Design of robust nonlinear autopilot for helicopters with handling quality requirements

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Helicopters are required nowadays to possess high levels of agility and maneuverability and the capability to operate in degraded visual conditions over a wide range of airspeeds. Furthermore, for certification purposes, they need to satisfy some stringent reliability and performance requirements (the so-called handling qualities, HQ's) set out by the US Army's Aeronautical Design Standard ADS-33 (more precisely ADS-33E-PRF). From the control engineer point of view, designing helicopter flight control laws is a very challenging problem: Helicopters present highly nonlinear and fully coupled dynamics that are inherently unstable. Existing mathematical models vary greatly from one flight region to the other and present severe uncertainties due to the empirical representation of aerodynamic forces and moments. Still, for most rotorcraft flying today, the control laws have been developed using predominantly classical single-loop frequency response and root locus design techniques. These techniques are based on linearized models about a set of trim points within the flight envelope and provide only local stability. Moreover, the cost and time associated with refining the flight control law for each trim point and then «stitching them together» with gain schedules to cover the full flight is substantial and there is still much room for improvement in achieving consistent HQ's in the extreme portions of the flight envelope. This research proposes to develop a nonlinear autopilot which does not rely on small signal approximation nor on gain scheduling and is based on recent results on nonlinear adaptive regulation. It is capable of achieving accurate tracking for vertical/lateral and longitudinal dynamics in the presence of severe uncertainties. More importantly, we propose to investigate means to integrate ADS-33-based requirements in the autopilot design. The design will be validated on two platforms: a MIMO Twin Rotor system resembling a helicopter available in our laboratory and HELISIM, an industry standard flight simulator for helicopters developed by the Quebec-based company Engenuity Inc.