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DIY Foil Kites

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kitexpert
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Re: DIY Foil Kites

Postby kitexpert » Sun Jan 30, 2022 8:22 pm

Ok, sounds good and if/when you have own experience it is hard to argue against that.

Those bridle angles just look to my eye a bit too radical, esp. in the wingtips. They are quite different to what FS or other quality kites use. Are you sure cells don't compress when kite flies and is loaded? Certainly there is some point when bridle is just too short, but of course this point isn't sharp or precise.

So kite just starts to fly worse when bridle lines become too short. At some point it is certainly possible to see that kite is not well supported, it becomes less smooth than it could be.

Difficult thing can be to tell why kite possibly underperforms. Is it because of too short bridle or something else? Making a conventional bridle one issue can be avoided.

Your bridle total length (kite to mixer) is not too short, more like opposite it is quite long.

When primary bridle lines follow the orientation of the ribs it is optimal but in practice some deviation from that can't be avoided.

IMO bridle is best done when everything else is finished, but design itself sets the basic dimensions and possibilities how to cascade. Then you have more time to look and think about it, or even get some opinions elsewhere.

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Schietwedder
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Re: DIY Foil Kites

Postby Schietwedder » Mon Jan 31, 2022 9:26 am

Yes the wingtips could be an issue (which would be easy to fix).
I will make some hi res in flight pictures or put a GoPro inside the kite to see if the spanwise straps are loose.

For the other primaries further in I don't see an issue. The method I use for determining the length of the inner bridle is to see that the angles the lines enter the canopy is towards the center or 90° to the canopy and if possible not outward.
When that's not possible smaller than an angle of 80°outward (so 10deg from 90° to canopy) (where Sinus (alpha) is still practically 1 so the horizontal part (cosinus alpha) is still practically 0)

Meaning that if the angles of the primaries are 90° or less the kite is supported in the direction of the aerodynamic lift or some spanwise tension on the straps is engaged when farther inward.

With that much canopy curve that state is achieved with quite short primaries (just not so easy in the tips)

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Re: DIY Foil Kites

Postby kitexpert » Fri Feb 04, 2022 6:18 pm

I'm not at all against short bridles - as long as they are not too short. Traditionally all foil kite manufacturers have used very long bridle lines but they have improved and got them shorter and more refined. Certainly the design of kite itself is a factor there, for example if spacing is 1313... it allows much shorter primaries than constant 3 spacing. But then there will be higher number of bridle lines and also inner structure of the kite will be different.

Canopy curve certainly affects.

Hi res pictures of kite in flight are definitely useful.

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Re: DIY Foil Kites

Postby kitexpert » Fri Feb 04, 2022 6:34 pm

BTW you have some unsupported ribs, they will show "through" the kite. But IDK how big deal that is, probably not at all. You could support them by adding some bridle attachment points. A and possibly B could be fully supported.

If you design line rows separately they don't have to be similar. It is quite usual D lines are quite different to others because they are not part of the pulley system and they don't have much restrictions by the inner design of the kite. You can also argue that brake line row is not needed at the center of the kite.

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Re: DIY Foil Kites

Postby Schietwedder » Tue Feb 08, 2022 12:04 am

Yes I started with bridle spacing constant 2 for the 11m
Then I went 2 and the 3 center ribs with a Bridge diagonal for 6 and 9m which are basically the same design.

I then had various types of arrangements and Ideas for the 16.5m. I also tried the popular 3-1-3-1 bridle spacing.
It turned out that for my design philosophy with relativelymid- low cell count for a given AR I have almost the same length of bridle lines with 3-1-3-1 spacing as opposed to constant 2 spacing which is superior In weight because I can leave out some more structure for the bridge diagonals.
I then went for mostly 3-2-3-2 spacing at least in center as this is a good compromise structure weight Vs bridle drag.

If it works out I will have significantly less lines(10%) and a relatively light but effective structure with a lot of spanwise tension.


Meanwhile the 10 cells in the centre are done. Maybe I can get it flying within the next 2 or 3 weeks before the days get longer for kiting after work again.
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Re: DIY Foil Kites

Postby kitexpert » Thu Feb 10, 2022 7:01 pm

1313 spacing is fully supported, 2 can be or it can be only half supported. Ozone used 2 spacing in first Chrono without D-ribs and I thought it was cheap. You could see unsupported ribs on top surface and kite also compresses slightly spanwise. However cell count in Chrono was so high these issues weren't at all serious.

Anyway, newer versions have been fully supported designs. FS kites have been fully supported for a very long time.

CA kites have used very simple method: every rib is bridled and there is no diagonal ribs at all. That is the way foil kites were 25 years ago. Amount of bridle line and line drag increases a lot but at least in some cases it doesn't matter. Kite certainly is fully supported :)

I stopped making kites with unsupported ribs after early years because I thought they are too primitive. For a long time I mostly used classic 3 spacing with full D-ribs because it allows simple low drag bridle. However 1313 is attractive because it makes primary lines very short. Mixing 2 and 3 is of course usable as well. There is some more exotic possibilities but it is questionable if they are worth it.

You could have had that kite fully supported by adding some bridle lines. But very hard to say how beneficial it would have been.

Different spacings for different line rows can be worth studying.

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Re: DIY Foil Kites

Postby Schietwedder » Fri Feb 11, 2022 12:20 am

Yes I totally get your point.... It is definitely some risk involved having unsupported ribs.
However they are unsupported where I expect very low diagonal tensions due to canopy shape and bridle angles.
It is basically nothing new I have done this before on my 6 and 9m just that now the d ribs bridges have a spacing of 3 ribs with a bridge and not 2 with dribs.
So if ou go back to the hi res pictures of the 6m you can't see any deformation in unsupported ribs which also had the spacing of 2. Nothing new here. Because of sufficient spanwise tension in the straps the deformation is negligible would be my suggestion.

If I find the time I will do a quick 2d finite element analysis with finite cable elements that can only take tension not compression just like sailcloth of my bridle layout and the kite structure then we know exactly :) I'm pretty confident though that it will work because spanwise tension should be bigger than aerodynamic forces forcing the unsupported ribs in membrane stress state forming a parabolic or triangular distorted shape if you do a quick evaluation by thumb.

Defensively a nice research topic to dive into!

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Re: DIY Foil Kites

Postby kitexpert » Sat Feb 12, 2022 12:14 am

I wrote a bit wrong earlier: I of course meant if you have 2 spacing without D-ribs your supported ribs will show through to the top surface. Unsupported ribs just bulge upwards - because they are unsupported. How much it depends many factors, mostly cell width and amount of canopy curve. Every second rib bridled is or has been very common in parachutes and they look quite rough because they are flat and have wide cells. But it doesn't matter for that purpose much if anything.

IDK if it is possible to avoid this without diagonal supports. But it can be quite small like it was in Chrono V1. Your kite has wider cell than Chronos have, but on the other hand most of the ribs are bridled. So I don't think it is a risk but it can make you wonder could it be otherwise, and it could.

Anyway this is a small detail of your design and there is certainly much more important things you must have thought and hopefully done appropriate decisions. I am still at least a bit suspicious about the primary bridle length because it is so different to what is usually used in kites. One interesting possibility to study different length bridle lines could be to make left and right sizes of kite different but having total bridle length same of course.

For diy guy kite prototyping is unfortunately quite difficult or at least it demands really good motivation. It just takes so much time and work.

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Re: DIY Foil Kites

Postby Rein de Vries » Tue Feb 15, 2022 3:49 pm

Progressing :)
IMG20220214222727.jpg
Ultra light 11m2 for the upcoming season!

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Schietwedder
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Re: DIY Foil Kites

Postby Schietwedder » Wed Feb 16, 2022 7:03 pm

Looking good Rein!


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