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DIY Foil Kites

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jaros
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Re: DIY Foil Kites

Postby jaros » Wed Jul 22, 2020 6:22 am

Congrats on your deserved success. You sure have the skills and patience! :thumb:

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Re: DIY Foil Kites

Postby JakeFarley » Thu Jul 23, 2020 10:57 pm

Has anyone attempted to build a DIY single skin foil kite? Seems like it should be simpler to build.

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Re: DIY Foil Kites

Postby kitexpert » Sat Jul 25, 2020 4:33 pm

Single skin kite is easier to sew but it has more bridle line which adds details and line work. As a concept I wouldn't say it is easier than normal foil kite, more like opposite. I've done couple of them with usual wing tip issues and after that designing Peak 4 type wingtips which helped, but it is still a single skin kite. Because I value dynamic feel (powered turns etc.) and boosting ability it is not my favorite kite type.

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Re: DIY Foil Kites

Postby Schietwedder » Tue Jul 28, 2020 7:00 pm

I´m working on a single skin as well. Will be sewn next winter for snowkiting season.
Yes, its easier to sew, easier to change once finished (which is quite important when doing prototypes, but more laboriuos to design at least with SP, maybe Paratailor/Single skin work better, I will give them a try as well before I print patterns, see what program is best to draw what I have in mind so I don´t have to do so much manual work.

Some Lessons learned with the 12m Foil:

After 4th time packing or so when i wanted to show it to some kiters on m local beach, already some glass fiber leading edge battens had snapped in two or cut through the end of their pockets, although I had reinforced them double with extra thick Dacron. They have to have quite a lot of tension to follow the form of the nose radius.
Maybe it has to be packed like a paraglider wing as well, but the cutting trough the pockets and breaking can also occur when crashing on the leading edge, which should not but can happen with a freeride foil.

I removed all battens now, and will test how the kite works without.
All the battens saved me 350g of weight at least.

I ordered some Nylon battens today, that are used in paragliders, from Kontest GmbH, a service Company for PG wings in North Germany. They are also used in most flysurfers and ozones. I hope its Nylon 6.6 which has better Spring Capacity and durability than Nylon 6

Then I can compare how the wing works with GF battens (not very practical but really nice tension and shape of the LE), without (light), and with nylon battens (not so much tension, but they dont break)

Here is an article in German about battens in PG wings, for kite design the aspects of dynamics in collapse situations maybe are not so important, but on A line positions, performance and stability in lulls there is some effect. I will see once I try the kite without. The 12m has the A lines on 10% chord which is quite standard. It could be a bit more backwards with stiff battens, maybe its a bit too far backwards without battens, and maybe a good position with the nylon rods, we will see. (Hard to tell though without other protos, or more connection points, on A and B at least. (Most Protos have couple of connection possibilities built in for good reason, I was too lazy though-or too stupid)

https://www.dhv.de/fileadmin/user_uploa ... ebchen.pdf

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Re: DIY Foil Kites

Postby kitexpert » Sat Aug 01, 2020 7:46 pm

Thanks for sharing your experiences. When you make kites by yourself it is a burden to make it any more complicated than it has to be. I mean for instance if you double A line row attachment points.

I once did that, there was perhaps 4-5cm distance between the connections. However kite flied and behaved pretty much same in both cases so I came to conclusion usual location of A row works or it isn't that critical.

There is a better material for battens than usually used nylon chord or thin GF rod, but I keep it for myself at least so far. But those two are usable too, however glass fiber rod I don't like that much because of its fragility.

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Re: DIY Foil Kites

Postby kitexpert » Sat Aug 01, 2020 7:58 pm

I disagree single skin kite is more difficult to design in SP, you just have to use it a bit unconventionally. Making typical ss flares to the ribs is easy to do manually after patterns are printed.

But to design hybrid kites in SP is difficult, then you have to stretch certain parameters to the limit and use program creatively because it is not designed for that kind of work.

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Re: DIY Foil Kites

Postby Rein de Vries » Wed Aug 12, 2020 12:59 pm

With some great input from Schietwedder and Kitexpert I finalized my design! Plan is to first make a 5m2 for stronger winds on HF, TT or buggy, and later on a larger more racy model ~11m2 for lower winds. :)

Area flat 5m2
AR 6.3
Span 5.6 m
41 cells

Airfoils:
17% thick @ 27.6% chord, 1.4% camber in the centre
13.5% thick @ 27.7% chord, 0.6% camber on the tips, some reflex
Similar to what Schietwedder used

Canopy:
Circular arc with 83% proj. span over total span (or 122 deg.)

Structure:
Billow ~5%
Skin tension at LE and TE ~ 2%
Straps at A level, and in between the B,C and D levels
Arched D-ribs, H ribs and mini ribs
Two 3D seams on the upper skin nose
LE battens

Bridle:
AoA centre 3 deg
AoA tip 6 deg (so 3 deg sewn in twist)
1:2:4 pulley mixer
Towpoints at 19% chord
final design.png
Design 5m2
I just started cutting paper :D

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Re: DIY Foil Kites

Postby Freddyforlaunch » Tue Aug 18, 2020 10:17 am

Great design Rein! Why cut paper? I lay the fabric on top of the paper and heat cut the fabric. You will see the lines through most colors. Saves effort.

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Re: DIY Foil Kites

Postby Schietwedder » Tue Aug 18, 2020 10:54 pm

Nice to see some more DIY guys doing it.

With a more racy design than mine, higher cell count (41) and AR you could (but don´t have to) maybe go down a bit in billow if not too late already. If you´re doing H/bridge diagonals adding billow manually by strap tension gets complicated as the diagonals and bridges have to be shortened as well. Going the opposite way by using a billow function for the top and bottom skins could be easier. I´m not so familiar with the billow function in SP but maybe it does exatly what I just described.
The 3 primaries on the outside tip could share a cascade in the bridle, bit more economical, nothing major.
Otherwise I´m looking forward very much seeing your design progress, high AR and small size is very ambitious combination for a DIY wing as it naturally has to be performing in gusty strong winds where stability is important, but even more amazing if it works.

And remember that probably equally important than a good design is clean manufacturing, fitting alignment points, clean nose seams, reinforcements, and low seam shrinkage. Do a lot of sewing test with all your material, thread, stitchlength and tension combinations before moving onto the build.




Meanwhile I tested and tweaked my 12m a bit more.
I am very happy with overall performance, depower and stability. No issues at all.
In light winds for me as a twin tip beginner getting going is a bit harder than on the SOUL 12 because turning is a bit slower and it stalls earlier. I described the problem and what could be done to improve it below:

You can see that when you downloop it in light wind, it has less to no power in the first half of the turn and then pulls like a maniac. If you know how to handle its special behavior and are better on the board control than me it can be quite nice, because you can rotate it on the spot, even fly it backwards to where ever you want in the wind window, release the bar and it lifts you directly.
For me on the buggy and snowkite where I can concentrate more on kite control this is actually quite fun to play around aggressively with kite control and release the power on the spot. However as a water kite beginner it makes it hard for me to water start in the low end, when I loop it has low power than a lot in the downloop and then low power again when it flies to the side of the wind window. Once I have better (TT) board control and can maintain my glide and apparent wind I will have no issues with it.
But since I also tested the 12m Soul, which turns a bit better with cleaner and more constant power, also with less stall when oversheeted in light wind I will look at following aspects in my design:

Firstly I changed from an Ozone pro tune mixer to a Flysurfer mixer. The Flysurfer seems to fit my kite better, I don´t know why but when mixertest is at 0 the Ozone mixer seemed to pull C before Z, even when shortening Z and extending C 2cm. On the Flysurfer mixer the kite seems to fly a bit more balanced when sheeting in.

-It has quite a flat arc shape which naturally results in less turning ability. When sheeting in one side of the kite a tip is angled at 60° when viewed from the front.
It has quite a considerable amount of vertical lift in relation to the sideforce needed for the steering.
You hereby stall it quite fast when the force for the steering input you want has always to be a lot higher than needed if you had a 90° tip which produces no vertical lift (C kite).

-The turning theory above though is not fully true as a stand alone as its also affected by bridle entry angle and not arc shape only. The angle at which the bridles enter the kite (especially on C and Z) is ultimately the direction of pull.
If the bridle line is not entering 90° into the canopy, it pulls in a different axis than the AoA altering of the ribs. With a very horizontal entry angle in the tips you get a bigger steering input than the flat arc of the wing would do naturally if I understand correctly?
Thereby I could design a new Z bridle which is shorter and has different entry angles.

You can turn a flat wing in a very tight radius when only pulling the brakes (Z/D brildes) as its done in 4 line open cell kite buggy handlekites and paragliders. You stall it a bit by only pulling the brakes but only on the back part though so you quickly get the power back when loosening the brakes. Methods to use this technique:

-Shortening Z. It stalls the backlines up to the center though and not only when you want to turn but also when you jump, or want more power, its not favorable for general behavior.

-Only shortening the outside Z bridles. I have done so but it also changes the towpoint of the brakes at least when I do it in the lower cascade (3rd cascade).
I´m not sure why and if correct, but it seems to load the inside brakes even more when the towpoint is moving out.

-Moving the brake positions from 96%of chord to 100% of chord for the outer Z connection points, and maybe even moving b and c forward towards the tips. I could sew some connection points to 100% onto the closing seam but for B/C I would need another proto.
The moving Z back towards the tips has been done on Speed 5 and on Soul, not on any racekites though. Easy for me to try out the effect, worth a try for sure.

-Cut out or extend (5cm) one or two Z bridles in the middle as they can harm the turn. Less lift in the center but maybe better steering and a faster kite (less drag).

-On flying photos I have quite a lot of billow on the top skin especially in the 1st 30% of the chord, probably because of my manual billow, 3d seam and because the bridle is designed for a flat wing without my billow/strap tension measures. I could increase the arc of my kite by the bridles a bit therefore my top sail would flatten and I´d have a more clean surface especialy in the LE where thickness is biggest. Therefore I wouldn´t stall the wing as fast in light wind as the top sail is the most important area to have a clean surface. Easiest try would be to just shorten the 3rd cascades, I don´t know how "clean" this technique is though. I don´t want to sew a whole new brildle if I can experience the effect of the top skin billow first in a different way to see if relevant.

-I could thicken the profile. It is quite thin for a mid AR wing of 5.3. This would give me more drag but also more stability and later stall in light winds. Stability is really not an issue at all with this wing, I never managed to frontstall it, but having later backstall in light wind when steering hard could be a benefit. The soul for example is a lot thicker but has a lot of performance still. Thicker airfoil is of course also slower, which needs to be considered for turning as well but as long as it turns, slower moving with consistent power in a loop for example is ok, especially when learning on the water. When flying up and down it could be a bit annoying with a slow wing.

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Re: DIY Foil Kites

Postby Rein de Vries » Wed Aug 19, 2020 8:16 pm

Thanks Freddyforlaunch! On some pieces it may indeed be easier just taking it directly without cutting.

And thank you Schietwedder for another interesting post. On the steering ability and the mixer action I can imagine the Z points on the chord play a role w.r.t. the flight characteristics. On my Sonics I noticed that, interestingly, they find their position further to the back in the middle of the kite. ~88% on the tip ~94% in the centre. This will have it's effect on the steering (turning) and power distribution in flight I suppose, so playing with it may help to fine tune.

On the billow I indeed plan on using the billow function in Surfplan, adding some thickness to the width of the cells. The design without billow is then just the skeleton of the kite with its ribs and straps exactly matching that baseline form. I tested SP and it prints bridles, ribs and airfoil shapes completely independent of the billow settings. And this is what I printed so far, so I can still choose the settings of billow that only affect the upper and lower skins.

Regarding the values I tend to be conservative as well, I would not want to risk taking a to low billow percentage and ending up with a kite without strap tension. On the Sonics (2) again I measured also in the order of 5% (of the width of the cell) on the bottom skin. Following that and tapering of to zero at the LE and TE for (skin) tension at the edges, I hope it will be fine. Furthermore I also tend to follow the settings from the foilmaker manual in the skin tension section for the percentages on the chord where my billow tapering starts. Which is on the upper skin at 10 and 85% and on the lower skin at 15 and 75%, resp.

Any further comments ideas or suggestion on these choices are very much appreciated!


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