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DIY Foil Kites

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Rein de Vries
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Re: DIY Foil Kites

Postby Rein de Vries » Tue Jun 02, 2020 6:27 pm

Fully agree Schietwetter, also for me it is all about passion to understand and get it right. And i think, since a lot of professionals are far ahead and show how it is done, the diy community should be more open. Discussing ideas and sharing designs is the way forward I would say. :jump:

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Re: DIY Foil Kites

Postby kitexpert » Tue Jun 02, 2020 8:43 pm

Yes, it is huge challenge to make diy foil kite which is comparable to commercial kites. I sometimes estimated I had used 2000 hours for that "hobby" and probably even that is an underestimate. I don't even remember all my kites: failures, too ambitious designs, bad materials etc. etc. It is kind of pity I've lost many original designs because my computers have changed so many times and my file saving practice has been somehow bad :) But this is more like emotional lost than any real lost because I have had all major parameters of foil kite design in my head for quite a long time and therefore I can make new designs pretty fast. However I suggest to keep all designs saved and even write down basic or essential parameters of them (unfortunately for example SP does this insufficiently).

I don't see it reasonable to try to compete with race kites, they are now so far developed and so sensitive, difficult and laborious concepts that they are practically out of reach for diy hobbyist. I could design and test that kind of kites too but I'd have to make some prototypes so it would immediately need foreign workforce.

Trying to tailor kite for some specific use (where pure high performance is not all that matters) is much better approach IMO. And it is very possible to reach or even surpass the level of usual mid AR kites like Frenzy, FRS, Pulsion and other simple kites. It should be remembered low AR wave foil kite is the easiest kite type there is.

There is also plenty of room for optimizing designs: what is the best cell count for certain AR, how much AR should be had, what inner structures are needed or is it better use more bridles, how much canopy curve and where, how wide or narrow wingtips, how much camber in airfoil/how thick airfoil, how airfoil is morphed, how cell width is morphed, how much billow in cells, how much coning in canopy, should kite be made of DCF/or perhaps Aluula (IDK where it can be bought), how many levels in the bridle, how inner parts are designed (fabric orientation, weight cuts), how big kite is reasonable (I think 12m for diy worker is reaching the limits, and perhaps smallest size could be 3m) etc. etc.

I'm still a bit surprised Schietwedder has separated bridle design from "kite design", but apparently he has had his reasons and he has also given some explanations of the process. In my design process bridle design is quite central part because cell count and inner structures and also air intake locations are so closely related to bridle, or vice versa. Bridle design shouldn't be any brainer because kite itself is finished and bridle attachment points are there already.

Sharing designs is an interesting idea. But it is two different things to share a kite plans or to share all design knowledge behind it. To share design file or to share patterns of a kite are too different things. Of course quite a lot can be found out from the patterns if someone understands them, but design file shows of course everything.

Many questions like how much AoA kite should have are very specific, some design needs more some less: how much canopy curve, how much AR, how stable airfoil, how many cells are the first things that comes to my mind which have to be answered as well. Discussions of different concepts and solutions are always useful, even though giving precise answers is often too difficult.

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Re: DIY Foil Kites

Postby aravind » Tue Jun 02, 2020 10:48 pm

If wave foil kites were easy to design then why are they so poorly designed? I mean I think you would have to be able to wave kite first or at least surf before you could understand what you needed. Not being able to surf and think you understand how to design a wave kite is kind of like I don't know obsessing over the lengths of bridles, and number of bridle attachments, when bridles make up less than 10% of the drag on a kite...Which is like the old proverbial ignoring the elephant in the room, the elephant being the super fat profile you stuck on kites because you can't understand math. The same math that if you understood would help you understand the relationship of I don't know.... drag. Let me simplify it, basics, you double the size or more specifically the frontal area of something and you double it's drag, triple the size, you triple the drag. It's called a linear relationship in math, but hey I wouldn't want to complicate you too much with expert terms like that. But do go on to continue designing things with a wrong foundational knowledge. All those fossilized cars we dig up do certainly prove the car came before the wheel! or was it the neuron came before the brain? I am confused again, Dumbdy dumb.

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Re: DIY Foil Kites

Postby kitexpert » Wed Jun 03, 2020 10:03 am

aravind wrote:
Tue Jun 02, 2020 10:48 pm
If wave foil kites were easy to design then why are they so poorly designed? I mean I think you would have to be able to wave kite first or at least surf before you could understand what you needed. Not being able to surf and think you understand how to design a wave kite is kind of like I don't know obsessing over the lengths of bridles, and number of bridle attachments, when bridles make up less than 10% of the drag on a kite...Which is like the old proverbial ignoring the elephant in the room, the elephant being the super fat profile you stuck on kites because you can't understand math. The same math that if you understood would help you understand the relationship of I don't know.... drag. Let me simplify it, basics, you double the size or more specifically the frontal area of something and you double it's drag, triple the size, you triple the drag. It's called a linear relationship in math, but hey I wouldn't want to complicate you too much with expert terms like that. But do go on to continue designing things with a wrong foundational knowledge. All those fossilized cars we dig up do certainly prove the car came before the wheel! or was it the neuron came before the brain? I am confused again, Dumbdy dumb.
Wave foil kites are a niche product and they are not at all very interesting design projects. They obviously need good stability and drift, also pretty fast turning speed is preferred. Small and simple low aspect foil kite has these qualities almost mandatorily. Because it is perhaps the easiest kite type it is very good choice for the first diy kite.

Nowadays some use hand held wings with hf and very probably someone could take a sheet of styrofoam between his hands and use that, or perhaos an Ikea bag. This kind of primitive wings tell it is not that much about the traction device but your hf skills. Hydrofoil is just so efficient it doesn't matter much If kite or wing has poor performance (in traditional sense).

Why small wave foil kites should have thin airfoils? Nowadays any foil kite has, not even race kites. And all small kites are pretty fast anyway, thin or thick. I must say it is pretty obvious KF member arawind doesn't understand much about traction kites or how to design them.

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Re: DIY Foil Kites

Postby aravind » Wed Jun 03, 2020 12:57 pm

You are ignoring much of the argument, but I will entertain you. Wave sports are actually one of the largest water sport segments in the world. It is not niche to have a product for these. What has hydrofoiling with ikea bags got to do with surfing? Open swell riding is really a new form. You are diverting the argument in a weird way. The reason any airfoil is to be thin is to reduce drag. At higher wind speeds drag is greater where a smaller kite is used more. The fact you seem to don't understand why drag should be reduced or why lower drag would be good for a wave kite, suggests that it is pretty obvious KF member kitexpert is not an expert, has no idea about surfing, and doesn't understand much about traction kites or how to design them.

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Re: DIY Foil Kites

Postby downunder » Wed Jun 03, 2020 1:49 pm

Hi FH :bye:

Rein de Vries
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Re: DIY Foil Kites

Postby Rein de Vries » Wed Jun 03, 2020 8:14 pm

:argue: come on! Can we go back to R&D please :lol:

Imo all of you had your points, both Kitexpert and Foilholio helped me a lot allready at least. Thanks!! So may we proceed in peace and harmony please :allbegood:

How about this question. On most foilkites the towpoint center kite is at about 18% chord. The CoP much further down however, close to 25%. Why is it done so? I would expect the best support of the wing with the tp at the CoP.

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Re: DIY Foil Kites

Postby aravind » Thu Jun 04, 2020 5:13 am

I am a bit confused about what you are talking about but a FH to you too!

As to TP and CoP etc. You must be referring to a fixed bridle foil kite as depower kites can vary their TP more. You also must be referring to the virtual TP. The TP should match with the CoP when the kite is static, ie at the window edge. If it is moving in the window it is not matched. If you stuck the TP too far back the kite would stall or fly backwards. Which is what happens when you oversheet a depower kite. The TP moves rear ward. TP can be counter thought of as the center of gravity on a kite, though a kite does also have CG too but that is only really applicable when there is no line tension, like riding a wave at the kite. With a glider if you move the CG forward it flys faster and similar with kites you move the TP forward and it flys faster.

Another important thing to remember is CoP are not static. They change with airfoil shapes and then also move with AoA and in different ways depending on the airfoil.

Another important thing is the pitching moment from an airfoil, or it's torque.

If you are getting at bridle placements on kites, then that has a myriad of considerations. At thick parts need less support or can have more internal structure and then I am sure the pressure distribution has a large part to play. It is quite possible to reduce the attachments a lot, even have only 3 bridle rows. Drag is the only benefit, except as well you can use alternate mixers, the simplest being 1 pulley. With the predominate cause of drag coming from the canopy, bridles I think shouldn't have too much importance placed on them, though not forgotten at all. Then as well bridles have much less weight than internal straps... Lots of things to consider, but overall I think the wrong airfoils and sizes are chosen for the wrong reasons. Most all kites are ruined by thick airfoils or tubes that slow them down. It is almost a conspiracy to sell more kites by reducing their wind range...not everybody has the strength or weight to hold down 18m tubes in 30knots like the airstyleman.

Rein de Vries
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Re: DIY Foil Kites

Postby Rein de Vries » Sat Jun 13, 2020 1:53 pm

For the lower skin I made the following design. 4 rows for the bridles and also various straps on the billowed surface. I used the ideas on the latest commercial models here and there.
lowerS.png
Also applying H ribs on the center cell and 75% finger ribs on the other cells I have now quite some internal structure for stiffness of the wing. In SP it looks like this. AR ~ 6.3 using 45 cells on 5m2 flat.
HF5.png
Using the billow settings below, I expect about the right strap tension is found. Thinking about dacron band (3-4 cm) for the straps. Is this the commonly used material for it?
billow.png
My uncertainty is still mainly with the airfoil shape. I changed a bit the measured shape using the idea below to find a more positive moment as well.
adjust.png
adjust.png (17.87 KiB) Viewed 420 times
airfoil2.png
I would expect if you know what to look at in the lift drag diagrams from Xfoil / Javafoil, one could develop the airfoil in a more educated manner. Does anyone have further insights on this? I still feel it is a rather large guess to just start with a certain shape, be it this measured and adapted shape or some known airfoil (e.g. MH91).

The top developers of this world probably use computational fluid dynamics (CFD) programs to test on the stability of their designs I would expect. My experience is that the camber is quickly to large leading to instability like front collapses. On the other hand I measured up to over 3% of camber on some very succesfull models, so it should be possible. If I use the mod as given above, the camber is reduced but the pitching moment goes up, leading to stability at the cost of lift. :roll:

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Re: DIY Foil Kites

Postby downunder » Sat Jul 11, 2020 3:41 am

Hi guys

Quick question, I think some sort of double sided tape is used for panels, or LEI tube. Which kind of it is it? Do I order one from extremetextil

And how is it possible that the needle is not "gunked" with a glue?

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