Aeroacoustic impact of rotor geometry variation in side-by-side configurations for urban air mobility vehicles

This study experimentally and numerically investigates the aerodynamics and far-field acoustics of side-by-side, contra-rotating rotor pairs for electric vertical takeoff and landing distributed electric propulsion, testing diameter ratios λ=Dr1/Dr2=1, 7/8, and 3/4 across lateral spacings 0.1≤s/Dr2≤2 using thrust, acoustic, and wake measurements together with reformulated vortex particle method simulations. The matched pair (λ=1) produces the most symmetric inter-rotor wake; diameter mismatch breaks this symmetry, with λ=3/4 showing the largest velocity deficit and λ=7/8 the smallest. Despite its smaller deficit, among the three diameter ratios tested, λ=7/8 generates the highest overall sound pressure level (OASPL), the most pronounced blade-passing-frequency tones, and the strongest departure from Gaussian pressure statistics, showing that acoustic coupling depends more on wake-impingement coherence than on deficit magnitude. The matched pair instead produces the highest broadband noise, and OASPL rises by 5–10 dB over isolated-rotor levels at close spacing, peaking near s/Dr2≈0.4–0.6. These findings suggest that strategic diameter mismatch can be utilized to modify the spectral character of side-by-side rotor noise, providing valuable insights for the acoustic design of quieter multirotor propulsion systems.