Fault tolerant flight control for a fixed wing unmanned aerial vehicle with partial horizontal and vertical stabiliser losses
In the study reported here, a fault tolerant flight control system for a fixed wing unmanned aerial vehicle with partial stabiliser loss is designed, analysed, implemented and verified. The partial stabiliser damage changes the natural dynamics of the aircraft and causes asymmetry. The control system must maintain aircraft stability and transition from the healthy to the damaged configuration without depending on in flight knowledge of the change in dynamics. The control system must also provide satisfactory transient performance for both the healthy and the damaged configuration. Using existing reference frames and conventions, a six degrees of freedom equations of motion model of the aircraft is derived that can model the effects of the partial horizontal and vertical stabiliser loss on the aircraft dynamics. This model considers the changes in the mass, moment of inertia, aerodynamic model, control authority of the aerodynamic control surfaces, as well as the shift in the centre of gravity. The altered aerodynamic coefficients are calculated using vortex lattice techniques for the different damage configurations. In order to determine the trim states and inputs of the aircraft as a function of the partial horizontal and vertical stabiliser loss, a multivariate Newton Raphson technique is applied to the equations of motion. The required trim actuator deflections are compared to the physical actuator limitations to establish the feasibility of maintaining trim flight for each damage case. Assuming feasible trim states and inputs, the system is linearised and the open loop dynamics of the aircraft are investigated as a function of partial stabiliser loss. A combination of classical and acceleration based control architectures are designed and implemented. The stability, performance and robustness of the flight control system are verified in simulation for damage cases up to 70% left horizontal stabiliser loss and 20% vertical stabiliser loss. The fault tolerant flight control system is verified with flight tests. A release mechanism is designed and manufactured to allow 70% of the left horizontal stabiliser and 20% of the vertical stabiliser to be jettisoned in flight. The flight control system is implemented on a practical unmanned aerial vehicle and successful reference tracking is demonstrated. Practical flight tests showed that the flight control was stable for both the healthy and the damaged aircraft configurations, and able to handle the transition following an in flight partial stabiliser loss event. iii Stellenbosch University https:/scholar.sun.ac.za