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.
Drag is the resistance or force that opposes the motion of the airfoil through the air. Increased speed causes increased lift because a larger pressure differential is produced between the upper and lower surfaces. Lift does not increase in direct proportion to speed, but varies as the square of the speed. Thus, a blade traveling at 500 knots has four times the lift of the same blade traveling at only 250 knots. Lift also varies with the area of the blade.