Flight control system design considering rate saturation

Abstract

In this paper the design process of modern flight control systems is discussed with respect to pilot-in-the-loop oscillations (PIO) due to rate saturation. At first the PIO phenomenon is characterised briefly. Well-known design criteria with respect to flight control system stability and handling qualities based on linear aircraft models are presented. A new design criterion considering the nonlinear effects of rate limiting is described. The criterion is based on the ‘open loop onset point’ (OLOP) of the rate limiter in a Nichols chart. The background for the development, and the verification of the OLOP-criterion are presented briefly. In order to demonstrate the applicability of the OLOP-criterion in the design process a simple example is examined. The flight control system parameters of a basically unstable aircraft are optimised with respect to the linear design criteria and the OLOP-criterion. Two alternative designs were discussed, a ‘nominal design’ and a ‘low gain design’ with reduced feedback loop gain. The low gain design is favourable with respect to rate limiting in the feedback loop of the flight control system, which is proved by nonlinear simulations in the time domain. Finally, the utilisation and the design of a rate limitation in the forward path of the flight control system are discussed.