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Technically "airborne" can just mean to move through the air, but I would like to know how high you have to be before you are entirely supported by air in a helicopter-like machine, as opposed to benefiting from the reaction from the earth (or whatever platform you are taking flight from). I am effectively asking for an equation that will tell me how high I have to be before the effect demonstrated in the following image is effectively zero.

enter image description here

I assume that this has something to do with mass, but am unclear on how to proceed beyond that.

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  • $\begingroup$ Are you asking about ground effect? $\endgroup$ – user10851 Dec 17 '15 at 0:01
  • $\begingroup$ The term is ground effect. For fixed-wing craft, it's related to the length of the wing. Very little effect above a height equal to the wing length. faatest.com/books/flt/chapter17/groundeffect.htm I'm sure a helicopter is similar, but there may be other considerations in a hover. $\endgroup$ – BowlOfRed Dec 17 '15 at 0:01
  • $\begingroup$ I suppose all my googling was for the opposite, for air-only effect, hence I didn't find anything. I'm surprised at the lack of sourceable data for this, although I suppose that could be due to companies treating this as trade secrets. $\endgroup$ – Lewis Goddard Dec 17 '15 at 0:39
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    $\begingroup$ I think it's likely because pilots don't really bother with what point it goes to zero, but what point it becomes effective. I think most helicopter performance charts will show OGE data (out of ground effect) and IGE data, with a fixed height assumption (usually 2 feet) like faa.gov/regulations_policies/handbooks_manuals/aviation/…. $\endgroup$ – BowlOfRed Dec 17 '15 at 1:07
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I wasn't able to find a well-sourced table, but this page has a chart that shows a helicopter has almost zero ground effect advantage when hovering at a height greater than 1.25 times the rotor diameter.

That fits well with this page from a testing company that states that ground effect on a fixed wing craft reduces drag by only 1.7% when at an altitude equal to the length of the wing. The reduction is much greater when the wing is closer to the ground.

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The effect is never zero, unless you happen to stop producing lift.

Lift is produced by air being continuously pushed downwards. This downward motion continues for several minutes, but is dissipated eventually. But in all cases the motion produces a pressure increase on the ground underneath. This is clearly audible for aircraft flying at supersonic speeds, because then the pressure change will not spread ahead of the aircraft. For subsonic aircraft, the pressure change is much more gradual and widely spread, but still there nonetheless.

To answer your question: Eventually, the aircraft is always supported by the earth.

Maybe you know the question about the closed truck full of birds. Someone bangs at the outside, so all birds fly up. Will the truck be lighter? Same story: The truck will not change weight, because now the air transmits the weight of the birds into its structure.

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  • $\begingroup$ While this does answer the question, with a metaphor that's much easier to visualize, but I was after the points at which the effect is "effectively zero". Specifically, the point at which an aircraft can move over the edge of a cliff and notice no change in performance. As it turns out that appears to be extremely low, as little as a rotor length. Upvoted all the same. $\endgroup$ – Lewis Goddard Dec 17 '15 at 22:13
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The usual rule of thumb in flying is that ground effect starts to appear within about one wingspan of the ground. av8n always has a good intuitive explanation. He explains it as a mirror-image aircraft beneath you. In flight training, it becomes important in soft-field takeoffs, where you transfer weight to the wings at as slow an airspeed as possible and get into ground effect. Then you accelerate to climb speed.

Aside: In Lindbergh's flight from New York to Paris, at times he made use of ground effect to save fuel. At a height above the water of 30 feet, more or less, its height is automatically stable, and it just putts along at low power like a sewing machine.

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The ground effect is only present in winged craft because in a flight with wings there is a much high pressure under the wings when closer to the ground this increases the normal lift effect. In propeller lifted craft the effect still applies because the propellers are just air foils twisted ,but they are also smaller which means less surface area to have an effect on. The smaller the surface area the close to the ground they have to be and Most craft like helicopters have there propellers on top of the craft for stability thus making to far away from the ground to have the ground effect apply. Also you should check out back spin lift driven craft.

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