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Tornado-Cat

The winning foils

3,407 posts in this topic

On 7/5/2017 at 0:56 PM, weta27 said:

Here they are - thanks Jason!:

35685973206_68980834af_o.jpg

34884220264_f4ef07407f_o.jpg

From his latest video:

 

at the risk of reposting the lovely foils...the reflections on the bottom concave section are sublime...I would love to see these in person some day in a museum...hopefully the Auckland Maritime :)

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Thanks for posting these nice pictures of ETNZ Foils,

What is interestiong is to imagine the lift distribution of the upper part of the foil:

And according to the "Horizontal" part and the tip part kinked upwards, one could imagine or "reverse engineered" the design philosophy, or a part of it.

Among many sources of drag there is a more or less marginal component of drag for a foil which is the WAVE DRAG, and digging in the theory, one might find that this "foil wave drag" is prop of squared  lift coef and inverted proportional to foil depth. (Not 100% sure, but that's the spirit / check by yourself)

So the flat part gives the max lift and the kinked tips have their lift decreasing in order that the couple of variables (local lift, depth) remains similar all along the foil span, in order to minimize wave drag along with induced drag, and friction drag

Sorry if not that clear, hope some specialist would provide clearer explainations

Regards

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^ My understanding of wave drag is that it only applies to compressible flow - specifically aerodynamics at or near the speed of sound. Are you talking about the deformation of the water surface directly above the hydrofoil, and reducing the energy being dissipated that way by making the chord length proportional to the submersion of the foil (distance below water surface? 

This is something we are constantly looking at in kite foil designs due to the huge cant angles (windward heel) - at the moment (thanks to low prototype cost) we're just 'throwing darts' at the problem, but I'd love to have a better understanding of what's happening.

 

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Foil wave drag is addressed (among all other foil drags) in a Cheasapeake workpaper about the Moth.

It is a few years old document, but still interesting, cannot find it among the hundreds of CFD  pdf I have on my laptop. 

Should be findable on a Moth thread.

Regards

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^ Found it, thanks - that's a really interesting study. Wave drag increases as you get closer to the surface - it's the deformation of the water surface that I was guessing about - but according to the study, the reduction in drag resulting from having less of the foil mast submerged more that offsets it, at least on T-foil set ups like the Moth.

https://www.scribd.com/document/213283260/International-Moth-Hydrofoil

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Note that ETNZ foil was sailed with the tip horizontal.

The kink enabled the horizontal tip to be both deep & nearly half of the span vs Orifice who did have a similar tip but much smaller, with most of the foil operating much closer to the surface & on an angle.

 

AC TEAMS DAGGERBOARDS.png

@darth reapius image :)

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Thanks Hoom,

If ETNZ used to sail with the tip horizontal, then my former remark is totally irrelevant.

Sorry, I apologize for this disinformation, just forget it.

Obviously, the reverse engineering of winning foil is not a trivial exercice.

Regards

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