Experimental Characterization of a Force-Controlled Flexible Wing Traction Kite

In order to use flexible wing kites effectively for power generation their size must be increased and aerodynamic performance should be systematically improved. Validated models are essential for an efficient design process. The largest uncertainty in current modeling approaches is the apparent wind speed va experienced by the kite. In this work we develop an in-flight measurement setup for the apparent wind speed vector. An air data boom with a Pitot tube and a pair of wind vanes is used to measure the relative flow below the wing, at sufficient distance not to be influenced significantly by the wing itself. The experimental data supports the current quasi-steady model of a pumping kite power system [1]. In this work we propose a mechanistic model for the resulting aerodynamic coefficient cR. A sole dependency of this coefficient on the angle of attack, as it is common for rigid airfoils, must be rejected. Instead, we find that the coefficient strongly dependsonthe wing loading and toa lesser extent also on the power ratio, i.e. the non-dimensional pitch parameter of the wing and the inflow angle, as assumed in [2]. We further observe that the force-controlled characteristic of the system majorly affects the flight dynamics. Important findings are that by changing the tether reeling velocity to obtain constant traction force the ground station balances wind gusts which thus hardly affect the relative flow vector but the power output. Sudden controller-induced changes in reeling velocity cause inflow angle variations similar to fluttering observed on rigid airfoils. Further can cR and va not vary independently for any force-controlled phase. The presented relation of different parameters affecting cR can be used for flight path optimization as well as kite and control systems design.