Systematic Development of a Limited Authority Flight Control System for Helicopters

Over the last three decades, there has been increased interest in full authority (FA) control systems for helicopters. Despite their great potential, it will be some time before all operational helicopters are able to benefit from such flight control systems. Therefore, interim methods to deliver similar levels of performance are required, and it is natural to explore ways of improving current limited authority (LA) systems to achieve this. The study described herein investigates this topic and contributes a new LA design method. The first part of the thesis describes the construction of the baseline FA controller for the AgustaWestland EH101 helicopter. The FA controller was designed using the 2DOFH∞ loop shaping methodology. Details of the controller design are presented, together with an analysis of linear and nonlinear simulation results. These reveal the controller’s abilities to provide good levels of performance, across the portion of the flight envelope tested, without the aid of gain scheduling. The second half describes the derivation of state-space formulae that enables the transformation of the above (FA) controller into LA form such that the latter controller replicates the small-signal FA responses. Extensive analysis and simulations are carried out for this architecture, both at the design point and at other conditions throughout the flight envelope. Also, the results are contrasted against a simpler LA architecture and the effectiveness of the proposed LA controller over this is shown. Both FA and LA results are complemented by an evaluation of handling qualities information against the design standard ADS-33. It is believed that the results presented in the thesis show the strengths of the proposed LA architecture and suggest that current helicopter hardware should be able to provide much of the functionality of FA systems without requiring a complete overhaul.