There are four forces that act on an aircraft in flight: lift, weight, thrust, and drag. The motion of the aircraft through the air depends on the size of the various forces. The weight of an airplane is determined by the size and materials used in the airplane's construction and on the payload and fuel that the airplane carries. The thrust is determined by the size and type of propulsion system used on the airplane and on the throttle setting selected by the pilot. Lift and drag are aerodynamic forces that depend on the shape and size of the aircraft, air conditions, and the flight velocity.
Because lift and drag are both aerodynamic forces, the ratio of lift to drag is an indication of the aerodynamic efficiency of the airplane. Aerodynamicists call the lift to drag ratio the L/D ratio, pronounced "L over D ratio." An airplane has a high L/D ratio if it produces a large amount of lift or a small amount of drag. Because under cruise conditions lift is equal to weight, a high lift aircraft can carry a large payload. And because under cruise conditions thrust is equal to drag, a low drag aircraft requires low thrust. Thrust is produced by burning a fuel. To generate high thrust, large amounts of fuel are burned. A low thrust aircraft requires low fuel usage. As discussed on the maximum flight time page, low fuel usage allows an aircraft to stay aloft for a long time, and that means the aircraft can fly long range missions. For gliders (aircraft with no engines), a high L/D ratio again produces a long range aircraft by reducing the steady state glide angle.
As shown in the middle of the slide, the L/D ratio is also equal to the ratio of the lift and drag coefficients. The lift coefficient is described on a separate page, and the drag coefficient is quite similar. Lift and drag coefficients are normally determined experimentally using a wind tunnel. But for some simple geometries, they can be determined mathematically.
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