Non-powered flight vehicles such as kites can provide a means of transmitting wind energy from higher altitudes to the ground via tethers. At Delft University of Technology, construction and testing of such a high altitude wind machine is ongoing. The concept is called the Laddermill. It generates energy by pulling a line under high tension from a drum with a kite and retrieving it under low tension. The change in tension is achieved by changing the angle of attack and flight pattern of the kite. Although there is increased world wide interest for systems to extract wind energy from higher altitudes with kites, research into kite properties is limited. Such research however is required to enable the design of purpose-built kites for energy extraction. This paper presents testing of surf kites that was performed for model validation. Various models are discussed: a point model, a flexible multi-plate model and a rigid-body arc shaped model. Mathematical modeling and numerical simulations of the kite based on test data are presented.