Ground effect is observed at what altitude relative to rotor span?

• A. More than the rotor span
• B. Less than a quarter of the rotor span
• C. Less than half the rotor span
• D. No fixed altitude; it varies with rotor design.

Answer: (C)

Ground effect is the lift-boosting influence of the ground on the rotor’s downwash when the rotor is close to the surface. The practical boundary used in this context is about one rotor radius above the ground, because the interference effects fade as you move higher away.

Since rotor span is the rotor’s diameter, one rotor radius equals half the rotor span. Therefore, being at an altitude less than half the rotor span places you in ground effect, where the rotor operates with higher lift efficiency and lower induced losses.

Being higher than roughly that distance moves you out of ground effect, with the induced power rising again. While some variation with rotor design and operating conditions exists, the standard guideline used here is within half the rotor span.