Lift and Drag

ift and drag are two of the four forces that act on an airplane. (The other two are thrust and weight.) Lift and drag are aerodynamic forces, which means that they are produced by the effect the shape of an airplane has on the air it is traveling through. Lift is an upward force produced by the shape of an airplane’s wings. Drag is a back­ward force produced by air resisting the movement of an airplane traveling through it. Lift and drag are both vec­tors, or quantities with size and direction.

Lift and drag are closely linked. Lift is necessary for flight, but drag tries to stop a plane from flying by slowing it down. More speed means more lift, but it also means more drag. These two forces are in constant conflict. Lift and drag is useful in determining how effi­cient an airplane is.

_________________________________________________ /

Lift-to-Drag Ratio

To figure out the efficiency of aircraft, designers and engineers use a measure­ment that combines and compares lift and drag. This measurement is called the lift-to-drag ratio. (A ratio is a pair of numbers that show how two quantities are related to each other.)

The lift-to-drag ratio also is known as the L/D ratio, or just “L over D.” It is simply the amount of lift divided by the amount of drag. As the second number of the ratio is usually one, it is some­times omitted and only the first number is given. The better a plane is at produc­ing lots of lift with little drag, the bigger the L/D number is. The best gliders have an L/D of 60 or 70, compared to an air­liner’s L/D of about 15. Birds, such as the albatross, that are good at gliding for long distances, also have a high L/D.