In order to assess aerodynamic performance of wind turbines an in house Blade Element Momentum (BEM) code is developed in Python programming language. Blade element momentum theory is a powerful tool to predict aerodynamic performance of wind turbines and it requires relatively low computational time compared to other aerodynamic performance analysis tools. Therefore, it is frequently used by wind turbine manufacturers especially, at the initial design phase. In this method, wind turbine blade is divided into multiple stations and it is assumed that there are no aerodynamic interactions between these stations. Blade element computations are performed for a given wind speed, rotational speed, blade radius, number of blades, airfoil profiles, twist angle and chord length along the blade span. Aerodynamic data, related to airfoils are taken from experimental studies in the literature. Later, airfoil data were updated by integrating National Renewable Energy Laboratory (NREL) airfoilprep.py tool to in house BEM code. Airfoilprep.py contains stall delay model and Viterna extrapolation. Stall delay model refers to correction of 2D airfoil data to account 3D effects caused by rotation of blades. Viterna extrapolation is used to extend airfoil foil data obtained from experiments or computational tools to 360o angle of attack range since wind turbines often operate at high angles of attack values. Finally, interaction of rotor and tower to power production is implemented in BEM code. Effect of tower presence is modelled by applying potential flow to non-rotating cylinder. Lastly, results obtained from BEM simulations are compared with the data obtained from NREL's Phase VI unsteady experimental study.