Unsteady aerodynamics of biplane airfoils with a feather-inspired flexible flap under fluid–structure interaction

With the rapid development of micro air vehicles (MAVs) in low-altitude application scenarios, biplane airfoils are introduced to conceptual designs for novel multi-wing configuration of aircraft. However, the aerodynamics of biplane airfoils is inevitably subject to strong wake interference between the upper and lower wings in the unsteady flow with high angle of attack (AOA) and low Reynolds number. It is urgent to find a new method for flow separation control of biplane airfoils. In this paper, the effects of the feather-inspired flexible flap on unsteady aerodynamics of biplane airfoils at high angle of attack is studied employing lattice Boltzmann–finite element method. The coupling mechanism of flow separation and vortex interaction between biplane airfoils at different spacing is systematically investigated, and the difference of unsteady flow control between rigid flap and feather-inspired flexible flap is compared. The results shown that the feather-inspired flexible flap can effectively optimize the lift distribution of biplane airfoils, reduce the lift discrepancy between the upper and lower wings, mitigate abrupt lift transients, reduce the total drag of the upper wing, and improve flight stability without significantly sacrificing aerodynamic efficiency. Moreover, the stiffness of the flexible flap has a notable impact on aerodynamic response. The fluid–structure interaction effect of the flexible flap in unsteady flow separation plays an extremely important role in improving the autonomy and adjustability of flow control, which improves the flow control strategy of the aerodynamics of biplane airfoils and provides support for the efficient development of MAVs in complex flows.