FAA Part 107 Remote Pilot (UAG)Loading and PerformanceEasy
A multirotor pilot slowly lifts off and hovers about 2 feet above a smooth concrete pad before climbing higher. During this brief low-altitude hover, the aircraft seems to require less power than it does once it climbs above one rotor-diameter in height. What causes this?
- ATranslational lift increases near the ground
- BDensity altitude is lower near the ground
- CLoad factor decreases near the ground
- DGround effect increases lift efficiency near the surface
Show answer & explanationAnswer & explanation
Correct answer: D. Ground effect increases lift efficiency near the surface
Ground effect occurs when rotor downwash interacts with a nearby surface, reducing induced drag and increasing lift efficiency, so less power is needed to hover. This effect typically diminishes once the aircraft climbs beyond about one rotor diameter above the surface.
Why the other options are wrong
- A. Translational lift applies to forward airspeed transitioning from hover, not surface proximity.
- B. Density altitude relates to air temperature/pressure/altitude, not surface proximity.
- C. Load factor relates to bank angle and G-forces, not hover height.
Ground Effect
A cushion of increased lift efficiency experienced by rotorcraft when hovering close to the ground, caused by reduced induced drag from disrupted downwash.
- Most pronounced within about one rotor diameter of the surface
- Allows more lift for the same power (or less power for the same lift)
- Diminishes and disappears as altitude increases beyond ground effect zone
Memory trick: Close to the ground, the drone rides an invisible air cushion.