed mismatch significantly alters the acoustic signature by splitting the blade-passing frequency (BPF) into two distinct components. This reduces tonal coherence and decreases overall noise levels by approximately 2 dB under static and high advance ratio conditions, and up to 5 dB at lower advance ratios. Psychoacoustic analysis indicates that perceived noise is governed by spectral characteristics rather than overall sound pressure level alone. Non-synchronized operation reduces tonal prominence, leading to decreases in psychoacoustic annoyance of up to 15% at lower advance ratios. However, asynchronous operation introduces an aerodynamic penalty, with efficiency losses of approximately 5% at ΔN = 50 rpm, increasing with larger mismatches.