Which altitude marks the boundary of ground effect?

• A. One rotor disc diameter above the surface
• B. Two rotor diameters above the surface
• C. One rotor disc diameter measured from rotor disk to surface
• D. Half rotor diameter above the surface

Answer: (C)

Ground effect is the enhanced lift and reduced power required that occurs when the rotor is close enough to the ground for its downwash to be influenced by the surface. The boundary of this effect is reached at a height equal to one rotor diameter from the surface. In other words, when the distance from the rotor disk plane to the ground is about one rotor diameter, the strong ground interaction ends and performance starts behaving more like it does in free air.

The best statement to describe this boundary is the one that explicitly says the distance is measured from the rotor disk to the surface and equals one rotor diameter. The other options propose different distances or less precise phrasing, which do not match the standard definition of the ground-effect boundary.