Definition
The lift-to-drag ratio is the amount of lift an airfoil or airplane produces compared to the amount of drag it generates at a given angle of attack. It is calculated by dividing lift by drag, and it expresses how efficiently the airplane converts forward motion into useful lift. The angle of attack that produces the highest L/D value is called L/Dmax, and at that angle the wing is operating at its most aerodynamically efficient point.
Plain English
It is a number that tells you how much lift the airplane is getting for the drag it has to push through. A higher number means the airplane is flying more efficiently — more lift for less drag.
Context Anchor
Seen in glide discussions when choosing the speed that gives the airplane its best glide distance.
Derivation
The ratio is written as a fraction — lift on top, drag on the bottom — so a wing producing 10 units of lift for every 1 unit of drag has an L/D of 10. The shorthand 'L/D' is read aloud as 'L over D.'
Why Pilots Care
The highest L/D determines the speed that gives maximum glide distance in an engine-out situation, directly affecting how far a pilot can reach a suitable landing site.
Analogy
Think of rolling a bicycle downhill. If it rolls smoothly with little resistance, it goes farther from the same hill height. Best L/D is the airplane’s most efficient glide condition.
Intuition Check
Do not read L/D as simply “more lift is better.” It means lift compared with drag; the best value is the balance that gives the most glide distance.
Example Sentence 1
After the engine failed, the pilot pitched for best glide speed to fly at L/Dmax and stretch the glide as far as possible.
Example Sentence 2
A sailplane's high L/D ratio lets it travel many miles while gradually descending between rising air currents.