A toroidal upper rotor for performance enhancement and noise reduction of contra-rotating system

Contra-rotating rotors have been widely observed in multicopters to increase the maximum take off weight (MTOW) and compactness. However, the lower rotor encounters vortices shed from the upper rotor, inasmuch there is strong blade vortex interaction (BVI). Consequently, there is BVI induced unsteady loading and noise emission. A recently proposed idea of toroidal rotor has shown to decrease the noise emission in comparison to a conventional rotor due to the decreased BVI. This benefit is further explored by applying the toroidal design as the upper rotor and maintain the conventional lower rotor, which is intended to decrease the impact of upper rotor vortex on the lower rotor. Utilizing the genetic algorithm, a toroidal upper rotor is designed. The results indicate that the aerodynamic efficiency of the toroidal 2-blade rotor is lower than both the conventional 2-blade and 4-blade rotors. While its noise level is lower than that of the conventional 2-blade rotor, it is higher than that of the conventional 4-blade rotor. Additionally, its tip vortex structure is similar with that of the conventional 2-blade rotor.