Control of tethered airfoils is investigated, in order to devise a new class of wind generators to overcome the main limitations of the present wind technology, based on wind mills. A model from the literature is used to simulate the dynamic of a kite whose lines are suitably pulled by a control unit. Energy is generated by a cycle composed of two phases, indicated as the traction and the drag one. The kite control unit is placed on the arm of a vertical axis rotor, connected to an electric drive able to act as a generator when the kite pulls the rotor and as a motor in dragging the kite against the wind. Control is obtained by "fast" implementation of Nonlinear Model Predictive Control (NMPC). In the traction phase the control is designed such that the kite pulls the rotor arm, maximizing the amount of generated energy. When energy cannot be generated anymore, the control enters the drag phase and the kite is driven to a region where the energy spent to drag the rotor is a small fraction of the energy generated in the traction phase, until a new traction phase is undertaken. Simulation results are presented, showing encouraging performances. (C) 2008 Elsevier Ltd. All rights reserved.