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.

NASA is a research leader in aeroservoelasticity, including recent advances in innovative experimental excitation mechanisms for more accurate vibration data, signal processing, nonlinear system identification, and robust flutter boundary prediction. The proposed work naturally follows and complements these topic areas. The resulting LCO suppression control solution will benefit the many NASA programs that involve the design, analysis, and test of air vehicles. This includes flight test programs that use aircraft ranging from high speed fighters to low speed transports, encompassing both manned and unmanned platforms. NASA does not develop aircraft; therefore this solution is attractive due to the fact that it is minimally intrusive by design and can be applied to the existing aircraft systems in NASA's fleet. A control solution for free-play induced LCO will result in a reduction in certification requirements, providing for more concentration on the primary objectives of the flight test programs.

The resulting LCO suppression control solution will benefit both commercial and military aircraft that suffer from free-play induced LCO. This solution is applicable to both manned and unmanned aircraft. The initial target market envisioned for this product is the worldwide aircraft manufacturing industry and the civilian and military flight test facilities. A desirable and unique aspect of the proposed approach is the generalized applicability that is unobtrusive to existing aircraft flight control systems by design. This opens the market up to a vast number of existing aircraft that are plagued by free-play induced LCO. New aircraft programs will also benefit from this solution by providing relaxation to the stringent free-play requirements from the beginning. This solution will result in significant cost savings to the aircraft industry associated with the relaxation of stringent requirements resulting in reduced manufacturing, inspection, and hardware replacement cost.

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