Conceptual design and passive stability of tethered platforms [electronic resource]
Keywords: Aeronautics and Astronautics

Motivated by recent commercial development of airborne wind energy, the conceptual design of tethered platforms is investigated. Airborne wind energy focuses on harvesting power from wind through nontraditional methods. Some of the ideas include commercial scale electric turbines for the upper atmosphere, using tow devices to assist in shipping or portable light wind generators. Different from current turbines, these systems do not have large masts and blades but instead use a tethered flying body to transmit energy. The vehicle at the end of the tether varies depending on the energy harvesting method but all fall under the broader category of the towed body. In contrast to modeling and analysis techniques specifically tailored to evaluate performance of individual types of towed bodies, a methodology for designing efficient, stable wings for towed bodies is developed. Static performance can be characterized by a minimal set of design parameters. This minimal set is derived through dimensional analysis showing that the static performance is dependent on a minimum of 3 parameters. These are chosen to be the ratios of effective lift to drag of the end vehicle, tether cross-flow drag to tether weight and effective lift to tether cross-flow drag. An experiment was performed to verify that two systems of different sizes with matching dimensionless design parameters would have similar dimensionless performance. To demonstrate the kind of design analysis that can be done, altitude is chosen as an example performance goal. Sensitivity studies show that for all the design parameters, there are value regions that are highly sensitive to changes in design parameter and areas that are flat. This means that increasing the effective lift to drag ratio beyond a certain threshold, for example, will give small increases in altitude. While the performance analysis is for generic towed bodies, the stability analysis focuses on rigid wings. Although similar to aircraft, attaching a tether to a flying object increases the amount of information to track and drastically alters modal and stability characteristics. By adapting some airplane analysis techniques to tethered systems the design complexity has been reduced, facilitating intuitive understanding for conceptual design. Static stability criteria is established similar to that of unrestrained aircraft, demonstrating the effect of bridling on static stability. Terms that relate trim and static stability are derived. A simple linear model is also developed for quick evaluation of stability. Stability derivatives from unrestrained aircraft analysis are combined with tether derivatives that give the change in tether force at the attachment point only. The linear model has 12 modes, 6 for the translation degrees of freedom and 6 for the rotational degrees of freedom. A method is presented for determining an optimal performance design. This is done by forming a constrained optimization problem to minimize the cost. The method is demonstrated in the design of a kite to break the single-line kite altitude record. The cost function includes estimates of tether cost and the price of materials for kite construction. Constraints include limits on aerodynamic and structural characteristics along with performance constraints such as required altitude. A stable planform and bridle position are then chosen using the analysis tools developed. Although a full scale design is not built, a sub-scale model, scaled according to the rules derived in the thesis, is flown to demonstrate the feasibility of the design. Using this methodology, parameter studies for environmental inputs and material properties are done for the optimized system.