Definition
The ratio of the lift produced by an airfoil or aircraft to the drag it produces at a given angle of attack. It is a measure of aerodynamic efficiency: a higher L/D means more lift is generated for each unit of drag. The maximum value (L/D max) occurs at one specific angle of attack and corresponds to the most efficient flight condition for that aircraft.
Plain English
It is a number that tells you how much lift the wing gives you for the amount of drag it costs you. The bigger the number, the more efficiently the aircraft flies.
Context Anchor
Seen in discussions of gliding performance, airplane efficiency, best glide speed, and performance planning.
Derivation
A straightforward ratio of two forces — lift divided by drag. The form 'L/D' is read aloud as 'lift over drag.' Knowing it is literally a division helps: an L/D of 10 means ten pounds of lift for every one pound of drag.
Why Pilots Care
Determines the best glide speed, maximum range, and fuel efficiency; operating near the peak ratio allows the aircraft to travel farther with less power or in an engine-out situation.
Grounding Statement
In a glide, an aircraft with a 10:1 L/D can travel about 10 units forward for each 1 unit it descends, assuming still air.
Intuition Check
Do not read L/D as simply “more lift is better.” The ratio matters because extra lift that also creates too much drag may not improve performance.
Example Sentence 1
After the engine quit, he pitched for best glide speed to fly at the aircraft's maximum L/D and stretch the glide to the nearest field.
Example Sentence 2
At cruise, the aircraft is flown at the speed that yields its highest lift/drag ratio for maximum fuel economy.