Introduction to Glider Flying > Aerodynamics of Flight > Forces of Flight > Lift > Bernoulli’s Principle

An airfoil with a positive angle of attack develops air circulation as its sharp trailing edge forces the rear stagnation point to be aft of the trailing edge, while the front stagnation point is below the leading edge. [Figure 3-5]

Figure 3-5. Stagnation points on an airfoil.

Air flowing over the top surface accelerates. The air-foil 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 3-6]

Figure 3-6. The upper surface of an airfoil is similar to the constriction in a venturi tube.

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 pres-sure 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 3-7]

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 perpen-dicular 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).