Figure (a) compares the ideal fluid case static-pressure distribution at the airfoil surface and centerline streamline with the real fluid case. Figure (b) illustrates that, in the ideal fluid case, the net static-pressure force acting on the front surface of the airfoil (up to the shoulder) parallel to the free stream exactly opposes and cancels that acting on the rear surfaces of the airfoil. In the real fluid case, shown in figure (c), the net result is a drag force due to the asymmetric pressure distribution called pressure drag. Figure (d) illustrates the lift and drag for an airfoil producing lift in both an ideal and real fluid case. ¾

Credits - NASA