Two models of the yaw behavior of horizontal axis wind turbines are presented and discussed. Emphasis in this paper is on the description of the models and underlying assumptions with details of the equations and solution methods referenced in technical reports. The more complex model (YawDyn) considers the coupling of blade flap motions and yaw motions which result from temporal and spatial variations in the approaching wind speed. The new methods are unique in that they simultaneously model the effects of skewed wake aerodynamics and blade stall. Both of these effects must be considered if yaw behavior is to be adequately understood. The models are validated by comparison with other prediction techniques, wind tunnel tests and full-scale atmospheric tests. In all cases the models are shown to give excellent qualitative agreement and reasonable quantitative agreement. It is concluded that the new models represent a significant improvement in the methods available to the wind turbine designer for understanding yaw loads and motions.

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