Pumping kite power systems are a promising way of harnessing clean energy from high-altitude winds. A large kite flies crosswind developing a strong pulling force through a tether which is connected at the ground to an electrical generator. The power output of such innovative systems is directly related to the wing aerodynamic properties which therefore play a key role. Moreover, the latter, often assumed with best guess, are also required for computational simulations at development stage. In this context, the research group of TU Berlin recently designed a car towing test bench for evaluating flexible airfoil performances in an automated and repeatable manner. By towing the kite at a prescribed speed, relevant information, such as tether force and elevation angle, are extracted and can be converted into aerodynamic properties thanks to a suitable analytical model. With the purpose of improving the assessment of aerodynamic properties and their exactitude, the present work takes advantage of the TU Berlin test bench by investigating the Kitepower wing. An alternative testing methodology is suggested and compared against the current procedure, indicating a dynamic behaviour of the kite. A 2D quasi-steady point mass model is implemented to compute the kite aerodynamic properties by taking both wing and tether mass as well as tether aerodynamic effects into consideration. Eventually, a detailed analytical model of the tether is developed to assess its sag and effects on the kite performance. Finally, an attempt to provide polar curves with re-spect to the angle of attack rather than the power ratio is experimentally carried out.

The aerodynamic characterization of kites is of paramount importance in the analysis of kites applied to wind energy generation because it is a key component of flight simulators. However, due to flexibility effects of the kite structure and the typically large sideslip and attack angles, severe difficulties arise. This work applies a Estimation-Before-Modeling (EBM) method, a widespread technique in aerospace engineering that makes use of data obtained during flight testing, to the aerodynamic characterization of power kites with four control lines. The procedure includes two main steps: an estimation phase and a modelling phase. The estimation phase involves a Kite Estimator (KE) that receives a comprehensive set of measurements and provides the time history of the state vector of the system. In this phase, aerodynamic forces andmoments are components of the extended state vector of the kite and they are estimated fromthemeasurements by using stochastic filtering and smoothing techniques. The KE has been fed with experimental data obtained during a test campaign with two different power kites (10m2 and 13m2). In the modelling phase, a multivariable regression algorithm has been used to determine the nonlinear coefficients of the aerodynamic model. The regression algorithm compares the aerodynamic forces and torques provided by the KE and the one computed theoretically with a flight simulator of a four-line kite. The accuracy of the aerodynamic model obtained with the EBM technique was assessed by comparing time histories of the experimental trajectories and the one provided by the kite simulator updated with the new aerodynamic model.