Power Off Descent
This diagram shows an aircraft that is in a power off descent (a glide). Since there is no power, there is no thrust and we now have only three forces acting on the aircraft.
In a steady power off descent, the combined forces of lift & drag oppose the weight and the aircraft descends at a constant speed down a straight line.
The gliding performance of an aircraft is determined by the angle between its descent path and the ground.
L/D Ratio
Since there is zero thrust and weight will not change, the glide performance of the aircraft is determined by the two remaining forces – lift and drag. The ratio of lift to drag will determine the aircraft’s glide performance – this is known as the L/D Ratio. The L/D ratio is effectively a measurement of the wing’s efficiency.
If lift is high compared to drag, the L/D ratio will be high and the glide angle will be shallow.
If lift is low compared to drag, the L/D ratio will be low and the glide angle will be steep.
Maximum L/D Ratio
As we know, the pilot can alter both lift and drag by changing angle of attack and speed. In a power off descent, the pilot controls both angle of attack and speed with the pitch attitude. Therefore, the L/D ratio will change as the pilot changes pitch attitude in a power off descent.
Your aircraft flight manual will list a best glide speed. This is the speed at which the aircraft has the maximum L/D ratio and the aircraft will glide the furthest.
