Nonlinearities in aircraft mechanisms are inevitable, especially in the controlsystem. It is necessary to investigate the effects of them on the dynamicresponse and control performance of aeroelastic system. In this paper, based onthe state-dependent Riccati equation method, a state feedback suboptimal controllaw is derived for aeroelastic response and flutter suppression of a threedegree-of-freedom typical airfoil section. With the control law designed,nonlinear effects of freeplay in the control surface and time delay between thecontrol input and actuator are investigated by numerical approach. A cubicnonlinearity in pitch degree is adopted to prevent the aeroelastic responsesfrom divergence when the flow velocity exceeds the critical flutter speed. Forthe system with a freeplay, the responses of both open- and closed-loop systemsare determined with Runge-Kutta algorithm in conjunction with Henon's method.This method is used to locate the switching points accurately and efficiently asthe system moves from one subdomain into another. The simulation results showthat the freeplay leads to a forward phase response and a slight increase offlutter speed of the closed-loop system. The effect of freeplay on theaeroelastic response decreases as the flow velocity increases. The time delaybetween the control input and actuator may impair control performance and causehigh-frequency motion and quasi-periodic vibration.
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