© 1997, American Institute of Aeronautics and Astronautics, Inc.Unsteady flowfields over a stationary/flapping airfoil combination in tandem are computed using a Reynolds averaged Navier-Stokes solver. The stationary leading airfoil has an experimental profile which promotes flow separation at its trailing edge. The effect of the flapping airfoil on the leading airfoil and the flow reattachment at the trailing edge are investigated by varying the amplitude and frequency of the flapping motion. A single deforming C-H type grid and overset grids are employed in the computations. Unsteady flowfields are presented in terms of Mach number contours and time histories of unsteady aerodynamic loads. The high frequency flapping motions, where the experimental data show complete flow reattachment at the trailing edge, could not be performed due to the time step limitation of the solver. However, the flow configurations studied produced a significant amount of flow entrainment at the trailing edge.