Textbook

Lift

To understand the lift of an airfoil let's apply Newton's Third Law of Motion. The airfoil deflects the air going over the upper surface downward as it leaves the trailing edge of the wing. According to Newton's Third Law, for every action there is an equal, but opposite reaction. Therefore, if the airfoil deflects the air down, then the air deflects the airfoil up - the resulting opposite reaction is an upward push. Deflection is an important source of lift. Planes with flat wings, rather than cambered, or curved wings, must tilt their wings to deflect the flow. You saw this deflection in the tennis ball wind tunnel test.

An alternate explanation focuses on Bernoulli's Law. Bernoulli's Law says that for an increase in velocity there is a decrease in pressure and a decrease in velocity will have an increase in pressure.

On a typical wing there is more movement of the air towards the top surface because of its curved shape. More air needs to pass through the same space on the top of the wing. (Basically, the flow is squashed on the top side of the wing.) This means there needs to be a higher velocity on the top side of the wing, and a lower velocity on the underside of the wing.

On the underside of the wing this lower velocity creates a higher pressure and at the top the higher velocity creates a lower pressure. (This is the application of Bernoulli's Law.) With high pressure on one side and low pressure on the other there is an imbalance in the forces on the wing. In the case of the airplane wing the lower pressure on the top generates lift. But, on an airplane's wing, its not that the pressure on the underside is pushing the wing up, its that the low pressure on the topside is so strong that the wing is actually being being pulled up as if you held a small piece of paper in front of a vaccuum cleaner.

How can you create more lift? A pilot can increase lift by changing the angle of attack, or tilting the leading (front) edge of the wing up. This is how the Wright brothers were able to get off the ground. They tilted the wings of their flyer to create lift.

This strategy can be used for either cambered or symmetric wings. This is why an airplane rotates (pitches its nose up) slightly at takeoff; the pilot is increasing the angle of attack to generate more lift. There is a limit to how much lift can be generated, however, and angling too much can result, paradoxically, in a drastic drop of net lift force.

Lift is also used by race car designers who have created airfoil-like surfaces to generate "negative lift", or downward-directed force. This force, combined with the weight of the race car, helps the driver maintain control in the high-speed curves of the race track.

What you'll see as we explain lift on a tennis ball is that underspin acts like an airplane's wing and topspin acts like the race car design with "negative lift".

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