Exploration of stacked rotor designs for aerodynamics in hover

Abstract

The development of rotorcraft with multiple propulsors has increased interest in co-rotating coaxial rotors, also known as stacked rotors, which can generate high thrust while maintaining a compact aircraft design. By optimizing the pitch angle and the blade arrangement, stacked rotors can reduce aeroacoustic noise and vibration while improving aerodynamic performance. This study leveraged a high-fidelity numerical solver to analyze the aerodynamic performance and underlying physics of various stacked rotor configurations in hovering conditions. With three rotor design variables — index angle, stacked distance, and pitch angle difference — together with high-resolution simulations, two key physical phenomena were identified: the inflow effect and the blade-vortex interaction effect. Specifically, the blade-vortex interaction at the lower blade was found to play a critical role in improving hover performance by generating upwash, which increased the local power loading of the lower blade. Also, the unsteady simulations revealed that stacked rotors produced significantly lower peak-to-peak thrust values compared to counter-rotating coaxial rotors, indicating potential reductions in aeroacoustic noise level. Finally, a design optimization based on deep neural networks was performed to successfully extract design rules to take full advantage of inflow and blade-vortex interaction effects, which can be used as a guideline for the future design study of stacked rotors.