Unsteady aerodynamics of a micro flapping rotary wing in forward flight

Flapping rotary wing (FRW) is a promising wing layout applicable to micro air vehicles design due to its capability in high lift production. Previous aerodynamic investigations of FRW have mainly focused on hovering flight, while forward flight has been relatively ignored. Therefore, this numerical study aims to address the unsteady aerodynamics of FRW in forward flight based on a FRW configuration with a forward-tilted rotational plane. Results show that compared with hovering flight, forward flying FRW shows a rotational-cycle-averaged thrust and lift reduction and an enhanced rotating moment, all of which can be mainly ascribed to the changes in the aerodynamics on the retreating side. For FRW rotating at a specified rotational speed, the larger advance ratio and severe forward tilt of its rotational plane are both disadvantageous to thrust production and can lead to significant lift reduction. However, they are beneficial for rotational moment enhancement and conducive to FRW rotating at higher speeds, thereby further compensating for the losses in thrust and lift. By further taking a real FRW at near-zero rotating moment into consideration, high thrust and lift can be achieved at the same time in cases of high advance ratio and large rotational plane tilt angle due to fast passive rotating, which differs from flapping wing and rotary wing. These findings can expand our understanding of the unsteady aerodynamics of flapping wing with complex kinematics in forward flight and provide guidance to the design of an FRW micro air vehicle that is capable of forward flight.