Textbook

Lift

Lift is generated differently on an aircraft than on a tennis ball. The spin of the tennis ball is responsible for its lift. Fortunately for the passengers and crew, aircraft don't have to spin to generate lift. We'll explain how lift is generated on an airplane and then for the tennis ball.

Lift is the force that pushes an airplane or a bird up against the weight and is created by the movement of the air around the wings. (The lift created by the body or tail is small.) The word "lift" is a little misleading, because one meaning of the word is "to rise". On an airplane normally that's "up". But in aerodynamics lift has a very specific definition and is not always "up" as you'll see.

How can you increase lift? What factors affect the amount of lift a vehicle can obtain? The weight and direction of the aircraft determines how much lift is needed. When going up, lift must be greater than weight. In level flight, lift must equal weight, and going down lift must be less than weight.

The equation used for lift is:

where C_l is the coefficient for lift, the Greek letter rho (which looks like the script letter "p" is the density of the air, V² is the velocity squared (velocity multiplied by velocity) and S is the "projected area" (the area that you see when looking at the object upstream). For a sphere, like the tennis ball, S is equal to (pi)r². Values for C_l are calculated experimentally and generally published in tables. It is fairly typical to look up the C_l value from a table based on either the spin rate or Reynolds number.

The velocity of the object is the most important element in producing lift. If the velocity of the airplane is increased, the lift will increase dramatically. If you double the velocity, you get 4 times the lift; if you triple the speed and you get 9 times the lift.

The figure below shows two streamlines around a typical airfoil (or wing); one travels over the top of the airfoil, the other moves underneath it.

The shape of the airfoil (wing) is important for lift, and is designed carefully. Most airfoils today have camber, or curved upper surfaces and flatter lower surfaces. These airfoils generate lift even when the air flow is horizontal (flat). The Wright brothers used symmetric (both sides were a mirror image of one another) airfoils in their airplane design. (A tennis ball is symmetric.) Since the upper and lower surfaces were the same the pressures on either surface (top or bottom) are the same, so the net combined force on the airfoil is zero and there is no lift! How, then, did the Wright brothers get their airplane off the ground? How does the symmetric tennis ball generate lift?

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