Airfoils

An airfoil is any surface, such as a wing, which provides aerodynamic force when it interacts with a moving stream of air.

Circulation of the airstream about the airfoil is an important factor in the generation of lift. Circulatory flow affects the pressure distribution on an airfoil. The physical principles just discussed help explain the circulation of air around a wing and the pressures distribution on the wing's surface. A combination of the forces described by these principles create the total lift generated by an airfoil.

The airplane wing's shape is designed to take advantage of both Newton's laws and Bernoulli's principle. The greater curvature on the upper portion of an airfoil causes air to accelerate as it passes over the wing.

According to Bernoulli's theorem, the increase in speed of air on the top of an airfoil produces a drop in  pressure and this lowered pressure in a component of total lift.

In addition to the lowered pressure, a downward-backward flow of air also is generated from the top surface of the wing. The reaction to this downwash results in an upward force on the wing which demonstrates Newton's third law of motion. This action/reaction principle also is apparent as the airstream strikes the lower surface of the wing when inclined at a small angle (the angle of attack) to its direction of motion. The air is forced downward and therefore causes an upward reaction resulting in positive lift.

The coefficient of lift (Cl) is a way to measure lift as it relates to angle of attack. Cl is determined by wind tunnel tests and is based on airfoil design and angle of attack. Every airplane has an angle of attack where maximum lift occurs.

This page was last modified June 11, 2000
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