Flapping insect flight generates high mean lift by the interaction
of the wings with the shed vorticity. Typical Reynolds numbers
of insects are around 5000. The high lift of an insect is not
usually explained by conventional quasi-steady aerodynamics.
In this study, we compute unsteady viscous flows, governed by
the Navier-Stokes equation, about a flapping wing, which undergoes
various translational and rotational motions. We show an optimal
range of flapping frequency, which results from the unsteady
effect and the dynamics of both the leading and trailing edge
vortices. We further offer a simple physical picture of the
frequency selection mechanism.