Identification of nonlinearities responsible for limit cycle oscillations of fighter aircraft

Higher-order spectral analysis is implemented to identify quadratic and cubic nonlinear aeroelastic phenomena associated with Limit Cycle Oscillations (LCO) encountered in a specific flight test maneuver that consists of a straight and level flight followed by a windup turn at a Mach number near 0.95 and an altitude of 10000 ft. The results show that non-linearities leading to Limit Cycle Oscillations are mostly present at the forward locations on the wingtip and underwing launchers. In the vertical direction, the nonlinearity is cubic and leads to the generation of a third harmonic component. In the lateral direction, the nonlinearity is quadratic and leads to the generation of the second and higher-order harmonics. Cross coupling between the major frequency in the vertical acceleration component and its second harmonic in the lateral acceleration is also detected. Physical explanations of identified nonlinearities and relations to vibration modes of the different components are presented.