Definition
The ratio of the lift produced by an airfoil or airplane to the drag produced at a given angle of attack and airspeed. It is a measure of aerodynamic efficiency: the higher the L/D, the more lift the airplane generates for each unit of drag it must overcome. Each airplane has a specific angle of attack at which L/D is at its maximum, called L/Dmax, where the airplane is operating most efficiently.
Plain English
How much lift the airplane gets for the amount of drag it has to fight. A higher number means the airplane is flying more efficiently. Every airplane has one sweet-spot angle where this number is at its best.
Context Anchor
Seen in performance, glide, and aerodynamic discussions, especially when comparing airplane efficiency or finding best glide performance.
Derivation
The letters come from common aerodynamic symbols: L for lift and D for drag. The slash means “divided by,” so L/D literally means lift divided by drag.
Why Pilots Care
A higher L/D ratio directly increases glide distance after engine failure and reduces fuel burn in cruise.
Grounding Statement
An airplane has a high L/D when it can keep producing lift while wasting relatively little energy overcoming drag.
Intuition Check
Do not read L/D as “lift and drag” separately. It means lift compared to drag, as a ratio.
Example Sentence 1
After the engine quit, the pilot pitched for best-glide speed to fly at L/Dmax and stretch the glide as far as possible.
Example Sentence 2
The pilot checked the POH to confirm the L/D ratio used when planning a power-off approach after an engine failure.