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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 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).
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