A glider's best glide speed corresponds to an L/D ratio of what?

A glider's best glide speed corresponds to an L/D ratio of what?

• A. 10 to 1
• B. 15 to 1
• C. 20 to 1
• D. 25 to 1

Answer: (C)

Best glide speed is the airspeed at which the lift-to-drag ratio is maximized, so the glider can travel the farthest horizontally for a given loss of altitude. The lift-to-drag ratio, L/D, directly determines the glide distance: the higher the ratio, the flatter the glide and the more distance covered per unit of altitude lost. At the speed where L/D is at its maximum, you get the smallest possible descent angle and the greatest glide distance. In this context, the maximum L/D is about 20 to 1, meaning for every unit of altitude you lose, you can move about twenty units horizontally. The resulting glide angle is roughly arctan(1/20), about 3 degrees, a very shallow descent. If you go faster, drag due to skin and form increases (lowering L/D); if you go slower, induced drag from the wing’s lift increases (also lowering L/D). So the best glide speed corresponds to an L/D ratio around 20 to 1.

Best glide speed is the airspeed at which the lift-to-drag ratio is maximized, so the glider can travel the farthest horizontally for a given loss of altitude. The lift-to-drag ratio, L/D, directly determines the glide distance: the higher the ratio, the flatter the glide and the more distance covered per unit of altitude lost. At the speed where L/D is at its maximum, you get the smallest possible descent angle and the greatest glide distance.

In this context, the maximum L/D is about 20 to 1, meaning for every unit of altitude you lose, you can move about twenty units horizontally. The resulting glide angle is roughly arctan(1/20), about 3 degrees, a very shallow descent. If you go faster, drag due to skin and form increases (lowering L/D); if you go slower, induced drag from the wing’s lift increases (also lowering L/D). So the best glide speed corresponds to an L/D ratio around 20 to 1.