Unlike the tail of a bird, regarded as a separately controlled aerodynamic surface, the membrane tail of a bat is operated as a dynamic trailing-edge flap. We investigate the effects of a dynamic trailing-edge flap on unsteady lift by numerically solving the Navier-Stokes equations around a bio-inspired flapping wing. The peak of the lift coefficient in the downstroke is considerably affected by the phase difference between the dynamic trailing-edge flap and the elevation. A quasi-steady formula is proposed to model the effects of phase difference on lift. The model is consistent with numerical results and experimental observations.