This picture also shows airflow lines that illustrate how the air moves about the airfoil section. Notice that the air is deflected downward as it passes under the airfoil and leaves the trailing edge. Since the air is being deflected downward, an equal and opposite force must be acting upward on the airfoil. This force adds to the total aerodynamic force developed by the airfoil. At very low or zero angles of attack, the deflection force or impact pressure may exert a zero positive force, or even a downward or negative force.
Air passing over the top of the airfoil produces aerodynamic force in another way.
The shape of the airfoil causes a low pressure area above the airfoil according to Bernoulli's Principle, and the decrease in pressure on top of the airfoil exerts an upward aerodynamic force. Pressure differential between the upper and lower surface of the airfoil is quite small - in the vicinity of 1 percent. Even a small pressure differential produces substantial force when applied to the large area of a rotor blade. The total aerodynamic force, sometimes called the resultant force, may be divided into two components called lift and drag. Lift acts on the airfoil in a direction perpendicular to the relative wind.