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I was looking at a helicopter performance chart and it listed two values in feet, identified as "Hover in ground effect (HIGE)" and "Hover outside of ground effect (HOGE)."

What do these numbers mean, and why are they significant?

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Ground effect is a condition where the performance of the aircraft increases while operating near a firm surface. In general, a helicopter is considered to be in ground effect when it is hovering less than one-half of its rotor diameter from the ground and the effects become negligible when the helicopter is more than a rotor diameter above the ground.

While hovering near the ground, the induced velocity at the rotor is reduced, which reduces the power requirement for a given rotor thrust (lift); equivalently, the rotor thrust is increased for given power due to ground proximity. Also, near ground, the rotor tip vortex is reduced.

Due to this, the helicopter can hover at higher gross weight (equivalently, at higher altitude for given weight) in ground effect (HIGE) compared to out of ground effect (HOGE).

Ground effect

Image from copters.com

Usually, the altitude at which a helicopter can hover at a given weight (and atmospheric conditions) is given for both HIGE and HOGE conditions. HOGE is much more restrictive compared to the HIGE. For example, the following image compares the hoer performance of R44 helicopter in and out of ground effect.

R44 HOGE

Data from helicopterforum.verticalreference.com and aneclecticmind.com

In a sense, HOGE represents the absolute limit of the helicopter performance and it should be considered for operational calculations where there is no availability of rigid surfaces for vertical takeoff and landing or their presence is not confirmed.

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