The performance and stability requirements for a robust flight control system design are presented in the form of a 'design challenge.' The challenge includes description of specific vehicle failures that are to be accommodated by the flight control system. The vehicle chosen for the design is the innovative control effector vehicle, and both longitudinal and lateral/directional degrees of freedom are included. Two flight conditions are considered: Mach number 0.3 and altitude 15 000 ft; Mach number 0.9 and altitude 35 000 ft. No scheduling of the flight control law is permitted in the design. After the performance and stability requirements are described, a solution to the design challenge is presented in the form of a sliding-mode control system offered as an alternative to reconfigurable designs. The performance of this system is then evaluated through analysis and computer simulation, including significant failures and damage.

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A brief review of sliding mode control is undertaken, with particular emphasis upon the effects of neglected parasitic dynamics. Sliding mode control as implemented with boundary layers is then interpreted in the frequency domain. The inclusion of asymptotic observers and reference model “hedging” is shown to reduce the effects of neglected parasitic dynamics. Application of the resulting observer/hedging-based sliding mode technique to the design of a robust longitudinal control system for a highly unstable aircraft is described. The sliding mode controller is shown to exhibit stability and performance robustness superior to that of a classical loop-shaped design when significant changes in vehicle and actuator dynamics are employed to model airframe damage.

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A team comprised of the Air Force Research Laboratory (AFRL), Northrop Grumman, Lockheed Martin, and the NASA Langley Research Center conducted three aeroservoelastic wind-tunnel tests in the Transonic Dynamics Tunnel to demonstrate active control technologies relevant to large, flexible vehicles. In the first of these three tests, a semispan, aeroelastically scaled, wind-tunnel model of a flying wing SensorCraft vehicle was mounted to a force balance to demonstrate gust load alleviation. In the second and third tests, the same wing was mated to a new, multi-degree-of-freedom, sidewall mount. This mount allowed the half-span model to translate vertically and pitch at the wing root, allowing better simulation of the full span vehicle's rigid-body modes. Gust Load Alleviation (GLA) and Body Freedom Flutter (BFF) suppression were successfully demonstrated. The rigid body degrees-of-freedom required that the model be flown in the wind tunnel using an active control system. This risky mode of testing necessitated that a model arrestment system be integrated into the new mount. The safe and successful completion of these free-flying tests required the development and integration of custom hardware and software. This paper describes the many systems, software, and procedures that were developed as part of this effort.