FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsEasy
A helicopter hovers at a skid height less than one rotor diameter above a firm, level surface. Compared to an out-of-ground-effect hover, why does the helicopter require less power to hover in this condition?
- AGround effect reduces induced airflow (downwash), increasing the effective angle of attack of the rotor blades
- BThe rotor RPM automatically increases when close to the ground
- CThe ground surface increases air density beneath the rotor
- DTail rotor thrust requirements are eliminated near the ground
Show answer & explanationAnswer & explanation
Correct answer: A. Ground effect reduces induced airflow (downwash), increasing the effective angle of attack of the rotor blades
Ground effect occurs because the ground surface restricts the induced flow (downwash) beneath the rotor disc, which reduces induced drag and effectively increases the angle of attack for a given blade pitch. This allows the rotor to produce the same lift with less power near the ground compared to a hover out of ground effect.
Why the other options are wrong
- B. Rotor RPM is a pilot/governor-controlled parameter, not automatically changed by proximity to the ground.
- C. Ground effect is an aerodynamic airflow phenomenon, not a change in air density.
- D. Tail rotor thrust is still required in ground effect to counter torque.
Ground Effect
An increase in rotor efficiency when hovering near the surface (typically within about one rotor diameter) due to reduced induced airflow through the rotor disc.
- Most pronounced within about one rotor diameter of the surface
- Reduces induced drag and required power to hover
- Diminishes over soft, uneven, or sloped surfaces and tall grass/water
Memory trick: Close to the ground, the air cushions the blades — less push needed.