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Old Dec 13, 2012, 04:13 AM
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Quote:
Originally Posted by richard hanson View Post
Ground effect defined- Just tip over a piece of plywood - note the slow contact with the floor.
Simple explanations are usually better than complicated ones, but good ones accurately capture behavior over a wide range of conditions. The above explanation suggests ground effect results from air having to “move out of the way” of a wing in the same the way it has to move out of the way of a piece of falling plywood.

I suppose this analogy might be used to explain why a plane experiences less drag (and a higher lift curve slope) in ground effect, or why a helicopter needs less power to hover in ground effect.

When you play around with an RC helicopter, you quickly notice that it’s easier to find a power setting for a steady (constant altitude) hover when you are within a rotor diameter or two of the ground (compared to the middle of the room). As the helicopter settles toward the ground, the upward force on the rotor increases, contributing to altitude stability.

If you try to find a power setting for a steady hover within a rotor diameter or two of the ceiling, you will find it MUCH more difficult compared to the middle of the room (try it). As the helicopter rises toward the ceiling, the upward force on the rotor doesn’t decrease, it increases. This contributes to altitude instability.

If you were to try to rapidly push a piece of plywood against a ceiling, you would find that the air provides resistance to your effort (it certainly doesn’t try to suck the plywood up toward the ceiling).

Bottom line: the falling plywood analogy would suggest a helicopter would be easier to hover near the ceiling, not more difficult. While it may be visually compelling, this analogy completely fails to capture behavior with just a slight modification to the conditions.
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