Natural frequencies of rectangular plates in the post-buckling states are analytically and experimentally studied as a fundamental study of the supersonic panel flutter. For panels which are simply supported for bending and constrained for in-plane displacement at four edges, the natural frequencies of the first and second modes are approximately calculated by using two modes approximation and GALERKIN method. The variation of the natural frequencies of a square plate with the amount of the buckling deformation is shown in figures for four cases of the in-plane stress condition of the two directions. For comparison with experiment, the first and second natural frequencies of clamped rectangular panels are obtained by two modes approximation. Furthermore, a higher approximation is studied for the lowest symmetrical mode, and the eflect of initial imperfection of the plate is also studied. Experimental results for a square panel firmly clamped at four edges are in good agreement with the present analytical results.
