Performance enhancement of gust load alleviation systems for flexible aircraft using H_infinity optimal control with preview

Abstract

Most of the gust load alleviation systems (GLAS) of currently-operational aircraft are of feedback-only control architecture based on inertial measurements. In few other aircraft, aerodynamic measurements from air data sensors are additionally included, or presently considered for inclusion, as they usually result in improving the performance of the GLAS. In both sensor types, the control system has very little time to react; and therefore, the performance of the GLAS would be further enhanced if the turbulence or gust could be measured at some distance ahead of the aircraft. Doppler LIDAR (LIght Detection And Ranging) sensors could enable such preview of the turbulence or gust, at a short range (typically between 30 and 200 meters) ahead of the aircraft. In this paper, the availability of a vertical wind profile ahead of the aircraft is assumed, and the paper focuses on the design of a load alleviation controller that exploits this information. The proposed methodology of designing this controller is based on the application of the H_infinity optimal control techniques to a discrete-time preview control problem. Minimizing the H_infinity norm of the transfer function from wind input to loads output, directly leads to the design of an effective load alleviation function. The preview-control formulation enables the design algorithm to synthesize a combined preview-capable feedforward and feedback load alleviation function. For a practical reason, the developed methodology is applied in the course of this paper to a flexible sailplane model (DLR's Discus-2c), although it is intended to be applied to larger airplanes (e.g. transport airplanes and business jets).