Diameter Ratio Effects on Aerodynamics and Aeroacoustics of Coaxial Contrarotating Rotors

This study investigates the effect of diameter ratio on the aerodynamic and aeroacoustic performance of coaxial contrarotating (CCR) rotors in hover. Tests were conducted with diameter ratios (DRs) (1.0–2.0) at fixed upstream rotor diameter, under both constant rotational speed and constant thrust, with varied axial spacing. The results show that increasing the diameter ratio significantly reduces noise emission at the fixed rotational speed, yet with a loss in thrust. At the fixed combined thrust, the CCR rotors exhibit the lowest noise at [Formula: see text] with a noise reduction of up to 2.8 dB, as a result of attenuated interaction noise and downstream rotor self-noise, yet with a loss of aerodynamic efficiency. A noise reduction of about 1 dB in overall sound pressure level was observed at [Formula: see text] compared to the isolated single rotor at the same thrust. The rotor spacing with minimum noise was found to be 0.25 times the upstream rotor diameter for most configurations, though the noise level becomes nearly insensitive to spacing at very high diameter ratios. These findings provide insight into the experimentally observed noise/thrust tradeoff and offer practical guidance on the design of low-noise CCR systems in urban air mobility applications.