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IGE, OGE and Recirculation


Wrote for the Helicopter History Site by Glenn Beare

In Ground Effect (IGE) is a condition where the downwash of air from the main rotor is able to react with a hard surface (the ground), and give a useful reaction to the helicopter in the form of more lift force available with less engine power required.

What is occuring is the air is impacting with the ground and causing a small build up of air pressure in the region below the rotor disk. The helicopter is then "floating" on a cushion of air. This means that less power is required to maintain a constant altitude hover. IGE conditions are usually found within heights about 0.5 to 1.0 times the diameter of the main rotor. So if a helicopter has a rotor diameter of 48ft, the IGE region will be about 24 - 48ft above the ground. The height will vary depending on the type of helicopter, the slope and nature of the ground, and any prevailing winds

Out of Ground Effect (OGE) is the opposite to the above, where there are no hard surfaces for the downwash to react against. For example a helicopter hovering 150ft above the ocean surface will be in an OGE condition and will require more power to maintain a constant altitude than if it was hovering at 15ft. Therefore a helicopter will always have a lower OGE ceiling than IGE due to the amount of engine power available.

Published performance figures for a given helicopter may state something like:

Hover Ceiling at Max Weight = 4000ft OGE and 6000ft IGE.

This means that the fully loaded helicopter can hover at 4000ft above the ocean (ie. no hard surfaces close below), and can hover at 6000ft above a tall mountain top where there is the ground close below (within 0.5 - 1.0 rotor diameters). Mountains this high are common in Papua New Guinea for example.

Recirculation is a condition which can occur during a low hover in ground effect. Imagine the airflow which was directed to the ground to create the air cushion in a ground effect is now rebounding off the ground and going back up into the top of the rotor system. When it passes back through the rotor again it gets accelerated. This process may continue with the air velocity increasing each time it passes through the rotor. Eventually the velocity is so great that the air going into the rotor from above causes a loss of lift and the helicopter will sink toward the ground unless the pilot increases power. This means that if recirculation is occuring, the helicopter will need more power to hold a constant height. Recirculation will not always happen but will be aggravated by the type of ground or nearby obstacles causing the air which is trying to escape out to the sides of the helicopter to be directed back up toward the rotor system. The result is a "recirculation" of downwash air

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