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Foil design idea

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Herman
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Re: Foil design idea

Postby Herman » Thu Mar 24, 2022 10:43 pm

I doubt that you will have enough control of the build parameters to justify doing lots of maths, and in any case throw a couple of bubbles in the fluid and you would have to redo the maths. Plenty of good designs have come from intuition. Moreover, particularly for fluid dynamics, sometimes what looks right works better than what a more mathematical approach may have suggested. Not that the maths is necessarily wrong, it was probably some of the assumptions etc.

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Schietwedder
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Re: Foil design idea

Postby Schietwedder » Thu Mar 24, 2022 11:42 pm

Yes, this is very advanced physics and even a bit bionics....companies /institutes working years with dozen of engineers to solve those kinds of problems.
Let alone building it like it was calculated as you say.
Nevertheless remarkable that we have some kind of an engineering thread in a kite forum, with some kind of scientific sources/citations.
At least it's remarkable to me if compared to other forums.
Keep it coming!

That it's hard to engineer and build something like that is quite clear otherwise everyone would do if it was easy.

That in the end someone will just build something which works or doesn't is quite clear for a board sport. Its not americas cup here. But inspiring for sure and i hope that I'm wrong.

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Re: Foil design idea

Postby yenice » Fri Mar 25, 2022 8:47 am

My original post when I opened this thread was making a hydrofoil with carbon or glass sandwich over a foam core. This was the idea I had in mind, because many foils are made this way. But after looking at some whale swimming videos, and their fluke movements, I started thinking that maybe rubber or reinforced rubber would be a better option to suit the flexible foil.

It will enable me to stick to the original thick foil profile without sacrificing the flexibility. The profile flex characteristics can be dictated by the foil profile itself. More flex towards the wing tips, because of increasing taper towards wing tips and also more flex towards wing trailing edge due to decreasing foil profile thickness. That means 2 dimensional flexing possibility, which would bring me closer to organic world. I can than use metal or glass or carbon rods, wires, pipes or plates imbedded in rubber, to reinforce some parts like leading edge or the fuselage connection points as need arises making some sections stiffer than other parts of the wing. I can also use different quality rubber hardness to change flex, buoyancy ect.

Rubber flippers are already in the market, but they are mostly used by swimmers submerged in water. A foil acting flexible as a rubber flipper, but the person using it out of water is a completely different case in which drag of swimmers body is discarded leaving only the drag of mast and the drag of foil itself.

I made a search for a rubber hydrofoil but could not find anything yet. Would be happy to have your opinions or experiences on this.

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Re: Foil design idea

Postby evan » Fri Mar 25, 2022 9:39 am

Remember the front wing has to be strong and stiff enough to at least support you entire weight pushing down from the fuselage in the center for sustained foiling and bend up or down from that equilibrium.

Like others said, this is a very difficult design challenge is you want want decent flexibility without breaking anything. Best approach is to build a lot of prototypes, but that won't be cheap ;)

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Re: Foil design idea

Postby tomtom » Fri Mar 25, 2022 10:38 am

Get me some designs and will think about how to make it flexible. Right now im building strapless snowboard which is 5cm thick and yet flexible... It is possible. Rubber is too heavy.

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Re: Foil design idea

Postby tomtom » Fri Mar 25, 2022 10:40 am

And in context of 100-120cm span HA wing - i dont think we need very thick profile.

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Schietwedder
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Re: Foil design idea

Postby Schietwedder » Fri Mar 25, 2022 11:10 am

yenice wrote:
Fri Mar 25, 2022 8:47 am
I made a search for a rubber hydrofoil but could not find anything yet. Would be happy to have your opinions or experiences on this.
Do you know these Hobie Kayak/Trimaran pedal drive things?
They have two alternating rubber fins driven by foot pedestals.
It is remarkably efficient.

I think as well that for the main wing a high AR composite structure would come in handy to apply the principles tomtom has in mind. Maybe the rear 20-30% of chord flexible. But for the Stabilzer something hinged/flexible/flapped/rubber-ish would make sense... Now for some non engineering experience....

Has anyone experience with pumping monofoils without stab?

The flexible tail principle I can recall pumping in a small sailing dingy no breeze (and no sail) into the harbour. Rocking it from side to side hitting the boats rolling resonace frequency. It only worked if you let go of the rudder. Not completely but with a certain looseness (compare it with a rubber fixed to the tiller in 0° position being able to swing left and right 10-15° with progressive restoring force due to rubber elongation)

It also did not work at all with boats that have swept back centreboards or rudders interestingly (Laser for instance)
It did work very well with Europe dinghies (because the centreboard could be tilted freely in the centreboard case) especially well if you cant the centreboard straight or even forward. You could pump/rock it with 2 knots easily without sail. That is really remarkable for that type of craft/resistance/weight.
That would equal a forward swept main wing on a hydrofoil.

Now some engineering again:

Regarding profile choice worth looking into symmetrical or very lightly cambered sections. They have very good L/D because of little drag, little to no change in cm (pitch/profile moment) and work in both directions!
Combine that with a slightly bigger than usual nose radius and you gain a lot of tolerance against changing AoA. Overall a bit more drag for zero AoA but for a wide range of AoA less drag. (Shock free flow)

Best Regards Niklas

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Re: Foil design idea

Postby BalsaMichel » Fri Mar 25, 2022 11:51 am

yenice wrote:
Sat Mar 19, 2022 10:41 am
...
https://simpleflying.com/boeing-787-win ... -take-off/
...
In my opinion this article partly comes to wrong conclusions. I doubt that less drag is a direct result of wing flexibility.
Thinking this article describes it more accurate:

https://www.newfoxy.com/2016/07/25/boei ... ve-flying/

Quote from this article: "The influence (-of flexibility-) on performance is slightly negative, but this is a very weak effect. It can be compared to the rolling resistance of a stiff bicycle versus one with a spring-loaded frame."

Micha

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Re: Foil design idea

Postby fluidity » Wed Mar 30, 2022 7:05 am

BalsaMichel wrote:
Fri Mar 25, 2022 11:51 am
yenice wrote:
Sat Mar 19, 2022 10:41 am
...
https://simpleflying.com/boeing-787-win ... -take-off/
...
In my opinion this article partly comes to wrong conclusions. I doubt that less drag is a direct result of wing flexibility.
Thinking this article describes it more accurate:

https://www.newfoxy.com/2016/07/25/boei ... ve-flying/

Quote from this article: "The influence (-of flexibility-) on performance is slightly negative, but this is a very weak effect. It can be compared to the rolling resistance of a stiff bicycle versus one with a spring-loaded frame."

Micha
If you want to examine parasitic drag from fluttering, simply hold a strip of paper into the intake of a running vacuum cleaner :D


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