Why is more power required to hover OGE rather than IGE?

Why is more power required to hover OGE rather than IGE?

• A. Decreased induced velocity
• B. Increased induced flow velocity and larger rotor tip vortexes
• C. Higher air density
• D. Reduced rotor torque

Answer: (B)

The key idea is that hover power is dominated by induced power, which depends on the induced velocity through the rotor disk. In ground effect, the ground interferes with the rotor wake, reducing the induced velocity and making it easier for the rotor to push air downward, so less power is needed to generate the same thrust. When you’re out of ground effect, that interference is gone, so the downwash expands more freely and the induced velocity must increase to produce the same thrust. That higher induced velocity means more energy is being added to the airflow, i.e., more induced power. The wake also shows larger rotor tip vortices out of ground effect because the wake is less confined. So more power is required to hover OGE than IGE because of increased induced flow velocity and larger rotor tip vortices.

The key idea is that hover power is dominated by induced power, which depends on the induced velocity through the rotor disk. In ground effect, the ground interferes with the rotor wake, reducing the induced velocity and making it easier for the rotor to push air downward, so less power is needed to generate the same thrust. When you’re out of ground effect, that interference is gone, so the downwash expands more freely and the induced velocity must increase to produce the same thrust. That higher induced velocity means more energy is being added to the airflow, i.e., more induced power. The wake also shows larger rotor tip vortices out of ground effect because the wake is less confined. So more power is required to hover OGE than IGE because of increased induced flow velocity and larger rotor tip vortices.