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What the maximum size an object can benefit from ground effect?

Also known as riding a cushion of air created from flying close to the ground, some birds can use this effect to fly with less energy than any other method. Russia have designed planes that do this as well, but the water would have to be calm. In another realm of aeronautical science, a very large hybrid airship gains lift from both its shape and buoyancy. Could it also use ground effect to fly more efficiently safely over the ocean? With enough propulsion the same airship does not need ground effect to fly on it's own. When would ground effect not save energy needed to fly?

This question has characteristic that could be applied in the Earth Science, Engineering, and Aviation stack-exchange. Although, I believe the question may better find an answer here since it deals with the physical properties of fluid dynamics, mass, and shape.

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The water is usually never flat so the larger wing span is ideal to keep the object above the chop.

Justintimeforfun
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  • In this related question, I estimated that the downdraft from a particular helicopter should be non-negligible for about thirty meters below the aircraft. Airflow at the ground seems like a requirement for any ground effect on lift. You would have to do your own calculation for different flight geometries. I’m interested what responses you will get from folks with more avionics experience than I have; please add links if you eventually wind up cross-posting to other communities. – rob Sep 10 '22 at 22:39
  • Why should there be any maximum size? – D. Halsey Sep 10 '22 at 23:00
  • This seems more like an engineering or design problem than fundamental physics. The answer will be highly dependent on the specific shapes & specifications. – D. Halsey Sep 10 '22 at 23:05
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    I’m voting to close this question because this is really a design issue I think is more suited to [aviation.se]. It would likely depend on (at least) the mass, area and propulsion system but the devil would certainly be in the details. – StephenG - Help Ukraine Sep 10 '22 at 23:16
  • I have to point out that the image does nothing to help answer the question and uses up bandwidth that some members may not have. Not everyone using the site has access to fast internet. – StephenG - Help Ukraine Sep 10 '22 at 23:17

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In practical terms, a conventional airplane flies into ground effect when its altitude is about equal to its wingspan. This allows it to generate more lift with less angle of attack, which reduces induced drag.

niels nielsen
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Ground effect reduces one particular type of drag on the aircraft, but not others.

As the craft gets larger, skin drag is going to increase. At some point, it becomes silly to reduce a tiny induced drag with ground effect, but leave the massive skin drag. The faster you go and the greater the skin drag, the less benefit you gain from ground effect.

BowlOfRed
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  • Why are you saying the induced drag is tiny? It's usually a significant fraction of the total. As the craft gets larger it also gets heavier, so the induced drag may also increase (depending on how much the wingspan increases). – D. Halsey Sep 10 '22 at 23:25
  • This is in the context of a lifting airship. The more the buoyancy, the less lift generated by the wings. The larger the craft, the greater the parasitic drag. For some other aircraft, this equation would be different. – BowlOfRed Sep 11 '22 at 15:43