High Angle of Attack Landing of an Unmanned Aerial Vehicle
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Motivated by the limited landing space on board a ship, this thesis investigates the landingof the Skywalker X8 fixed wing unmanned aerial vehicle (UAV) in a net. Further motivatedby the way birds abruptly, and equally elegantly, reduce their velocity when landing ona perch or the branch of a tree, a perched landing strategy utilizing the increased dragexperienced for large angles of attack is used to minimize the velocity at impact with thenet. This is accomplished by expressing landing as an optimal control problem, takingadvantage of a nonlinear model of the X8 that is valid for high angles of attack. At thisstage, the optimal control problem is only concerned with the three longitudinal degreesof-freedom. It is solved in an open source optimization framework, using a nonlinearinterior point method. Further, a nonlinear model predictive controller (NMPC) that cancontrol the X8 throughout the landing is developed. At the heart of the optimal controlproblem lies a linear model, blended with a flat plate model to increase its validity forhigh angles of attack in lift, drag and pitch moment. The linear model is developed usingan easy-to-use computational fluid dynamics (CFD) modeling software. In addition a sixdegrees-of-freedom software-in-the-loop (SITL) simulator is developed for future use intesting of hardware-near implementations of the controller, and to allow for validationof the model through pilot testing. Comparison of six different landing scenarios yield awide range of landing velocities, depending on the constraints of the scenario. Simulationswith the developed NMPC show that the same performance is achievable through controlof the X8 under minor environmental disturbances. From this it is concluded that theperched landing strategy will lead to a considerable reduction in terminal absolute velocity,compared to a low angle of attack approach. It is found to be advantageous to start froma low altitude, landing into an elevated net. However, whether an equally large reductionis possible in practice depends on the capabilities of the real-time implementation andthe validity of the model, particularly the propeller model, the pitching moment and dragcoefficients. Finally it depends on how the lateral degrees-of-freedom are affected by thehigh angle of attack flight.