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Ground effect and hydrofoiling

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bragnouff
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Ground effect and hydrofoiling

Postby bragnouff » Wed Nov 15, 2023 9:57 pm

Ground effect is factored in many aerial applications (ex: Ekranoplan). Extra lift close to the boundary due to reduced induced drag. That's also quite noticeable with water when twin tipping and foiling, with some extra lift or altered flying behaviour when in the shallows. (and I'm not talking about burying the foil in the sand and faceplanting, which is of course noticeable !)

My question would be about what kind of foil design would make the most of the ground effect. Supposedly, it could have a natural tendency to ride at a constant height from the bottom. Going down would be compensated by extra lift due to getting closer to ground. In practice, it's a matter of compromise, you can't expect to bank on the ground effect at all times, and it needs to work also with full depth, for the starting phases, and of course because no spot has a constant depth. I guess, turning would have to be more reliant on yaw than roll.
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Trent hink (Mon Nov 20, 2023 12:47 am)
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Re: Ground effect and hydrofoiling

Postby mashimisha2 » Fri Nov 17, 2023 8:09 am

Ground effects work on the water because the surface is more or less level.

From what I understand, the bottom of the ocean is not level.
I have even hear it mentioned that there are boulders and coral reefs down there.

This might affect the performance of a wing that depends on proximity to a flat surface.
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roQer (Fri Nov 17, 2023 11:36 pm)
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Re: Ground effect and hydrofoiling

Postby ronnie » Fri Nov 17, 2023 10:35 am


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Peter_Frank
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Re: Ground effect and hydrofoiling

Postby Peter_Frank » Fri Nov 17, 2023 4:10 pm

Interesting - but difficult to see the "comparison", what the outcome was?

How much difference did they feel.

Do you think there is more explanation or a YouTube/Vimeo video where one can see it bigger/fullscreen instead?

8) Peter
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nixmatters (Sat Nov 18, 2023 8:53 pm)
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Re: Ground effect and hydrofoiling

Postby Regis-de-giens » Fri Nov 17, 2023 4:45 pm

I do not have a clue on which design is more prone to ground effect ; however I suppose that if it is the case, aspect ratio should count ; so I think that some answers could be found (or advised here ?) in planes users : if some pilot can tell us if gliders feel more ground effect than acrobatic planes when close to the landing track ? ...

At first sight, I doubt that ground effect has a massive effect for hydrofoil that leads to real optimisation (it has on a TT board yes since the quantity of displaced water "squeezed" between the board and the ground is far higher...) : indeed I personally like to ride in shallow water , for fun, so at low distance from the ground; but even feeling very close to touch the ground, say 10-20 cm) the additional lift does not feel that sensitive and i would say ... does not bring real pleasure to me except the "challenge" of mastering the precision of flight altitude ...

I like testing things but I'll remain far from that ground effect comparison study to preserve my material and my legs :-)

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Re: Ground effect and hydrofoiling

Postby BWD » Fri Nov 17, 2023 4:56 pm

That is cool! Looks like a lot of extra glide in the last shot especially.
I think I have felt this often kitefoiling as I go over sandbars gliding a lot and doing so is easier than it seems it would be.
Most of my rides end at a beach with 2-4 shallow (30-60cm) sandbars in front of it. Riding over them saves wading 50-100m, so I usually do it, no problem despite a low aspect wing (ketos 1200 kool) and wind shadow near shore. Nothing I can quantify but noticeable even with a low aspect wing, and follows the theory of less induced drag within a wingspan of the bottom.
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Trent hink (Mon Nov 20, 2023 12:50 am)
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Re: Ground effect and hydrofoiling

Postby ronnie » Fri Nov 17, 2023 10:12 pm

Peter_Frank wrote:
Fri Nov 17, 2023 4:10 pm
Interesting - but difficult to see the "comparison", what the outcome was?

How much difference did they feel.

Do you think there is more explanation or a YouTube/Vimeo video where one can see it bigger/fullscreen instead?

8) Peter
I doubt there is more video.
Horue might be the best chance of some info on it as he does a lot of pumping in swimming pools and water channels. With the new Foil drive, it should mean that there is a possibility of sustained foiling without pumping - which would make it easier to study the effect.








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Re: Ground effect and hydrofoiling

Postby Trent hink » Sat Nov 18, 2023 8:38 pm

What role does compressibility of the fluid play in this effect?

I know it is well-studied in aviation, and there are even aircraft designs based on ground-effect. I suppose skimboarders are also familiar with it...

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Re: Ground effect and hydrofoiling

Postby Schietwedder » Sun Nov 19, 2023 10:54 am

Yes ground effect applies,
I did some CFD simulation on a pumpfoil I designed and it was really remarkable.


You can also feel it in the real world really well if you have the right gear and conditions.

At our homespot we have a 30-40cm shallow sandbank which is about 50 m long and just sand without seaweed on bottom. If you go in relatively flat water or light winds on a High AR ish foil and you concentrate having even kite pull and approach the sandbank relatively low on mast without any heel and just leave your weight as it is sort of like freezing, you can actually realise as the water getting shallower the foil gets lifted up on the mast and brought back down when the depth gets deeper again.

I noticed the most when kiting with a sabfoil razor 820 on a 111cm mast as you have much mast that can be pushed up. It was like 30cm higher on mast on the sandbank without weight shifting.

As I have a CFD model of the razor as well I can do a simulation which should proof that in numbers when Im at the right computer tomorrow.

What does not work in that CFD Code particularly is reverse ground effect due to riding close to the free surface. Many people say that the foil rides more eficcient high on the mast but I’m not sure if thats just the fact that the mast is less draggy by being all the way out of the water or if theres also an effect with the flow having to squeeze though the gap between free surface and suction side andwhat effect that has on the overall circulation on the wing and the induced drag.
I would guess it would be less efficient than the ground effect with a sandbank if theres any effect because also the water surface is pulled down resulting in a wake which causes wave drag of some sort. You can see that water deforming quite well when people pump or on surface piercing hydrofoils of sailing vessels. On normal ground effect the ground does not “bend” so theres no or less energy loss by deformation.

i can ask the software developer if he can plug in a free surface module, in panel method thats quite straight forward.

BR Niklas

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Re: Ground effect and hydrofoiling

Postby ronnie » Sun Nov 19, 2023 9:28 pm

They would probably come across ground effects concerns in a hydrofoil test tank. They would probably be wanting to avoid them affecting their results.

https://www.youtube.com/shorts/mhwIxqiuukw



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