Study on lift enhancement of a flapping rotary wing by a bore-hole design

A novel design of a micro air vehicle with flapping rotary wings was recently proposed. In this paper, the investigation is focused on aerodynamic enhancement of a bore-hole and an elastic cover added onto an intact flapping rotary wing. The aerodynamic force and the pitching motion of the perforated wing are both acquired in experiments. In comparison with an intact wing, a typical perforated wing shows a significant decrement in negative lift and thus a higher mean lift. As a supporting process, the computational fluid dynamics method is then employed to analyze the flow around the wing and the hole, and thereby make clear the underlying physical mechanism. It is found the hole opens due to the cover deflection in a passive manner and produces a secondary starting vortex on the wing lower surface of the cover during upstrokes. Together with the tip vortex around the lateral edges of the cover, the resulting vortex significantly reduces the magnitude of the negative lift, and thus explains the mean-lift enhancement in a flapping cycle. Additionally, the mean-lift enhancement correlates with the geometric parameters of the hole. An increment up to 40% in maximum mean lift is identified for a perforated wing in comparison with an intact wing. These results indicate that a bore-hole design could be a promising approach to enhance the aerodynamic lift of a flapping rotary wing.