Advanced helicopter flight control using two-degree-of-freedom H(infinity) optimization

Abstract

We report on the design and ground-based piloted simulation testing of a high-performance helicopter flight control system. An observer-base d multivariable controller was designed using a singular-value loop shaping method based on a two-degree-of-freedom H°° optimization, formulated to give robust model following and sta- bility. The controller provided wide envelope stability and almost total decoupling, consistently allowing level-1 Cooper-Harper handling qualities ratings to be achieved for mission task elements performed on the large mo- tion simulator at the Defence Research Agency, Bedford, England. Although designed for hover and low speed, it was successfully tested at speeds of up to 90 kn. In the words of one of the test pilots, the controller performed outstandingly. HE continuing drive to extend the operational capabilities of combat helicopters will require flight control systems with han- dling qualities tailored to the mission task. By removing the pilot from low-level stabilization and control loops, there will be consid- erable scope for improved mission effectiveness and survivability, particularly when required to operate in adverse conditions. The vehicle of interest is the Westland Lynx multirole combat he- licopter. This type of helicopter presents pilots with a high workload. It is open-loop unstable and exhibits high levels of cross coupling and variations in handling characteristics with flight condition. Use of automatic control for stabilization and reduction of cross coupling is standard in this type of aircraft, although the controllers tend to be low authority. That is, most of the available actuator authority— typically 80% or more—is directly in the hands of the pilot. In 1988, as part of an earlier collaboration with the then Royal Aerospace Establishment (RAE), Yue and Postlethwaite1 demon- strated the potential of H°° optimal control theory for the design of full authority helicopter flight control systems. They tested suc- cessfully using the ground-based facilities at RAE Bedford, now the Defence Research Agency (DRA), an 18-state control law de- signed for hover/low speed. This led to the first piloted simulation of a rotorcraft controller designed using H°° optimization. Other work based on the Lynx can be found in Refs. 2-4. In Ref. 2, the authors used a one-degree-of-freedom (1-DOF) H°° loop shaping procedure. In Ref. 4, the design and evaluation are described of a lin- ear quadratic Gaussian/loop transfer recovery controller for a 30-kn trim condition. That work did not, however, make use of nonlinear models or piloted simulations for the evaluation. The aim of this paper is to describe how better performance was achieved over a wide range of simulated flight condition, using the most accurate nonlinear models available. Piloted simulation was a vital part of this work, as was determining compliance with the Military Rotor- craft Handling Qualities Specification.5 The main contribution is the design of a novel multivariable controller giving high perfor- mance and robust stability. We demonstrate how specified quanti- tative handling qualities requirements can be incorporated into the design process. Results are presented from piloted simulations us- ing one of the world's most advanced simulation facilities: the large motion simulator (LMS) at DRA Bedford.