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Design Methodology and Control Strategy for Transitioning Vertical Takeoff and Landing Unmanned Aerial Vehicles for improved speed, range and payload capacity

Boek - Dissertatie

Unmanned aerial vehicles are increasingly deployed in various applications such as surveillance, mapping, inspection and even parcel delivery. The capability to vertically take off and land proves to be advantageous for applications in confined regions and is particularly interesting for deployment in autonomous applications. This capability compromises the flight performance in terms of speed, range and payload capacity, limiting their usability for highly demanding applications. Combining speed, range and payload capacity with vertical takeoff and landing capability is a challenging problem because of the very distinct flight phases. Several transitioning solutions such as tilt-wing, tilt-rotor or dual propulsion systems all impose additional weight and mechanical complexity to the design. Therefore, this work focuses on lightweight and reliable designs without tilting mechanisms or actuated surfaces. The only moving parts are multiple propellers that simultaneously propel and control the ‘multi-rotor’ vehicle. Until recently, multi-rotors were only designed and used for low-speed applications with optimized flight times and payload capacities. In an attempt to significantly improve speed and range, this work introduces a new concept with wings that fully or partially support the weight after a transition from hover to forward flight. A parameter selection method uses mass and performance models of individual components of the vehicle to determine the optimal conceptual design, based on mission-specific requirements. The propellers operate in a non-axial flow for which no prior data was available. Wind tunnel experiments are therefore used to validate several theoretical models. An improved model is derived that takes into account the efficiency loss in non-axial flow. Because of the distinct flight phases, also a new control strategy is designed, implemented and tested. This strategy allows intuitive control for a human pilot, can be applied to several different configurations and can be used by a path-planning algorithm. Multiple prototypes are designed, built and test flown. An automated test procedure allows us to estimate a constant wind speed and removes the need for an additional wind sensor. Wind tunnel experiments and flight tests confirm that decreasing the drag of the vehicle is most important to improve speed and range of a design. The propellers contribute largely to the overall drag at low transition angles. On the one hand, designs with larger wings fly at larger transition angles which improves the propeller efficiency and effectively decreases the power requirement in forward flight. On the other hand, a large wing adds extra weight to the vehicle, complicates operation in windy conditions and increases the difference in operating condition for the propulsion system, leading to an efficiency loss. In addition, a larger wing also reduces cruise speed and improves endurance. The best performing airframe in terms of speed, range and controllability are therefore lifting-body or blended-wing designs which have only a small lifting surface but a strongly reduced drag. Measurements with different blended-wing designs show a trend of higher range and speeds for heavier vehicles with the same geometry. This work predicted, measured, evaluated and improved the performances of multirotors in terms of speed, range and payload capacity. This facilitates their use in a wide variety of high-demanding applications.
Aantal pagina's: 260
Jaar van publicatie:2016
Toegankelijkheid:Open