Definition
The ratio of the lift produced by an airfoil or aircraft to the drag it generates at a given angle of attack and airspeed. It is a measure of aerodynamic efficiency: the higher the ratio, the more lift the aircraft produces for each unit of drag. Each aircraft has a specific angle of attack at which this ratio reaches its maximum value, written as L/Dmax, which corresponds to the most efficient flight condition and the best glide performance.
Plain English
How much lift the wing gives you compared to how much drag it costs you. A higher number means the aircraft is gliding more efficiently — getting more useful flight out of less air resistance.
Context Anchor
Seen in glide performance discussions, especially when learning why one glide speed gives the greatest distance over the ground in still air.
Why Pilots Care
The airspeed that produces the maximum lift-to-drag ratio delivers the greatest glide distance and is the target speed during an emergency glide after power loss.
Analogy
Think of it like fuel economy for a glider: more miles for less effort. A high lift-to-drag ratio means the wing carries the airplane a long way for a small cost in drag.
Grounding Statement
In a glide, the airplane is always coming down, but a better lift-to-drag ratio lets it move farther forward for each bit of height it loses.
Intuition Check
Do not read this as simply “more lift is better.” Lift-to-drag ratio is a balance: the useful lift is being compared against the drag that resists the airplane’s forward motion.
Example Sentence 1
After the engine quit, the pilot pitched for the airspeed that gave the best lift-to-drag ratio to stretch the glide toward the nearest airport.
Example Sentence 2
The instructor demonstrated that a clean airplane configuration improves the lift-to-drag ratio and therefore increases glide range.