Definition
The lift-drag ratio (L/D) is the comparison of the lift produced by an airfoil or aircraft to the drag it produces at a given angle of attack and airspeed. A higher ratio means the wing is producing more lift for each unit of drag, which is the measure of aerodynamic efficiency. The ratio changes with angle of attack, and each aircraft has a specific angle of attack that produces its maximum lift-drag ratio (L/D max).
Plain English
It's a way of measuring how efficient a wing is. For every bit of drag the wing creates, how much lift do you get back? The bigger that number, the more efficiently the airplane is flying.
Context Anchor
Seen when discussing flap effectiveness, glide performance, and airplane efficiency. In this chapter, it helps explain why flaps can add drag faster than they add useful lift.
Why Pilots Care
A higher lift-drag ratio improves glide distance, fuel economy, and the ability to reach a suitable landing site after an engine failure.
Analogy
Think of it like fuel economy in a car: miles per gallon tells you how far you go for each unit of fuel burned. L/D tells you how much lift you get for each unit of drag paid.
Intuition Check
Do not read lift-drag ratio as simply “how much lift the airplane has.” It is a comparison between lift and drag, so an airplane can produce more lift and still have a worse lift-drag ratio if drag increases even more.
Example Sentence 1
With the flaps fully extended, the lift-drag ratio drops, so the airplane descends at a steeper angle on final approach.
Example Sentence 2
At the published best-glide speed the aircraft reaches its maximum lift-drag ratio and covers the greatest distance over the ground.