Ground effect is most pronounced when the aircraft is at what height above the surface?

• A. Less than the wingspan
• B. More than two wingspans
• C. At least one wingspan
• D. On the surface

Answer: (A)

Ground effect is strongest when the aircraft is very close to the surface because the ground disrupts the wingtip vortices and the downwash behind the wing. When you’re within roughly a wingspan of the surface, this interference reduces the downwash and induced drag, so the wing produces more lift for the same angle of attack. That means you can fly with a higher lift-to-drag ratio and often at a lower airspeed or with less power. As you climb higher than about one wingspan, the ground’s influence fades and the aircraft behaves more like it does in free air. So the height at which ground effect is most pronounced is less than a wingspan above the surface.