Nonlinear Analysis of Aeroservoelastic Models with Free Play Using Describing Functions.

Nonlinearities in aeroservoelastic systems are a source of limit-cycle oscillations. Aerodynamic control surfaces and actuators may have excessive free play in connecting joints and hinges, which has the potential to cause limit-cycle oscillations associated with aeroservoelastic vibrations, impacting handling and ride quality and leading to structural fatigue. Because of these negative impacts, free-play limits have been established by the Joint Services Guidance Military Specification. These military specifications have also been adopted by the Federal Aviation Administration for commercial aircraft. This stringent requirement can be excessively conservative and is very difficult and costly to meet. Analytical tools using describing functions have been developed to establish free-play limits on aeroservoelastic aircraft systems without adverse consequences. These tools are robust and easy to implement, justifying relaxation or validation of the conservative requirements. It is shown that oscillation frequency and amplitude can be accurately estimated using describing functions with an analytical model of an aeroservoelastic system with free play. A novel analysis technique is also demonstrated, whereby free-play occurrence and limit-cycle behavior can be visualized using time-varying frequency content. Validation of the method is illustrated with an aeroservoelastic stabilator control surface model using nonlinear simulation, with free-play limits found in experimental conditions. [ABSTRACT FROM AUTHOR]