Adaptive LFT control of a civil aircraft with online frequency-domain parameter estimation

This paper describes the application of an indirect linear fractional transformation (LFT)-based state-space adaptive control scheme to a transport aircraft, within the context of the European project REconfiguration of CONtrol in Flight for Integral Global Upset REcovery. The principle of the scheme is to design and validate off-line a gain-scheduled controller, depending on the plant parameters to be estimated, and to combine it online with a model estimator, so as to minimize the onboard computational time and complexity. A modal approach, very classical for the design of a flight control law, is used to directly synthesize the static output feedback LFT controller, depending on the control and stability derivatives, ie, the parameters of the linearized aerodynamic state-space model to be estimated. Since the gain-scheduled LFT controller online depends on the parameter estimates instead of the true values, its robustness to transient and asymptotic estimation errors needs to be assessed using mu and integral quadratic constraint analysis techniques. A primary concern being an online implementation, a fully recursive frequency-domain estimation technique is proposed, with a low online computational burden and the capability to track time-varying parameters. Full nonlinear simulations along a trajectory validate the good performance properties of the combined estimator and gain-scheduled flight controller. To some extent, minimal guaranteed stability and performance properties of the adaptive scheme can be ensured by switching to a robust controller when the parameter estimates are not reliable enough, thus bypassing the Certainty Equivalence Principle.