Optimal control surface mixing of a Rhomboid-Wing UAV

Abstract

This thesis describes the development of an open-loop control allocation function also known as a mixing function for aircraft with an unconventional control surface setup (i.e. not consisting of a conventional elevator, rudder and ailerons) by using mathematical optimisation. The techniques used to design the control allocation and mixing used on the unconventional configuration when flying it without artificial stability or control augmentation is provided. A typical application of this control mixing would be to enable a pilot to operate an unconventional unmanned aerial vehicle (UAV) as if it was a conventional model aircraft during flight testing or as a backup mode should any sensor failures occur during a typical flight test program. The allocation can also be used to simplify the inner control structure of a UAV autopilot or stability augmentation system. Although this type of mixing would be straightforward on a conventional airframe, an unconventional configuration has several unique characteristics that complicate the modelling and design process. A custom six degree of freedom (6DOF) formulation for flight simulation was made available to model the aircraft and run various scripts to evaluate the aircraft response when the control allocation function is implemented. The simulation model was used to develop the mixing function that maps conventional input commands to the unconventionally situated control surfaces in the most optimal way. The design process was formulated as a multi-objective optimisation problem, which was solved using a custom sequential quadratic programming and custom leapfrog programming method. A methodology was proposed to define the constraints, which can be customised for a particular aircraft or application. The control allocation function was implemented in two different simulation environments to investigate the suitability of candidate designs. A robustness study was performed to evaluate the impact of actuator failures on the aircraft control response using the designed control allocation system. The proposed control allocation design methodology can also be used to design the inner control loops of more sophisticated control systems such as stability augmentation and automatic flight control, which is also briefly discussed in this thesis.