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Title: The Albatross UAV : propulsion by dynamic soaring for unmanned aerial vehicles
Author: Deittert, Markus
Awarding Body: University of the West of England, Bristol
Current Institution: University of the West of England, Bristol
Date of Award: 2010
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Abstract:
Dynamic soaring is an engineless flight technique, which extracts energy from a wind gradient. i.e. a change in wind speed with height. It is the key method by which albatrosses achieve their long endurance flight of several thousand miles, flown with hardly a flap of their wings. Such flying takes the albatross close to the water's surface and usually occurs in strong wind conditions. Development of unmanned aerial vehicles (DAVs) is driven by the need of long endurance flight. Techniques such as increasing the engine's fuel efficiency and increasing the amount of fuel carried on-board, suffer from diminishing returns. To achieve ultra long en- durance flight the limits of on-board energy storage must -be overcome by exploiting a source of ambient environmental energy. This thesis investigates dynamic soaring flight as a means of propulsion for DAVs of about 3 metres wingspan. Trajectory optimisation is the central method of investigation used in this thesis. A faster and more efficient approach is presented in this work. by exploiting the DAV's flight model's differential flatness property. The trajectory optimisation process is used to establish outer bounds on the range of feasible wind speeds. for a generic example DAV. From these outer bounds, the probability of favourable winds is investigated, in six candidate locations in the Antarctic Ocean. the Atlantic Ocean, the North Sea and the English Channel. Our example DAV's flight performance closely matches the albatross' flight performance in its natural habitat, the Antarctic Ocean. However, the achievable likelihood of favourable winds is too low in European waters to justify using dynamic soaring as a primary means of propulsion and it must be used together with other flight techniques and propulsion methods to achieve adequate reliability and robustness. By studying the sensitivity of the minimal and maximal wind conditions to flight path con- straints and physical parameters of the DAV model, further insight is gained into the technique of dynamic soaring. Previous work by other authors concentrated upon finding general tra- jectories, with flight models carefully normalised to reduce the number of parameters to one or two variables which represent the whole aircraft. In this thesis, parameters are considered individually. Key results are identification of the maximum airspeed limit as a key factor for the upper bound on feasible wind speeds and the height of the trajectory's low point above ground as a key factor for overall performance. Establishing outer bounds on the feasible wind speed range naturally raises the question of which range of wind speeds within the outer bounds can be exploited in practice. Wind speeds of sufficient magnitude for dynamic soaring are strongly associated with atmospheric turbulence. Because dynamic soaring trajectories are situated very close to the surface, trajectory errors are a particular concern. The ability of an DAV to fly in turbulent conditions is thus closely related to the ability of its control system to reject external disturbances. The performance of a periodic LQR-based controller is evaluated in simulation with particular care having been placed on the turbulence model. It was found, that stabilising the UAV on the nominal trajectory does not necessarily coincide with good energy management and a future control system design should explicitly address energy extraction and management.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.557598  DOI: Not available
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