Review of Multivariable Control Applied to the VAAC Harrier and Remaining Open Questions

Abstract

In the early nineties, a multivariable flight control law, designated FCL005, was developed for the Defence Evaluation and Research Agency (DERA), now QinetiQ, Vectored thrust Aircraft Advanced flight Control (VAAC) research Harrier XW175, and has been previously reported [1–3]. It was developed within the Cambridge University Engineering Department (CUED) Control Group, and much of its success depended on parallel theoretical developments within the group. The design itself used $\mathcal{H}_{\infty}$ loop-shaping [4], with increased understanding of how to select weights being deduced from the application. Gain scheduling was initially done using switching between controllers, but in piloted simulation the switching points were evident, even when using bumpless transfer techniques. At around this time it was shown that the $\mathcal{H}_{\infty}$ loop-shaping controller could be written in observer form (see for example [5]), providing an improved scheduling method. When it came to implementation of the control law on the aircraft, the lack of processing power necessitated discrete time implementation at a relatively slow rate compared to the bandwidth. To support this, the discrete time solution for the optimal $\mathcal{H}_{\infty}$ controller in observer form developed in [6] was used. Later research within the CUED Control Group looked at model (in)validation [7] using the Harrier as a vehicle for testing its practicality(.)