Flutter Analysis Using Nonlinear Aerodynamic Forces

The nonlinear effects of transonic aerodynamic forces on the flutter boundary of a typical section airfoil are studied. The flutter speed dependence on amplitude is obtained by utilizing a novel variation of the describing function method which takes into account the first fundamental harmonic of the nonlinear oscillatory motion. By using an aerodynamic describing function, traditional flutter analysis methods may still be used while including the effects of aerodynamic nonlinearities. Results from such a flutter analysis are compared with those of brute force time-marching solutions. The aerodynamic forces are computed by the LTRAN2 aerodynamic code for an NACA 64A006 airfoil at Mx = 0.86.