Lift is the upward force created by an airfoil when it is moved through the air. Although lift may be exerted to some extent by many external parts of the airplane, there are three principal airfoils on an airplane—the wing, propeller, and horizontal tail surfaces.
To understand how an airplane wing produces lift, Bernoulli’s Principle
and one of Newton’s Laws should be reviewed.
Bernoulli’s Principle states in part that “the internal pressure of a fluid (liquid or gas) decreases at points where the speed of the fluid increases.” In other words, high speed flow is associated with low pressure, and low speed flow with high pressure.
This principle is made apparent by changes in pressure of fluid flowing within a pipe where the inside diameter of the pipe decreases, similar to a venturi tube. [Figure 1-9]
In the wide section of the gradually narrowing pipe, the fluid flows at a lower speed, producing a higher pressure. As the pipe narrows, it still contains the same amount of fluid; but because the passageway is constricted, the fluid flows at a higher speed producing a lower pressure. This principle is also applicable to an airplane wing, since it is designed and constructed with a curve or camber. [Figure 1-9] When air flows along the upper wing surface, it travels a greater distance than the airflow along the lower wing surface. Therefore, as established by Bernoulli’s Principle, the pressure above the wing is less than it is below the wing, generating a lift force over the upper curved surface of the wing in the direction of the low pressure.
Since for every action there is an equal and opposite reaction (Newton’s Third Law of Motion), an additional upward force is generated as the lower surface of the wing deflects the air downward. Thus both the development of low pressure above the wing and reaction to the force and direction of air as it is deflected from the wing’s lower surface contribute to the total lift generated. [Figure 1-10]
The amount of lift generated by the wing depends upon several factors:
• speed of the wing through the air,
• angle of attack,
• planform of the wing,
• wing area, and
• the density of the air.
Lift acts upward and perpendicular to the relative wind and to the wingspan.
Although lift is generated over the entire wing, an imaginary point is
established which represents the resultant of all lift forces. This point
is called center of lift. [Figure 1-11]
This single point is the center of lift, sometimes referred to as the center of pressure. The location of the center of pressure relative to the center of gravity (weight) is very important from the standpoint of airplane stability. Stability will be covered in more detail later.