BERNOULLI’S PRINCIPLE
Air flowing over the top surface accelerates. The airfoil is now subjected to Bernoulli’s Principle or the “venturi effect.” As air velocity increases through the constricted portion of a venturi tube, the pressure decreases.
 

 
Compare the upper surface of an airfoil with the constriction in a venturi tube that is narrower in the middle than at the ends. [Figure 2-9]
The upper half of the venturi tube can be replaced by layers of undisturbed air. Thus, as air flows over the upper surface of an airfoil, the camber of the airfoil causes an increase in the speed of the airflow. The increased speed of airflow results in a decrease in pressure on the upper surface of the airfoil. At the same time, air flows along the lower surface of the airfoil, building up pressure. The combination of decreased pressure on the upper surface and increased pressure on the lower surface results in an upward force. [Figure 2-10]
 
As angle of attack is increased, the production of lift is increased. More upwash is created ahead of the airfoil as the leading edge stagnation point moves under the leading edge, and more downwash is created aft of the trailing edge. Total lift now being produced is perpendicular to relative wind. In summary, the production of lift is based upon the airfoil creating circulation in the airstream (Magnus Effect) and creating differential pressure on the airfoil (Bernoulli’s Principle).
NEWTON’S THIRD LAW OF MOTION
Additional lift is provided by the rotor blade’s lower surface as air striking the underside is deflected downward. According to Newton’s Third Law of Motion, “for every action there is an equal and opposite reaction,” the air that is deflected downward also produces an upward (lifting) reaction.
Since air is much like water, the explanation for this source of lift may be compared to the planing effect of skis on water. The lift which supports the water skis (and the skier) is the force caused by the impact pressure and the deflection of water from the lower surfaces of the skis.
Under most flying conditions, the impact pressure and the deflection of air from the lower surface of the rotor blade provides a comparatively small percentage of the total lift. The majority of lift is the result of decreased pressure above the blade, rather than the increased pressure below it.