At what altitude would the aircraft be in ground effect?

• A. Between 1 and 2 rotor diameters
• B. Greater than 2 rotor diameters
• C. At any altitude
• D. Less than 1 rotor diameter

Answer: (D)

Ground effect is the phenomenon where the rotor’s downwash is influenced by the nearby surface, which reduces the induced power needed to generate lift. This interference is strongest when the rotor is very close to the ground and generally persists up to about one rotor diameter above the surface. Within this zone, lift is more efficient and hover or low-altitude flight requires less power. Once you rise above roughly one rotor diameter, the ground’s influence diminishes and you’re out of ground effect, meaning the rotor must work harder (more induced power) to maintain the same lift. So, ground effect occurs when the aircraft is at an altitude less than one rotor diameter.