The estimation of aircraft control system stability boundaries by the describing function method

Abstract

In this paper, the conditions for periodic modes formation in the closed-loop pilot-vehicle system in the compensatory control mode are established. The case of high gain pilot task is considered under the conditions of a sudden disturbance, as well as under the influence of the system nonlinearities, such as actuator position and rate limit. Sinusoidal input describing function method and parametric resonance equation were used to determine the onset conditions for self-oscillations and forced oscillations. The case when this methods lead to erroneous results is established. The numerical limits of permissible pilot gain, time delay and reference signal at which unstable periodic modes do not arise are calculated.