Bernoulli’s Principle

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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 con-striction in a venturi tube that is narrower in the middle than at the ends. [Figure 2-9]

Figure 2-9. 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 2-10]

Figure 2-10. Lift is produced when there is decreased pres-sure above and increased pressure below an airfoil.

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).