H∞ control: from desk-top design to flight test with handling qualities evaluation

A combined team from the University of Leicester. UK Defence and Evaluation Research Agency (DERA) and the National Research Council (NRC), Canada, have flight tested a multivariable H∞ optimal controller on the NRC Bell 205 fly-by-wire (FBW) research helicopter. This is believed to be the first time that such multivariable control theory has been applied successfully in the rotary-wing world. The controller was designed by the University of Leicester with modelling. simulation and test pilot support provided by DERA. After succesfully completing functional checks on the Advanced Flight Simulator at DERA Bedford, the UK team went to Canada for the flight trial. Implementation. validation and verification took only a week to complete and successful in-flight engagement was achieved on the first flight. A further week of tests and handling qualities evaluation followed during which over eight hours of FBW flight time was accumulated. The paper provides an overview of the control design methodology, the conduct of ground-based simulation and in-flight testing and summarises the lessons learned and requirements for further research. The following conclusions can be drawn from the study: • The work has matured the application of modern multivariable control design techniques, closing the gap between theory and practice and further demonstrating the potential of active control technology (ACT) for helicopter flight control. • The H∞ Loop Shaping Design Procedure (LSDP) provides a relatively simple and intuitive framework for achieving robust stabilisation. Both performance and robustness can be addressed explicitly in the controller synthesis and the the designer can make trade-offs between them. • The design process involving the complementary use of desk-top design, ground-based simulation and in-flight testing was important in arriving at a successful design, and in reducing both development time and costs.