What is ground effect in helicopter flight?

• A. An increase in lift when altitude is less than half the rotor span.
• B. An increase in air drag near the ground.
• C. A phenomenon that only occurs at high speeds.
• D. An increase in tail rotor thrust near the ground.

Answer: (A)

Ground effect is the increase in lift and overall efficiency of the rotor system when the rotor disk is close to the surface. The presence of the ground interferes with the rotor’s downwash, creating a cushion of higher-pressure air beneath the rotor that reduces induced velocity and the energy lost in producing downwash. Because of this interference, a given amount of power can produce more lift when you are near the ground than you can higher up.

In practical terms, this effect is strongest while you’re within a relatively short distance of the surface—often described as within about a rotor diameter, with many references noting noticeable effects when the altitude is within roughly half the rotor span. That proximity means you’ll see an easier hover and less power required to stay afloat, which is why the statement focuses on an increase in lift when altitude is very close to the ground.

The other options don’t describe what ground effect does. It’s not mainly about drag near the ground, it isn’t restricted to high speeds, and while tail rotor thrust can be influenced indirectly, the fundamental phenomenon is the lift increase from the rotor–ground interaction.