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When comparing a hover in ground effect with a hover out of ground effect, the helicopter has (1) _____, but (2) _____ in hover out of ground effect.

  • A

    (1) The same total rotor thrust; (2) The same rotor required power.

  • B

    (1) The same total rotor thrust; (2) Increased rotor required power.

  • C

    (1) Increased total rotor thrust; (2) The same rotor required power.

  • D

    (1) Increased total rotor thrust; (2) Increased rotor required power.

Refer to figure.

In a still air hover, the airflow through the rotor disc begins at zero velocity above the disc and is accelerated by the rotors through the disc into the air below. Acceleration starts well above the rotor and continues through and slightly beyond it until the air is outside the influence of the high pressure region under the blades. Thereafter the airflow diverges and slows down as it loses energy to the surrounding air.

When the helicopter hovers close to the ground, the downwash meets the ground and is forced to diverge more sharply, this increases the pressure in the wake below the helicopter. The increased pressure opposes and thus reduces the induced flow.

When hovering out of ground effect the downwash and the ground is no longer supporting the helicopter. Therefore, the Induced flow increases which requires a higher angle of attack to produce the same thrust which requires additional power applied.

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