DIY Foil Kites

[Rein de Vries]

Thank you for your explanations! Bit by bit i also start to understand how things should be done to come to good perfomance. Interesting journey especially nowadays with all the innovations towards the olympics!

Find it a nice challenge to establish a state of the art diy foilkite in Surfplan to share openly. Aiming for some similar design as Schietwedder presents, meant for HF free ride but probably a smaller version.

One of my open points is on the amount of twist the wing should have. In the center region, with the mixer in default I would expect AoA of about 0-3 degrees built in. But how many degrees would be typical towards the tips? Assuming the towpoints are at 18% chord, would it approach 10-13 degrees or even more in a typical performance oriented surf foil?

The choice will effect both the skins and the bridle but I cant seem to find any typical numbers for it.

[kitexpert]

Thank you for your explanations! Bit by bit i also start to understand how things should be done to come to good perfomance. Interesting journey especially nowadays with all the innovations towards the olympics!

Find it a nice challenge to establish a state of the art diy foilkite in Surfplan to share openly. Aiming for some similar design as Schietwedder presents, meant for HF free ride but probably a smaller version.

One of my open points is on the amount of twist the wing should have. In the center region, with the mixer in default I would expect AoA of about 0-3 degrees built in. But how many degrees would be typical towards the tips? Assuming the towpoints are at 18% chord, would it approach 10-13 degrees or even more in a typical performance oriented surf foil?

The choice will effect both the skins and the bridle but I cant seem to find any typical numbers for it.

Hard to give any exact values for built-in AoA's but it should be remembered that amount of canopy curve affect it. If there is a lot of coning kite will have more grunt but on the other hand some depower is very probably lost. If kite has almost flat shape then increasing AoA of it doesn't make much sense, it only makes cells wider on the front.

To find out best values it would need testing, so many different prototypes. Fortunately foil kites can be adjusted by the bridle afterwards.

Traditional way is to increase AoA of the wingtips slightly, to get more stability there, but also airfoil for wingtips is usually modified.

[Rein de Vries]

My plan is to build a small free ride HF foilkite which may also be used in somewhat more wind on TT. Using Surfplan I aim for:
- Flat area 5m5
- AR 6.3
- 49 cells
- miniribs
- no d-ribs
- straps on lower skin at A,B and C level but also one on the upper skin, just above D level.
- 3-4% billow on both skins
- LE battens
- 1:2:4 pulley system
- Airfoil as below but probably with some more pitching moment to prevent front collapse.

I carefully looked at some commercial models to get an impression on what the profiles should look like and see that the tips are somewhat thinner then the center profiles. Also little less camber there I think. The pitching moments are little lower as on e.g. MH91 (cm 0.25 = 0.03).

Would it be adviceable to alter the airfoil a bit to higher pitching moments? And do the camber values seem reasonable or would this be to sharp I wonder

[kitexpert]

IMO those airfoils are too far from what is usually used in foil kites. Too thin, too much camber. Also shapes are a bit rough, perhaps (obviously) not enough way points. Of course it is very difficult to say how kite would behave with this kind of airfoils but I wouldn't want to try it.

49 cells is lot. High cell count is good but it means complex structure with many parts and also bridle line, especially if diagonals are not used. In short: lot of work.

AR 6.3 is high. It means kite is a pretty difficult or probability for success is quite low. Preferably several diy kites should have been finished before that kind of project and dozens of reasonable designs and quite a lot studying of existing kites would help also.

IDK what upper skin strap near D line row would do. Miniribs and LE stiffeners are beneficial.

Anyway good luck, or more time with the design process

[Schietwedder]

I would go to 30 cells maximum if its your first kite.
It would only add weight and construction errors.
It wont be a clean canopy with a lot of cells which prevents your first kite from performing good.

The most important thing to do right is sewing and being conservative when you never did a foilkite.
Those little details like straps, vents and LE battens take a lot of labour. Even until you know where to put them and what shape each one for every cell should take you quickly hit a good three to four long evenings for a 36 cell kite, when you never done it count twice of that, with 50 cells multiply it by two.

So just to figure this out a good 2-3 weeks of time are gone if you do it afterwork, without even cutting those bits out just making the patterns.
If you have good cad skills and a lasercutter at hand maybe one week incl cutting.

Lets take a 25 cell kite, and a 50 cell kite.
For a simple kite you cut a upper skin lower skin and a rib for every cell, we ignore patterns.

That would be 3×25=75 parts
with the 50 cells 3×50=150 parts

Now we do some nice add ons,

Dacron Reinforcements for Bridles and straps on Ribs:
4 Parts per cell maybe less if you only do it for the bridles

Straps
3 Parts per cell

Le battens incl reinforcements
5 parts per cell

Mini ribs incl reinforment
3 Parts per cell

3d shaping
extra 2-3 parts per cell let's say

Thats 18 parts per cell more! Remember we started with 3 parts per cell. We havent even looked at diagonals yet.

So with 25 cell you would have 21×25=525 Parts
With 50 cells 21×50= 1050 Parts.

You see my point? Easy to get distracted by those details.

Without Corona lockdown my 36 cell wouldn't have been finished before christmas.

It's actually not so important which size your kite is, its more how many details and cells you add which determines the overall time.

[Matty V]

I thought this was the Pansh R&D thread for a minute, then I remembered they haven’t done any

[Rein de Vries]

A few cells less would also be fine I think indeed. Reason to stick with so many is to stay close to some commercial bridles. But I may go for some fewer and use Surfplan indeed.

The strap on the upper skin may also be discarded yes. I figured it plays a similar role to maintain the original span while adding billow, but also see it is not always done. Is billow actually mainly something for the lower skin, in combination with the straps there? I would think SP allready takes care of the general upper skin shape according to the canopy so it seems a bit silly to add more to that. Also on my other DIY kites (some foils, arcs and flexifoils) I see allready a 'ballooning' upper skin so guess it is quite fine there. Or do I miss something now

Do also think the idea of the local upper skin 3D shaping is a nice addition as it is also seen now on the latest R1. The French foilivier site shows quite clearly how an ideal upper skin should look there, and it is indeed quite different from what one can get from 2D.

For the airfoil shape I will take some smoothening and some more waypoints into account, guess that this would suffice once the pitching moment is sufficiently positive. Possibly reduce camber a bit and make them a bit thicker as well. I did not find more posts on the forum of people sharing their airfoil shapes actually, apart from Stefano M using MH91 as a starting point.

[PrfctChaos]

Anyone making their own singleskins yet (peak 4 copies / improvements).

Wouldn't a single skin be the easiest DYI kite to start with? Half the cutting, easy access for sewing and they seem to be pretty resistant to bridle shrinkage / inaccuracy. Just asking out of curriocity, since i know lots of people would be interre sted in larger, higher aspect singleakins for low wind hydrofoiling.

[downunder]

Is it worth making a SS kite? for AU700, brand new!?

Large SS are just not up to a job. Plus, we can get a decent used Soul for AU1100.

Brand new Sonic v3 15m AU1600. That's for Euro riders price in Euros, even more.

Making a foil board is way more feasible, with a lot less unknowns.

It is unknowns which puts me off...

[Schietwedder]

I think no diy kite builder is trying to safe money I could have bought a lot of kite quivers for just a few diy kites, its just a very fascinating part where lots of aerodynamics, practical "engineering" and craftsmanship meets. You need to know/learn a lot in many different areas, always question your desicions, it takes hundrets of hours to make one but in the end the tools needed to make a good job are very accesible, just a computer and a simple sewing machine will do in a living room.
Its a very clean work (I hate sanding and painting/laminating)

It's more like an addiction once you have strarted, not a straight forward build project, you have to be a bit crazy. I've sewn all sorts of stuff, sails, tents, bags sun awnings but nothing even comes close to a foil kite in complexity and knowledge required, to just make one that works a little bit to ok.
Its so many parameters that need to be in tune that I would say its also a bit of luck required in the beginning, later after a few years you might be a bit more straightforward.
Very few have the time, and the motivation to do all that, that's why we're not seing so many diy kites.

If there is the interest, I would be open sharing my designs at one point (that ones that work good enough), or even sell a diy kit from the lasercutter to only have to sew which is already more than enough work for the average DIY kiter.
But right now none of the kites are up to a state where I am sufficently satisfied in order to let someone else put so much of his time and money into that.

We will see though with the latest 12m I'm quite confident once I get the bridle designed and sewn that it might me a good DIY kite as I was designing it over 1.5 years before I was confident enough to cut 600 EUR of cloth.
But that will have to wait a bit longer as now its summer and time to go sailing, doing the exams and sail making season has hit crazy after corona lockdown.