Definition
The ratio of the lift produced by an airfoil or aircraft to the drag it produces at a given angle of attack. L/D is a measure of aerodynamic efficiency: the higher the ratio, the more lift the aircraft generates for each unit of drag. Every aircraft has a specific angle of attack at which L/D is highest, called L/D max, and this angle produces the greatest gliding distance and the most efficient flight.
Plain English
A number that tells you how much lift the wing is giving you compared to how much drag is holding you back. A bigger number means the airplane slips through the air more efficiently and can glide further.
Context Anchor
Seen in glide discussions, especially when learning best glide speed and how far an airplane can travel after a power loss.
Derivation
Simply 'lift divided by drag.' The slash is read as 'to' or 'over.' If the wing produces 10 pounds of lift for every 1 pound of drag, the L/D is 10 (or 10:1).
Why Pilots Care
The L/D ratio directly determines glide range. Maintaining the airspeed for best L/D allows a pilot to reach the farthest possible landing site after an engine failure.
Grounding Statement
If the airplane has a better L/D in a glide, it travels more forward distance for each foot of altitude it loses.
Intuition Check
L/D does not mean the airplane simply has “more lift.” It means lift compared with drag; the useful idea is the balance between the two.
Example Sentence 1
After the engine failed, the pilot pitched for best-glide speed to fly at L/D max and stretch the glide toward the nearest field.
Example Sentence 2
The flight manual lists the airspeed that yields the highest L/D so the pilot can plan the glide to a suitable field.