Towards gain-scheduled H∞ control design for a tilt-wing aircraft

This paper describes the development of robust, multivariable H^∞ control systems for the cruise and hover operating points of the high-speed autonomous rotorcraft vehicle (HARVee), an experimental tilt-wing aircraft. Tilt-wing aircraft combine the high-speed cruise capabilities of a conventional airplane with the hovering capabilities of a helicopter by rotating their wings at the fuselage. Changing between cruise and hover flight modes in midair is referred to as the conversion process, or simply conversion. A nonlinear aerodynamic model was previously developed that captures the unique dynamics of the tilt-wing aircraft. The nonlinear model is trimmed, linearized and analyzed at the cruise and hover operating points. The similarities and differences between a tilt-wing and conventional aircraft are examined through modal analysis. The H^∞ design methodology was used to develop cruise and hover control systems because it directly addresses multivariable and robust design issues. The development of these control systems was governed not only by performance specifications at each particular operating point, but also by the unique requirements of a gain-scheduled conversion control system. The cruise and hover control designs form the basis of an eventual conversion control system and this guides the choice of the H^∞ weighting functions. The performance of the resulting cruise and hover closed-loop systems is analyzed in the frequency and time domains. Hover flight test hardware is described. A tilt-wing aircraft modeling, simulation, animation, and real-time control (MoSART) software environment provides 3D visualization of the vehicle´s dynamics. The environment is useful for conceptualizing the natural aircraft dynamics and for gaining an intuitive understanding of the closed-loop system performance.