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Aircraft A has a lift-to-drag ratio (L/D) of 10:1, and Aircraft B has an L/D of 15:1. Which question correctly compares the two aircrafts' glide characteristics?

  • A

    Aircraft A has a shallower glide angle.

  • B

    Aircraft B has a shallower glide angle.

  • C

    They both have the same glide angle.

  • D

    There is not enough information to ascertain their relative glide profiles.

Glide angle (and glide range) assumes no thrust, and therefore is determined directly by the lift over drag (L/D) ratio. The best L/D ratio occurs at VMD, the speed for minimum drag.

Glide angle is therefore directly proportional to CD / CL

This also means that glide distances are very easy to calculate if you know the Lift-to-Drag ratio.

  • In aircraft A, for instance, if you are flying at 6000 ft, which is effectively 1 nm above the ground, aircraft A has a L/D ratio of 10:1, which becomes a glide ratio of 10:1. For every mile you descend, you will glide 10 miles laterally. If you want to go the maximum distance in that glide, you trim for VMD and you should be able to fly 10 nm in still air.
  • Compare this to aircraft B, which has a greater L/D ratio of 15:1. Intuitively this means it has more lift/less drag, and we should expect that plane to be more efficient at gliding, which it is. If it also needs to glide from 6000 ft, then it should be able to glide 15 nm in still air, which is further.

Therefore, the glide angle of aircraft B is shallower than the glide angle of aircraft A.

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