This paper presents an approach to on-line control design for aircraft that have suffered either actuator failure, missing effector surfaces, surface damage, or any combination.The approach is based on a modified version of nonlinear dynamic inversion.

A modified derivation of nonlinear dynamic inversion provides the theoretical underpinnings for a reconfigurable control law for aircraft that have suffered combinations of actuator failures, missing effector surfaces, and aerodynamic changes. The approach makes use of acceleration feedback to extract information pertaining to any aerodynamic change and thus does not require a complete aerodynamic model of the aircraft. The control law does require feedback of effector positions to accommodate actuator dynamics. Both accelerometer and rate gyro failure detection and isolation (FDI) systems are implemented, allowing up to three independent failures for each FDI system as long as they are in different axes. Nonlinear simulation results show that the FDI systems improve the robustness to accelerometer/rate gyro uncertainties. An advanced tailless aircraft model is used to demonstrate the concepts.The simulation includes accelerometer and rate gyro noise and bias, failures due to accelerometers, rate gyros, and actuators, and modeled missing surfaces that cause airplane aerodynamic changes.

This paper describes a general form of nonlinear dynamic inversion control for use in a generic nonlinear simulation to evaluate candidate augmented aircraft dynamics. The implementation is specifically tailored to the task of quickly assessing an aircraft's control power requirements and defining the achievable dynamic set. The achievable set is evaluated while undergoing complex mission maneuvers, and perfect tracking will be accomplished when the desired dynamics are achievable. Variables are extracted directly from the simulation model each iteration, so robustness is not an issue. Included in this paper is a description of the implementation of the forces and moments from simulation variables, the calculation of control effectiveness coefficients, methods for implementing different types of aerodynamic and thrust vectoring controls, adjustments for control effector failures, and the allocation approach used. A few examples illustrate the perfect tracking results obtained.

A recently proposed method of on-line control design for aircraft reconfiguration is modified to mitigate the effects of effector ratelposition saturation and sensor noise in critical measurements while preserving some, perhaps reduced, level of flying qualities. The on-line control design, based on an incremental version of nonlinear dynamic inversion, does not require a complete aerodynamic model of the aircraft, but does require the local control derivatives along with feedback of accelerations and effector positions. Recovery from a variety of failure (stuck or missing effectors) is possible under the original design as long as the working effectors do not enter saturation for extended periods and critical measurements are relatively noise free-an unlikely situation. Here, an improved control allocator minimizes both effector rate and position, ' utilizing a multi-pass strategy to restore lost control power due to saturation using the remaining unsaturated controls. Command model flying parameters are adaptively manipulated online to comply with reduced levels of control power further reducing saturation. A classically designed compensator placed around each actuator underpins strategy to reduce jitter due to sensor noise in the control variable responses while preserving decoupling of original control. Improvements due to these modifications are demonstrated on an advanced tailless fighter.

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