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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 » Sat May 22, 2021 7:42 pm

Flying my 5m2 diy kite and talking to Niklas we both see that our kites (same profile) tend to be rather fast with a high power spike as compared to commercial high AR foil kites. The latter fly more slowly and retain pull all through the window. It puzzles me a bit why this is. We obviously need that on the water and towards my new project I consider a more cambered profile as a mixer setting with shorter Z and longer B and C are found to give the diy the desired continuous power in the window. It is like a bridle induced increase in camber. I like to aim for a diy kite that has this property by design and wonder whether this would resolve it. I obviously loose a little stability by trying and wonder whether this camber is the reason that the commercial kites fly little different or that it is just a bridling difference from starters we are looking at. For comparison; our diy projects have little above 1% camber whereas the commercial kites have up to 3% or so. Profile thickness and AR are pretty much comparable in this sense. Curious to hear your thoughts!

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Schietwedder
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Re: DIY Foil Kites

Postby Schietwedder » Mon May 31, 2021 12:49 pm

I would say yes, to a point.

First of all there´s two effective measures:

1. increasing the area between the camber line and chord line but staying at the same max. camber.
That is what you do with the bridle effectively just built in the profile.
So the disatvantage, it has a far bigger impact on lessening the stability for a given lift, but you also can pull it out better with your bridle if it´s too much.

2. Increasing the camber by percentage and doing so in the very front (between A and B) of the profile to minimize impact on stability.
It´s the cleaner method with regards on stability but i am not so sure how much you can pull out again if the kite overflies too much, cant be depowered so much or you want to have a bigger windrange.

That´s why I´m still struggling with myself back and forth also on doing the same as you increasing camber.
It´s not such big of an issue for the usability of my kites just shortening z a little and I have heaps of grunt.

Once learning how to foil I might wan´t to get rid of that grunt again slightly.
I will wait until my 9m is done and if it goes in the same direction as how my 6m is flying I won´t change much, maybe a little more grunty profile. But I will first increase profile thickness before touching on camber I guess. We have a lot of lee rotor because of a military pier and some high coastlines in windward, so stability is really important especially when its light winds or the breeze turning slight offshore in the evenings after work when doing the last laps.

Also fabrication improves a lot with every kite in the beginning, so most issues disapear without much change in design once having a good basis I figured out.

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Schietwedder
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Re: DIY Foil Kites

Postby Schietwedder » Sun Jul 18, 2021 1:20 pm

Some more impressions of the 6m in steady 18-20 knots.
Turning and reacting quite nicely, think the z-straps make a difference. If brake line doesn´t enter at 90° the steering line input does not pull out the billow first but directly the load goes into the strap.
The Z line is now shortened 3cm for Twintip. On the buggy or foil I have the Z lines 3cm more loose letting fly it a bit more forward.
Drifting also quite well on the surfboard with the light weight (not on video)


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Re: DIY Foil Kites

Postby Rein de Vries » Mon Jul 26, 2021 8:48 pm

Very much impressed! Great work :thumb:

What can you say about the difference you feel between flying commercial kites and your DIY's. Are you close :?:

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Re: DIY Foil Kites

Postby Schietwedder » Mon Jul 26, 2021 9:47 pm

Thanks Rein!

I must say I can't say much about how commercial kites compare as I use only my own at least in 6m size or close I don't have any comparison.
Would be interesting to fly a Kauper Falcon, a Hyperlink, Soul, Nova or similar to find out.
Not much foil kites up here in my local spot, only a 12m Soul I flew I can compare to my 11m.
A few weeks ago I kited together with a guy with a Kauper Falcon 13m and a door as I flew my 11m and normal Twintip. He had some more powerful turns and better low end in lulls of course but was very impressed when he asked what size it was and didn't expect to be an 11.
Would have loved to swap kites but stayed too long outside then he was gone already.

What I love about the 6m is that it loops forever, it fly's still flies when everything falls out of the sky with 1100g and drifts very easy when riding a wave or turning downwind with full speed in the buggy. That is mostly due the light weight and not any fancy design or engineering I think. I was amazed though how light weight you could achieve with very little effort. Just a few less cells that billow still looks fine and d ribs only where the bridle needs some, no fancy cutouts just the round standard holes from SP and only here and there a bit of seamtape to align things at the alignment points.

Has it's downsides also with 27g upper and lower skin cloth. Accidentally looped it with ground impact in the buggy in high speed and the cloth shaved through behind the LE battens at two cells in the buggy last weekend, but was a quick fix luckily.
Don't know though if 33g would make such a big difference though in that circumstance. Only bullet proof 40g+ at the LE and battens not all the way to the front would have like on SOUL probably.
Better fly careful is the easiest fix probably.

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Re: DIY Foil Kites

Postby Liam1 » Thu Aug 05, 2021 8:53 pm

(First post so I don't know if this is formatted correctly)
Hello all,

I don't even remember why but earlier this summer I decided I would like to build my own foil kite (I’m aware of how much time it’s going to take) but because I didn't think it would be any good, I wanted to make an unconventional kite.
That’s when I decided I wanted to take not only the wings of a bird but the entire thing (maybe plane), a little bit more like a show kite/single line kite but I actually plan to use it for kitesurfing (and maybe let my brother use it on hydrofoil) if it works.

Inspiration:
3D-Plane-180-Kids-Stunt-Kite-Adventure-Colours.jpg
3D-Plane-180-Kids-Stunt-Kite-Adventure-Colours.jpg (10.34 KiB) Viewed 431 times

Now that the origin of the kite has been established I would both like to get a bit of advice on where I can find certain pieces of information and maybe just general feedback on what I’ve already done/decided because I’m completely inexperienced with building and designing kites.

Information
Is there anywhere where I can get a list of tested airfoils that work for kites?
I’m currently using the profile PW104, I found it when looking for things about foilmaker and the website (https://members.tripod.com/daniel_gagnon0/id19.htm) said it was great for kitesurfers because of stability (which I think is especially important for my kite and the gusty conditions we often have).

Current (awful) design
Alright, now don’t laugh at how stupid it is and, of course, I know it’s not efficient at all.
Screenshot 2021-08-05 12.50.19.png
So this is currently what it looks like, it’s a very rough paint job but I feel like it sells the bird better than the standard red colour.

Size (might not be completely accurate or useful because of the dumb design):
Flat area: 10.02 m^2
Flat AR: 4.9
Flat wingspan: 6.7
Cells: 25 (Was a bit low I thought so it does have 40% miniribs)

Profile:
Centre, PW104 17.586 at 17.91
Tip, PW104 13.586 at 17.91
These are once again standard values because I don’t think randomly changing them would be a good idea but tip should be less than centre as far as I know so even though it contradicts the page I found the profile on.

Arc:
Arc of 112° even though that is more for a race kite than one I hope would fly.

Billow:
3.5% front starting at 15%
4% rear starting at 90 and 95% for upper and lower skin respectively.
I believe the billow settings are from Niklas’s tutorial because I didn’t really know which settings would be best and these are in line with the 3-5% I believe this thread said were okay settings.

Bridle:
I think it’s quite standard again.
AoA: 3% for both centre and tip
Screenshot 2021-08-05 21.26.58.png
Screenshot 2021-08-05 21.26.58.png (60.41 KiB) Viewed 431 times
Tow point at 30% because of the weird shape, I think, it seems to be at about 20% of chord if it was in a standard position.

I think those are the most important parts of the design. My idea on why it may work is that if the “body” doesn’t help anything (which it most likely won’t) the rest of the kite should still be strong enough to kite with. The main problems I would think of are front stalls because the “body” would deform the kite too much, the kite being ripped apart because the upper and lower skins don’t follow a smooth line (could be solved relatively easily by having “strings” where the upper and lower skins would’ve normally been maybe) or that the body would be flattened making it into a “standard” foil kite (which would actually be solved by the solution above as well I think.).

And if you would have to rate the chances of success, does it have a larger chance than 1% or is it just so incredibly dumb that I should give up on it?

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Re: DIY Foil Kites

Postby PugetSoundKiter » Fri Aug 06, 2021 5:29 am

Interesting idea. Form follows function except in art, form follows fun :)

I’ve flown & rode some large, closed cell, thick, low A/R, high drag kites (PL Arcs 6<->19m, Arc Stacks 9+11m, Flysurfer Titan 15.5m, Pansh Genesis 6<->16m), and several old big LEI C-kites & Bow. I’d suggest a 15m size to use with a twin tip. The size of a 15m adds stability, is useable in lower (less gusty wind) and gives enough pull for a twin tip to leverage against the added drag.

The bird wings should work fine, maybe add a couple inlets, to add inflation pressure and stability. If relaunch is not a design consideration maybe make it open cell to add even more pressure/flow/stability and quicker inflation.

It might look more like a bird as a high wing design, top skin of wing and body as one surface with the rest of the body hanging lower. Going for more anhedral in the shape should help stability. Using spars to keep the bird body shape might be needed. If relaunch is not an issue rigid spars might be the easiest design. Also adding bridle point to the bottom of the body could help keep it’s shape.

The bird chord is very long which will add torque to the frame, amplified with AOA changes. Spars as torsion-bars along the parameter and even cross braced might be able to help add rigidity. Adding spars (weight) and bridle points (drag) might be another reason to keep the kite size on the slightly bigger side, along with the fact that higher winds & gusts would probably distort the shape even more.

Maybe get a similar 15m kite (like a used Pansh A15) and modify it in stages to see what happens and what is needed for modification, like:
1. Making the center cell(s) to be much bigger below the kite
2. Making the bigger center longer aft
3. Making the bigger center longer fwd
4. Shaping and coloring the bigger center to look like a bird body
This trial and error might give good insights on tradeoffs before starting with a clean CAD design.
:bye:
BatKite2.jpg

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Schietwedder
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Re: DIY Foil Kites

Postby Schietwedder » Fri Aug 06, 2021 9:59 am

Hi Liam,
Very cool idea! Not as stupid as it sounds, with a few compromises it could totally work.
I actually thought about it too wondering how people would look like on the beach if some funky kite like a flying penguin or a giant pig or something overtakes looking at all the blown up animal kites on römö island.

Kitesurfers (including me) are often focussed a bit too much on the latest and greatest material so I love this approach making it a bit more fun.

Don't know if you came across the flying owl paraglider:


I think it shows a bit of the problems what you might be experiencing and might be able to avoid slightly knowing them before but also that it actually works.

Biggest issue is probably that how pugget sound kiter said the added drag. While you see the owl gliding just above the ground (there are more videos on his yt) the kite will fly quite far back in the wind window so it will go not as high upwind and have a lot of side pull.

If you go directly towards 15m is quite a lot of money and exponentially more time for the first time.
I like your approach with 10m and closed cell.
I would make the body a bit more gradual (3cells)

What you could do also:
Make the bird section with zipper, a fixed fifth line bridle and also full d rib and bridge diagonal so in case it doesn't work you can just zip out the center and attach the two wings again with the zipper (or sew them) and you have a fully functional kite and wasted not so much material.

On the profile choice and stuff I can analyse your profile in XFLR compared to some industry standards and the one Rein and me are using but I need some time for that.

The glider put in quite a lot of reflex/stability which I would do also if my life depends on it :D
In a kite it really depends on the effect the bird body has.
If you like I can also provide you with the profile we're using and I know works, just send a mail to info@schietwedderunlimited.de in that case.


Other things I would consider: The center of effort/gravity of the birds body has to be as such that it doesn't support front collapses by its weight, drag or lift. Front stalls though should be not suuuch a big issue with the thing sitting rather far back in the wind window, maybe a bit in very gusty conditions if the body is not pulling the rest of the wing aggressively in a direction it doesn't wants to be in.


Best Regards,
Niklas

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Re: DIY Foil Kites

Postby Liam1 » Fri Aug 06, 2021 11:43 pm

Sadly I don't have the time to give a very detailed answer/response right now but I do want to give a quick response to the advice/considerations given.

To Puget:
Going for a 15 meter wouldn't help too much I don't think, the spots tend to be just as gusty if not more so when I have the chance to use my 15 which doesn't happen very often, and going for a fifteen meter doesn't really allow for testing in "too little wind". When 15 meters are on twintip it could still be used on hydrofoil which would be good enough for testing, it could always be scaled up to 15m if the design works.
You also said it is very long and after seeing the video Niklas sent I decided to make it no so extreme, the owl and eagle both look nice even though they aren't as long as the actual birds are, same goes with the height of the wings, if you're underneath the kite you don't really see the difference (I did change the wing height in surfplan just to see what the difference would be and I don't think the difference is that noticeable).

To Niklas:

Seeing the kites at Rømø was when I was sure I wanted to make my own kite, the idea came a couple days earlier but just as an idea nothing serious. We went to Rømø just a couple of days after you uploaded your video but I didn't see your kite(s) sadly.

I had not come across the owl actually, probably because I was to focused on kites, and it already fought me you don't need to be completely true to the source material to make it look good. I now have the bird shortened a bit which makes the change in shape quite a bit more gradual already I think. That also helps with drag but I don't think the extra drag would stop it from going airborne, just makes the performance worse.

Cutting it in half and using the wings would've been plan b if I couldn't get it to work.

I'm currently trying to analyse the foil/design myself as well in xflr5, but it's my first try so I'm not sure how useful that's going to be... (Following tutorials on yt to try and understand)

Thank you both for the help already!

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Re: DIY Foil Kites

Postby Liam1 » Tue Aug 10, 2021 7:07 pm

So I think I may have finished some type of analysis, the xflr5 file is on my google drive since I can't attach it to this post: https://drive.google.com/file/d/103Xpog ... sp=sharing please check the file on something like virustotal (I'm a stranger on the internet so just in case.).

If my analysis is correct, and I interpreted it correctly, it should work okay (probably neither are correct), it would fly with an AoA of -2 degrees or more (simulated with 3kg for the wing but that shouldn't change the angle just the speed required I'm pretty sure.) and the moment would be nose up from 1.5 degrees up.

The drag of the body is only twice as much as on the cells next to it for viscous drag and about thrice for induced drag which is less than I expected. If you subtract lift from the body from the total lift vector it’s still positive so as long as the body doesn’t collapse or something it’s not too bad.

Is there a way in Surfplan to add d-rib to specific cells rather than to the whole kite? It may be interesting to add them to just the body to reduce billow and generally to hold its shape better, I don’t think it’s necessary for the other cells so would rather do without.

As I said in my last post I changed the size of the body a bit so here’s what that looks like.
Screenshot 2021-08-07 19.10.15.png
And this is what it looks like in xflr I think it's accurate enough for an okay analysis.
Screenshot 2021-08-07 19.54.15.png
If more information is required, for instance, if the xflr file can’t be opened I would be happy to give it but am unsure of what would be wanted/necessary.


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