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DIY Foil Kites

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elguapo
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Re: DIY Foil Kites

Postby elguapo » Mon Apr 05, 2021 3:38 pm

that's a nice moving kite, man...

(what the story on the woodie control bar?)

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Re: DIY Foil Kites

Postby Rein de Vries » Mon Apr 05, 2021 9:34 pm

Thanks Niklas for another great video and Kitexpert for your comments. Recognize the point that small foils are fast and quickly fly to the edge WW where they loose pull. This in contrast to LEI kites which have more constant pull at a lower speed while staying more in the window. I made this foil also rather thin (17% centre profile) which makes it even more extreme in this sense. Comparing back to back with a Sonic 2 6m2 shows that the Sonic also has more constant pull at a lower flying speed. Will tune the 5m2 to a setting with some grunt and stability at the same time to guarantee some pull when on the water. This can be done via the mixer I think; giving it some higher AoA (increasing stability and the kite remains deeper in the WW) and little longer B en C at the same time increasing camber (and herewith power) a bit. This would be my best guess before going onto the HF with it.

Another thing I notice is the subtle tip stalls while turning. Niklas's AR 5.3 (same profile) does not show this and I guess I should be able to get that out via the bridle but have no clear plan yet. Mine has it probably because of the higher AR and perhaps I should wait whether it is actually a point when on the water. On the other hand, it can be improved anyhow so ideas are very much welcome.

A last point is the tip collapses at the edge of the WW while turning. These show up occasionally, also in contrast to e.g. the Sonic 2 6m2 which is even higher AR. Would the difference be determined by the local AoA in combination with the TP's? I wonder what I miss here.

Altogether I am very pleased with my project results; the flying characteristics are pretty much as expected, the inner structure with straps and ribs give altogether a rather solid steering and the billow appears more or less as expected. :D

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Re: DIY Foil Kites

Postby downunder » Tue Apr 06, 2021 2:18 am

The bar was discussed, its a nice touch.

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Re: DIY Foil Kites

Postby Schietwedder » Tue Apr 06, 2021 10:02 am

kitexpert wrote:
Mon Apr 05, 2021 12:44 pm
IDK if you have much or at all experience with LEI's but they are easier because they sit deeper in WW. Single skin structure allows more grunt, back stalling later (or not at all) and recovering it faster. Small foil kite needs much more kite skill, you have to be able to feel what kite wants to keep it flying and at the correct part of WW and producing force. With slow high drag board kiting can become jerky, i mean controlling pull can be a challenge.
That´s exactly what I learnt and grew up with in the kite buggy as a child, to steer a rather small non deowerable handle-foilkite where it needs to be to control the power so I actually love that feeling of a foil on the water with TT. But yes you´re right if the waves get high and breaking like in the north sea it gets a bit complicated with small foils on TT as you need quite a lot of power on/off going through the wave above into the next one. Also the relaunch in breaking waves get´s an issue, so thats when I use my tubes. And yes they are easier and above 25/28 knots I´m rather on a tube. But thats another discussion in other topics tube vs small foil.

Here on the (most of the time) rather flat baltic sea I love getting up to speed with one/two power dives and have the pull so much forward still with the foilkite, being able to scream upwind when I want. Ultimately I designed the 6m for HF use but my riding skills aren´t progressing as fast as my sewing apparently :D (Otherwise there wouldn´t be a point in making it so light)

To go back to the design thread and also what Rein is experiencing:
We should discuss why that deep in the wind window physically is working so well with tube kites (on TT) and why a foilkite struggles a bit more in that respect as there might be some nice learnings to have foils that have a lot of grunt when powered with the bar but nice stability and high L/D when depowered so here´s my aspects:

-foil kites have much less camber in order to be stable in pitch motion (or not to frontstall in kite therms). They fly further to the edge of the WW (less AoA) and therefore are more prone to get negative or zero A0A and flip over if having too much camber when being depowered. That is just because drag is missing compared to a tube. So one needs to find a way (for freeride TT, slow HF, Waveboard, a race guy would call me stupid) to add drag but add a benefit to it as well (more grunt).

-lift is related to drag so adding AoA (sheeting in) brings the kite further back in WW but then with the average foil we experience earlier backstall as with tube kites. Why is that? My thoughts:
-we induce less camber with foil kites than with SS or Tubes when sheeting in... stall occurs earlier. (attachment points with larger distance from Z to C than from C to B and so on can help adding camber when sheeting in as well as certain mixer settings)
-due to billow we will never get such a smooth LE and uppersail panel like a tube, so also because of that we will always have earlier stall in the slow flying/high AoA domain with a foil as long as that issue isn´t solved. (Double membrane foilkite with smooth upper sail sewed onto the billowed skin, anyone?)

-Profile thickness and AR are two main drivers of drag. If on the very thin/high AR end thats where Rein is at atm. It´s nice though when being up to speed on a HF as you will always have a really far forward flying wing, getting going might be complicated getting up to speed with a race HF wing setup and 5m². But the main reason for Rein was a small and not so expensive/long to build test platform before moving onto a bigger kite project as far as I remember.
The thickness thing is probably one of the main aspects making the Soul so easy to use and accesable for beginners as its thick as, so it sits a bit deeper in WW among other "benefits" (for freeride) a thick foil has.
I´m quite sure and Rein and me discussed a lot in private mails that there is an important relation of profile thickness and flying speed/RE number what a kite has to match for a certain range of use/purpose in order to not fly out of the WW and stay there with not much pull until getting up to speed for slow applications without much apparent wind/downwind angles.

Also interesting to discuss a bit more the Tip collapse.
We´re both using quite agressive aerodynamic washout meaning that when I deveoped the tip profile I added a lot and more than usual reflex in the tips. (High pitch stability in tip). I wanted to get tip collapses off the list, and for my designs it never once occured, although even running same AoA over the whole wing (no geometric washout) as most have to increase AoA in the tips to keep the tips out.
So the biggest difference is the higher AR of Rein.
I also run 10% attachment on A (relatively far aft -> high stability again because counteracting bridle force gets closer to aerodynamic force and therefore not having such a big pitching moment as the lever of the moment is reduced. Don´t remember your( Rein´s) value. I run very stiffly sewed in LE battens in order for that to work and support the forward part of the LE, also distributing the loads of A-risers better
Another aspect could be sewing/billow related differences as it´s quite hard to sew those small tip cells together and small deviations have big effects due to size effects for high cell count, high AR, small wings.
Inflation pressure during flight due to vent layout another point to look for maybe

What we can (can we?) write off the list for tip collapse reasons is bridle layout, because the tips come in because they frontstall, and the bridle cant do much about it because it can only pull in the "wrong" direction for that matter. What the bridle can do though is induce higher AoA or reflex in the tips to resolve this issue.
It should be checked for safety before changing something else though that the bridle is close to "factory" settings after being loaded the first time to ensure that the profile is kept as it´s designed for in the tips and doesn´t have too much camber as B and C might be out of tune for some reason. Small devitions in the tip bridles have big effects again for the short chord.

Another point I could think of (if I remember correctly) is that in the tips your B lines gets the A lines as there is only 3 attachment points in the tips. If you depower B could be released so much that the tips become unsupported so they have no (A- or front line to flag out on). So for a trial connecting the tip bridles on A B and C instead of B C and D could be a thing to look out for (If I recall correctly again).
Let me know what you guys think!

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Re: DIY Foil Kites

Postby kitexpert » Thu Apr 08, 2021 11:02 pm

Both of you have reached quite fine results already and you can do a lot by getting familiar to how that certain kite behaves and how it needs to be flied.

Comparing to kite like Sonic3 is a bit too much because there is years or decades of knowledge and work of several people behind it, and thousands of hours of work to make it finished product: prototyping, testing and manufacturing kites. Usually diy kite maker does this all alone. However in some respects diy kite can reach pretty high levels and certainly using diy kite can be a pleasure - not only because it is diy.

Very nice to see how usable kites you have created, I can see it by how they fly in your videos.

Wingtips: To have them stable can be difficult and even finest kites like Sonic3 is not completely free of them folding down occasionally. Problem is if you make them rock solid it can decrease performance. Whole issue is complex because so many things are affecting: used airfoil, amount of canopy curve, wingtip width (chord length), locations of attachment points, built in AoA (washout), how wingtips are bridled (pulling ratios), internal pressure, battens, cell shape...

So I can't say or isolate one thing but if I had to pick one I'd say solid wingtips depend a lot of air foil shape. Wingtips of course have much smaller chord than center of kite so it is worth some thinking how to pull them. But again, there is many things which affect on what finally happens when kite is powered up.

Foil kite/LEI kite difference: In high AoA's and low (apparent) wind speeds single skin has an advantage, it tolerates higher AoA's without backstall. This gives more lift. Double skin foil kite is better when it flies fast or kiter gets more speed (higher apparent wind). There is also known differences of inertia and cleanness of surfaces etc.

Interesting idea of double membrane foil kite, I'd be happy if i had it. Probably not practical enough but well thought.

All in all I see good thinking, you are doing well

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Re: DIY Foil Kites

Postby Rein de Vries » Fri Apr 09, 2021 5:50 pm

Interesting discussions, and thanks so much for the compliments!

In my 5 I took A at 9% and towards the tips the prof. thickness goes down to 13.5%. Not so different as Niklas's settings and using the same profile I figure mine is more unstable in the tips due to the higher AR. My guess is that getting it right should come from a combination of Canopy, local AoA, TP's and the bridle settings as e.g. Sonic 2 has a much more cambered profiles which are found to be clearly very stable. So the profile cannot really be the issue I presume. Would it be that interesting upturn in the 'canopy of the LE' towards the tips that makes the S2 so stable? The tips basically stand a bit open towards the wind, whereas my circular canopy is rather inward. I think I see a similar difference between S2 and S3, S3 being also sensitive on the tip stability again.

My attempt to 'sloop' ABCZ in the mixer and increase AoA worked out fine. It does not overfly me anymore, is more stable and has some more grunt in a little less fast flying motion. I think this is better towards being on the water for starters.
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Re: DIY Foil Kites

Postby Schietwedder » Mon May 17, 2021 1:45 pm

Little update from my little diy kite studio here in Kiel:

Continuing here on my 9m in the evenings after studies/work.
It's basically a scaled up version of the 6m which works fine and I use on the water now regularly so not much new technical features to discuss about.

Only changes:
Ribs, D-Ribs and A,B Straps Skytex 40 Hard instead of 32
C and Z straps Skytex 32 Hard
Skins still Skytex 27.

Z straps at 96% proved to be really useful for direct handling as without I have the feel in my other kites when giving a steering input you first tend to pull out the billow and then something happens. To know how big this effect is is of course hard to tell without a second proto without this feature otherwise being exactly the same but it doesn't make handling worse for sure so a nice to have feature without a lot more work construction complexity.

Also have the possibility to switch from A lines at 7% to 10% chord to see the effect on depower and stability. At previous kites I went with 10%

On BCZ I have the possibilities to run with
deltaAB=deltaBC=deltaCZ or
delta AB<deltaBC<deltaCZ
to increase camber and delay backstall when pulling the bar.

Added C wires above bridles ribs, extended entry vent sleeves for less water intake.

Of course also fine-tune the bridle with towpoints so it matches an off the shelf bar, as I know the towpoints have to move a bit forward to not have to extend the backlines with pigtails again like on the 6m.

Staying with varying towpoints in chord and span direction for each bridle row (ABCZ to account for displacement by mixer and to let the brake act more outside on the wing.)

Maiden flight in a few weeks maybe if the wind stays this light so you can't kite and have to continue the winter hobby...

In autumn or winter, might build a Lasercutter and a 16.5m2 AR 6.5 lightwind machine is in the design process.

Best Regards
Niklas
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Re: DIY Foil Kites

Postby kitexpert » Mon May 17, 2021 10:23 pm

Schietwedder wrote:
Mon May 17, 2021 1:45 pm


Also have the possibility to switch from A lines at 7% to 10% chord to see the effect on depower and stability. At previous kites I went with 10%

On BCZ I have the possibilities to run with
deltaAB=deltaBC=deltaCZ or
delta AB<deltaBC<deltaCZ
to increase camber and delay backstall when pulling the bar.
Ok, I was a bit puzzled about this but then I saw you have doubled attachment points in B-C. That is nice for tests but you could have done it more, further apart from each other.

I've also tested different A line row locations but I couldn't find any difference so about 10% seems to work well.
Schietwedder wrote:
Mon May 17, 2021 1:45 pm
Of course also fine-tune the bridle with towpoints so it matches an off the shelf bar, as I know the towpoints have to move a bit forward to not have to extend the backlines with pigtails again like on the 6m.

Staying with varying towpoints in chord and span direction for each bridle row (ABCZ to account for displacement by mixer and to let the brake act more outside on the wing.)
I usually just move front line or A line row attachment on mixer. IDK if it is necessary to think tow points so much, just something reasonable and then depowerable concept with a mixer and stable enough wing (airfoil) takes care rest of it. Tow point moves a lot when kite is depowered/powered up and kite itself defines how large this AoA range is or can be.

Brake will act more closer to wingtips because chord length is shorter there. Quite often this is compensated a bit by having brakes for example at 95% at the center and 100% at the wingtips. This difference can be even more. However SP doesn't have a design option for this. Another method is to have different pulling ratios for wingtips like FS had in Sonic Race (unfortunately they didn't do it as well as they could have).

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Re: DIY Foil Kites

Postby Schietwedder » Wed May 19, 2021 3:13 pm

kitexpert wrote:
Mon May 17, 2021 10:23 pm
Ok, I was a bit puzzled about this but then I saw you have doubled attachment points in B-C. That is nice for tests but you could have done it more, further apart from each other.

I've also tested different A line row locations but I couldn't find any difference so about 10% seems to work well.
Yes I could have done more but would be outside of the straps then, as I came up with this from experience with the 6m after cutting straps already for the 9m.
Most kites I came across run from 7-10% though on A and that is the range I can adjust in.
In theory there should be a difference, which strongly depends on the profile though and the CP travel/resultant Lift force x position travel in the low AoA domain. Moment equilibrium will define the lowest AoA when flying static depowered on A. When using a profile with a lot of Cp travel (cambered) there might be a far bigger effect and pitch stability issues than a more reflex/symmetrical profile. So we´ll see in practice, but my profile is for sure more on the stable side of the spectrum hence there should in practice also be a more benign effect probably relating to your practical experience as well. But I´ll never find out without trying. If there´s more depower range, I´ll greatly take it, if I run into stability issues, I´ll move back to 10%.
Of course this is more important to smaller kite sizes when winds are gustier, they accelerate more, and everything happens quicker. When having a 6,9,11 quiver I also seldomly fully exploit the upper range of the 11 but on the 6m it can happen rather fast was my experience as force of course also rises quadratically with windspeed and a 5knot gust has far more effect in 25 knots of basewind than in 10 knots.

With the B C and D points it is rather low in importance and more a luxury than a must have adjustment (I think) because the mixer increases camber already a lot in higher AoA by its kinematics when used and set right.

The recent kites just feel very dialed now in general compared to my first trials (9m is my 7th DiY now), they fly out of the bag with little adjustment recently (more adjustments based on what I want it to behave, little that it has a really annoying feature that affect its flying ability or usability) so I feel I can concentrate or variate more on the little things to study these effects a bit more which leads then to a more round and thought through design in more aspects rather than a kite that just flies and works for personal use. (I was very happy if at all I could achieve the latter with first designs)

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Re: DIY Foil Kites

Postby kitexpert » Fri May 21, 2021 7:38 pm

Good answer. It really is a pity when diy kite maker is limited by practical issues like projects becoming too laborious for experiment freely.

Ideally kite bottom skin should have bridle attachment point on every 5% or so, then real mixer/camber testing would be possible. But having span wise strapping then would be quite a challenge and also having so many loops would mean a lot of work.

Stable airfoil (wing) doesn't care that much if A line row is at 7% or 11%, at least that was my conclusion.

In experimenting it should be best to go too far in both directions. Only then certainty of best adjustment could be had. Being too conservative or cautious is very probable - when you have to work tens of hours to get something finished you don't want to risk that much.

Why mixer (1:2:4) would "increase a lot camber in higher AoA" like you wrote?

Good to hear improvements in your design. I also noticed my best kites flied quite well immediately and there is not that much need for bridle tweaking etc.


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