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Ground effect reduces the induced drag of a wing principally because the ground plane

  • A.Increases the skin friction on the lower surface
  • B.Raises the free-stream dynamic pressure beneath the wing
  • C.Removes the bound circulation entirely
  • D.Weakens the downwash by constraining the trailing vortex system

Answer: D

Close to the ground the trailing vortices cannot develop normally, an effect modelled by an image vortex system below the surface. The downwash at the wing is reduced, the lift vector is tilted less, and the induced drag falls. Pilots notice the aircraft floating during the landing flare for exactly this reason.

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