Definition
The numerical relationship between the lift produced by an airfoil or aircraft and the drag it generates at a given angle of attack and airspeed. It is calculated by dividing total lift by total drag and is a direct measure of aerodynamic efficiency.
Plain English
A number that shows how much lift an aircraft gets compared to how much drag it has to push through. A higher number means the aircraft flies more efficiently.
Context Anchor
Seen in aircraft performance, wing design, glide performance, and discussions of aerodynamic efficiency.
Derivation
L stands for lift, D for drag. The slash means 'divided by,' so L/D is simply lift divided by drag — a ratio expressing how much lift each unit of drag is producing.
Why Pilots Care
Higher values allow greater range and longer glides, directly affecting fuel use and emergency options.
Analogy
Like a car's miles-per-gallon rating, where a higher number means traveling farther on less effort.
Intuition Check
Do not read L/D ratio as just “lift” or just “drag.” It is a comparison between the two: how much lift is being gained for each amount of drag being paid for.
Example Sentence 1
The pilot pitched for best glide speed to fly at the L/D ratio that would give the longest distance to the nearest airport.
Example Sentence 2
Maintenance teams check wing surfaces to keep the airplane's L/D ratio at its designed level.