H∞ robust control for gust load alleviation of geometrically nonlinear flexible aircraft

Abstract

This paper presents an H_∞ robust control synthesis framework for gust load alleviation of very flexible aircraft exhibiting geometrically nonlinear structural behaviour. The controller is synthesised on a compact reduced-order model (ROM) comprising eight degrees of freedom for the unmanned aerial vehicle (UAV) configuration and nine for the flying-wing, obtained through nonlinear model order reduction of the coupled fluid-structure-flight dynamics system, and subsequently validated against the full nonlinear model (540 and 1 616 degrees of freedom, respectively). The control architecture utilises trailing-edge flap deflection as the actuation mechanism and wing-tip displacement as the regulated performance output, incorporating an input-shaping weighting function K_c that governs the trade-off between the degree of structural load alleviation and the associated control effort. Results are obtained for a Global Hawk-like UAV configuration, demonstrating a 23.15% reduction in peak wing-tip deflection under discrete gust excitation and a 10.26% reduction in root-mean-square response under stochastic turbulence; the extensibility of the methodology to a larger, very flexible free-flying-wing platform is also demonstrated. The methodology establishes that H_∞ controllers designed on low-dimensional ROMs are capable of providing robust gust load alleviation when deployed on the high-dimensional nonlinear aeroelastic system.