In the autumn of 2000, the National Renewable Energy Laboratory (NREL) organised a 'blind comparison', in which wind turbine research centres worldwide were asked to provide stationary aerodynamic loads for the wind turbine Phase VI and aerodynamic coefficients for the S809 airfoil. These institutes participated in this benchmark with their wind tunnel measurements and numerical calculation programs. The results furnished by the participants showed a large discrepancy and scatter, in particular in deep stall. Based on the differences between the provided aerodynamic properties, the need for improvements on the current design codes and for implementation of new numerical method was imposed. The purpose of the present work is to reproduce the aerodynamic coefficients for S809 airfoil in attached flow and in-stall development, by using a coupled potential method and the boundary layers theory. For the fully stalled region, a novel technique was introduced, based on the AERODAS model developed by NASA. This model has been improved to consider the effect of Reynolds number, and to prevent numerical instability, which might be produced by the transition from pre-stall to post-stall regions. The results of our calculations were approximately in the mid-range of those from the 'blind comparison' participants while they were not highly disparate to the measurements.