Helicopter Flight Information

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Ground Effect - What is it, and how does it work?

I had a friend who failed his CFI oral in part because he stated that ground effect was a cushion of air beneath the helicopter. I am sure there were some other issues, however this was one reason noted for his failure. You will hear this cushion explanation many times in your career, and although the sensation of ground effect seems like a cushion, the cushion statement in and of itself is not definitive enough for a CFI applicant...

In helicopters, ground effect results for three basic reasons (actually number 3 below is the result of number 2):

1. An increase in the angle of attack of the rotor blades due to: In close proximity to the ground (most noticeably within 1/2 rotor diameter), the ground interferes with the down-flow of air beneath the rotor system (the air begins to stack up); this results in an increase in the angle of attack on the main rotor blades which results in a necessary decrease in collective pitch to maintain a given hover height above ground.

2. A reduction in the size of rotor tip vortice: Due to ground interference with the rotor down-wash,the vortices that encompasses the rotor blade tip are reduced in size relative to the helicopter height above ground. If you are hovering 10-feet above ground, the vortice will be larger than if you were hovering 3-feet above ground. The consequence of this decrease in vortice size is number 3 below.

3. An increase in the effective area of the rotor disk: As the size of the vortice is decreased, the portion of the blade producing lift is increased which results in a necessary decrease in collective pitch to maintain a given hover height.

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Last modified: 01/19/08