A reduced-order unsteady lift model for the local sweep morphing of avian-inspired gliding wing

Birds extend their flight envelope and adapt to time-varying aerodynamic demands by actuating the shoulder and wrist joints to morph the local sweep angles of the inner and outer wings. However, the local sweep morphing may cause unfavourable unsteady lift on the lifting surface. We investigate these unsteady lift responses using an avian-inspired wing with local sweep morphing under different morphing strategies. The unsteady lift is computed through numerically solving the incompressible Navier–Stokes equations. The results show that the local sweep morphing wing experiences a substantial maximum lift overshoot when forward sweeping and a notable maximum lift undershoot when backward sweeping. These lift over/undershoot phenomena can be alleviated by three measures: adopting smooth nonlinear morphing kinematics, initiating morphing from lift extrema opposite to the over/undershoot direction and prolonging the morphing duration. For the lift over/undershoot, the component obtained by subtracting the extended pre-morphing lift is attributed to the modification induced by local sweep morphing. To predict such lift over/undershoot, we develop a reduced-order unsteady model for the sweeping-modification lift coefficient, where Prandtl’s lifting-line theory is extended to the regime of variable translational velocity. The proposed model captures the symmetry of the sweeping-modification lift coefficient, dominated by the horizontal morphing velocity. Additionally, the vertical morphing velocity is found to correlate with the asymmetry of the sweeping-modification lift coefficient by modulating the leading-edge vortices. This study is expected to improve the understanding of surging-wing flow physics and support the design of bio-inspired multifunctional aircraft.