A hybrid Navier-Stokes potential flow methodology for modeling three-dimensional unsteady viscous flow over horizontal axis wind turbine configurations is presented. In this approach, the costly viscous flow equations are solved only in a small viscous flow region surrounding the rotor. The rest of the flow field is modeled using a potential flow methodology. The tip vortices are modeled using a free wake approach, which allows the vortices to deform and interact with each other. Sample results are presented for two rotor configurations tested by the National Renewable Energy Laboratory. Comparisons with experimental data, full Navier-Stokes simulations and blade element momentum theory are given to establish the efficiency and accuracy of the present scheme.