Prediction of pilot-in-the-loop oscillations due to rate saturation

Rate saturation in modern flight control systems has been shown as a main contributor to triggering of pilot-in-the-loop oscillations (PIOs). Oscillations with significant rate saturation effects are classified as category II PIG. A new method was developed to predict stability problems of rate-saturated closed-loop systems based on the rate-limiting element describing function. Jump phenomena are observed in the closed-loop system describing function after rate-limiting onset, which have been shown as highly correlated with location of the open-loop onset point (OLOP) of the rate limiter in the Nichols chart. The OLOP criterion and its background, physical significance, and determination are presented. Its relation to the negative inverse describing function technique is discussed. Application of the OLOP criterion to closed-loop aircraft-pilot systems in the roll axis was investigated by using aircraft models from in-flight simulations down on the NT-33 and the YF-16 first flight configuration. A high correlation was found between the OLOP criterion and the PIO susceptibility based on numerical investigations in the time domain with simple pilot models. Rate limiters in the forward path and feedback loop of the flight control system were investigated.