Robust H∞ control of a fly-by-wire aircraft: an LFT approach

The goal of this paper is to evaluate, by means of a practical benchmark problem, the potential benefits deriving from the application of modern multivariable techniques to the flight control laws design for a fly-by-wire small commercial aircraft. A reference dynamic model of the aircraft has been chosen in order to satisfy a suitable set of Flying Qualities requirements. Moreover the set of plants obtained in correspondence of a quite large region of the flight envelope around the nominal point has been parameterised with respect to Mach number and dynamic pressure exploiting the linear fractional transformation (LFT) approach; this allows to take into account, during the controller design phase, the variation of aircraft characteristics within the given region of the flight envelope. Then a model matching H∞ control problem has been solved via the μ synthesis approach, which allows to deal with disturbance rejection and robustness specifications. By repeating the procedure in a few further design points it is possible to cover the whole envelope via a small number of different controllers and render the following scheduling phase much more easy to accomplish. Numerical simulations show that the designed controller performs satisfactory.