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, the third is a result of the second):
- 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 preventing a free flow below the helicopter, the air begins to stack up so-to-speak; this results in an increase in the angle of attack on the main rotor blades.
- 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, etc.
- 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.
As the above items come into play, it becomes necessary to lower the collective to maintain a given height above ground.
It is a fact that some pilots can hover a given aircraft at a lower power setting than other pilots; or in other words, hover a heavy aircraft where another pilot couldn't. How could this be? If you think about ground effect as described above, and think about the process of hovering a helicopter, which involves thinking faster than the helicopter, where the pilot is constantly making small corrections to keep the helicopter in a given position over the ground. Then it is easy to understand that a pilot who is smoother on the controls or able to react faster to movements which results in smaller cyclic inputs can accomplish this feat. Still don't grasp it? When you make cyclic inputs in the hover, the rotor disk is moving about blowing the ground effect "so called cushion of air" out from under the rotor disk. END.