  We present a model appropriate to the initial motion (2-3 chords of travel)
of a flat-plate airfoil accelerating in an inviscid fluid. The separated flow
structures are represented as vortex sheets in the conventional manner and
similarity expansions locally applicable to the leading and trailing edges of
the plate are developed. The topological character of vortex sheets is
maintained rather than resorting to point vortex discretizations. Beyond this,
there are two theoretical novelties to our approach as compared to previous
studies. First, an expansion is applied to the attached outer flow rather than
the vortex sheet circulations and positions. This allows the asymmetric effect
of the sweeping component of the free-stream flow parallel to the plate to be
built-in to the same governing equation as the singular-order flow. Second, we
develop a time-dependent self similarity procedure that allows the modeling of
more complex evolution of the flow structures. This is accomplished through an
implicit time variation of the similarity variables. As a collective result,
the predicted vortex dynamics and forces on the plate compare favorably to
Navier-Stokes simulations. Lastly, the model is utilized to provide some new
intuition about the separated flow at the leading edge.
