Modeling Kite Flight Dynamics Using a Multibody Reduction Approach

Successful deployment of kite-based high-altitude wind power requires automatic flight control. Control law design is preferably based on a real-time-capable flight dynamic's model. The starting point of the analysis is a multibody-system representation of an arc-shaped leading-edge inflatable kite attached to a massless straight tether by means of two bridle lines with movable attachment points. This paper presents a reduction of this system to a rigid-body approximation. This process is performed in two stages: a state reduction and a model identification. The state reduction decreases the large number (similar to 400) of degrees of freedom of the multibody system to the six degrees of freedom of an equivalent rigid body on the basis of conservation of momentum. The model identification determines the aerodynamic and structural characteristics for the rigid-body model on a parametric basis. The flexibility of the inflatable kite is taken into account using variable aerodynamic derivatives and a quasi-static structural model. The state reduction is assessed by means of an error criterion. Simulation shows that the error is negligible. The rigid-body model is validated for symmetric flight maneuvers, comparing the multibody-system and rigid-body responses subject to small perturbations.