Effects of bore-hole design on the aerodynamics of a flapping rotary wing in forward flight

Abstract

Inspired by the folding of wing area in bird flights, a bore-hole design has been introduced to hovering flapping rotary wings (FRWs) to reduce negative lift generation during upstrokes. However, the contribution of this novel concept to FRWs in forward flight is still unclear. Using numerical simulations, the effects of bore-hole design on the force generation and flow structures of FRWs at various advance ratios (J) and tilt angles of disk plane (β) are explained here. Our results show that the FRWs with bore-hole design can retain their lift enhancement within certain limits of J and β. Moreover, the lift enhancement is remarkable in upwind flapping cycles while becomes negligible in tailwind ones. Like hovering scenarios, the lift-enhancing mechanism in forward flight is characterized by the release of pressure difference on the hinge and the formation of a secondary leading-edge vortex (LEV), despite the distinct flow structures in upwind and tailwind conditions. It is further found that a positive incident angle is preferred for the formation of a strong shear layer above the hinge, therefore promoting the release of pressure difference. Besides the lift enhancement, the bore-hole design can also result in a decrease in revolving torque whereas the wind-frame drag is barely changed. According to our findings, following research considering the fluid-structure interaction of the hinge is suggested and the material and structure design of the hinge is crucial for future FRW MAVs.