FAA Private Pilot Helicopter (PRH)Performance and LimitationsEasy
A helicopter is hovering 3 feet above a hard-surfaced runway. An instructor points out that less power is required to maintain this low hover than would be required to hover 50 feet above the same surface at the same gross weight. What is the primary reason for this?
- ADensity altitude is always lower near the ground than at higher hover heights
- BGround effect restricts downwash airflow, reducing induced drag and required power
- CThe rotor system is producing increased translational lift near the surface
- DTail rotor thrust increases automatically near the ground, reducing main rotor load
Show answer & explanationAnswer & explanation
Correct answer: B. Ground effect restricts downwash airflow, reducing induced drag and required power
Ground effect occurs when the rotor system is within about one rotor diameter of the surface. The downwash is restricted, which reduces induced drag and the induced airflow angle, allowing the rotor to generate the same lift with less power.
Why the other options are wrong
- A. Density altitude depends on pressure and temperature, not hover height above the surface.
- C. Translational lift requires forward airspeed, not applicable to a stationary hover.
- D. Tail rotor thrust does not automatically increase due to proximity to the ground.
Ground Effect
An increase in rotor efficiency that occurs when hovering within approximately one rotor diameter of the surface, caused by reduced induced drag.
- Effective within about one rotor diameter of the surface
- Reduces induced drag and required power
- Greatest over smooth, hard surfaces; reduced over tall grass, water, or slopes
Memory trick: Close to the ground, the air 'cushions' the rotor — less power needed.