Aircraft Motion
Physics of Aircraft
Weight and Thrust
Secondary Controls
Straight and Level
Aircraft Design Features
The Stall
Practice Exam

Lift/Drag Ratio

Assessing an aerofoil is a matter of looking at both the lift and drag it creates. An aerofoil with very high lift may also have very high drag and one with low drag may also have low lift!

The efficiency of an aerofoil really depends on the lift to drag ratio. An aerofoil that produces 10 times as much lift as it does drag would have a lift to drag ratio (L/D) of 10:1.

Both lift and drag change when we change angle of attack so the lift/drag ratio is not a constant value. Very low angles of attack produce both low lift & low drag, and very high angles of attack produce both high lift & high drag.

The maximum lift/drag ratio occurs at an angle of attack around +4°

We know that any aircraft in level flight must have a value of lift equal to its weight. Since weight does not change in flight, the lift must also be constant if the aircraft continues flying straight and level. So we can say that an aircraft in level flight has a constant value of lift.

If we change airspeed in level flight, we must change our angle of attack in order to keep the lift constant. Slower airspeeds need a higher angle of attack and faster airspeeds need a lower angle of attack.

Therefore, in level flight (constant lift) the best lift/drag ratio must occur at the airspeed for minimum total drag (VMD).

For a given weight, the minimum total drag angle of attack will occur at a particular airspeed (VMD). Flying at this minimum drag airspeed (VMD) results in:

Maximum gliding distance (in nil wind)
Maximum range (in nil wind)
Note that the minimum total drag occurs at a given angle of attack, but the airspeed at this angle of attack will change as weight changes. Heavier aircraft will fly faster at this angle of attack and lighter aircraft will fly slower.
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