DESCRIBING FUNCTIONS FOR HIGHER ORDER ACTUATOR MODELS WITH RATE LIMITING

Although powered actuation of aircraft control surfaces has been commonplace for decades, detailed models of the actuators themselves are often not readily available. For most applications including the analysis of flight control and pilot-vehicle systems, simplified lower order models are used with great success. When actuator rate limiting became an important focus of recent pilot-induced oscillation research, generalized describing functions representing the actuator dynamics in the presence of rate limiting were developed from a simplified first order model. However, work by NASA and others has shown that lower order models are not appropriate for aeroservoelastic analysis applications, such as flutter analysis, involving higher frequency dynamics. Furthermore, the generalized describing functions do not adequately capture the nonlinear effects of rate limiting at the higher frequencies that may be encountered with an active flutter suppression system. Using available higher order models for aileron and stabilator actuators from a modern, high performance aircraft, an attempt is made to develop generalized describing functions with rate limiting. The results indicate good success for these examples, especially in the more highly saturated region. Further work is needed to better understand the impact of forward path gain variations and additional higher order models are needed to better validate the normalized results shown.