Design of robust multivariable helicopter control laws for handling qualities enhancement

Control laws for a typical battlefield helicopter in the hover flight condition are presented. The control laws form the stabilization loop of a control structure for tailoring pilot commands to meet desired criteria. The performance and stability robustness of the control laws are presented in terms of singular values of specific frequency responses. The loop shaping procedure for designing the control laws using H^∞ optimization is presented and analyzed with respect to the singular values of the sensitivity and complementary sensitivity functions. Control of hovering is difficult due to the rapid and significant changes in dynamics when manoeuvring from this position. Throughout the design procedure a nominal model of the helicopter trimmed about the hover is used. The controller designed has fixed gains relative to the nominal model. The nonlinear time simulations show the design to be robustly stable to the unmodelled rotor and actuator dynamics and to the unmodelled dynamics due to changes in the flight envelope