 two-dimension panel flutter two-dimension panel flutter theori experi flutter buckl plate discuss increas initi deviat flat static pressur differenti plate rais critic reduc veloc applic galerkin method linear problem flutter unbuckl plate question author paper flutter condit formul form integr equat solv numer method iter method matrix approxim avoid constraint assum mode plate finit bend rigid confirm galerkin method approxim analysi limit form amplitud flutter motion buckl plate present 