Nature of Freeplay-Induced Aeroelastic Oscillations

Time- and frequency-domain analyses are used to revisit a well studied case of an aeroelastic wing with control-surface freeplay nonlinearity. The dynamic response in general is presented as dependent on the stability of two linear systems, with zero and nominal control-surface rotational stiffness, and values of two angles, freeplay and preload. The principal new findings include the following. A physical explanation is given to the bifurcation airspeeds and frequencies of the oscillations for the system with both zero and nonzero aerodynamic preload of the control surface. For the latter and more complex case, it is shown and explained why a certain amount of the preload is not only always destabilizing but may also be the reason for the otherwise unexpected higher-frequency oscillations. A conclusion thus is drawn that the aircraft control-surface freeplay analyses based only on zero and high quenching aerodynamic preloads are nonconservative. All results are compared with the experimental and other published theoretical analyses for the considered model. Finally, a suggestion is given for future experiments with a goal to observe the response changes with the preload and to determine the stabilizing control-surface hinge moment.