Aeroservoelastic Suppression of LCO due to Free-Play Using a Combined Analytical and Experimental Approach

Aerodynamic control surfaces with excessive free-play can cause limit cycle oscillations (LCO), a sustained vibration of constant amplitude that is caused by a combination of aeroservoelastic effects and free-play. The LCO can impact handling qualities, ride quality and can cause structural fatigue, ultimately leading to structural failure. Due to the negative impacts of free-play induced LCO, very stringent absolute free-play limits have been established for control surfaces on both military and commercial aircraft. Systems Technology, Inc. (STI) and Boeing propose to develop an innovative, robust, and reliable active control concept that alleviates the adverse effects of control surface free-play, relieving costly requirements associated with manufacturing, inspection, and part replacement. The solution involves a novel linear fractional transformation framework for relevance to models of varying complexity and a robust control approach that exploits the piecewise-linear nature of the free-play nonlinearity. To aid in control design and to provide practical real-world relevance, a combined analytical and experimental approach is proposed by the STI-Boeing team. The proposed solution is minimally intrusive, providing for application to a wide array of existing and future aircraft (including both high speed fighters and transport aircraft), ultimately resulting in significant cost savings and increased pilot safety.