DIY AR6 closed cell Quadrifoil Style

[Schietwedder]

Hi guys,
as already posted, (viewtopic.php?f=197&t=2391006&start=190) I´m an engineering student, sailmaker and started last year making my own designs on foilkites and made 3 smaller sizes (1.5m², 2.3m², 4m²) for kite buggy mainly. The 4m² is now depowerable and closed cell. Now after I´m feeling a little more confident in what I´m doing- at least confident enough to invest in materials for bigger kites on a students budget I did some more thorough research on aerodynamics and kite design as I was doing in previous designs. The first three kites were more to learn the manufacturing process and see how all the parameters work together.

What was working really well in those last 3 kites: manufacturing process, sewing accuracy, inflation, skin tension (without using D-Ribs and straps for simplicity), sewing and designing the bridle.
What I want to improve: I think i used a very shitty profile: (NACA2415 to 19%thickn. in foilmaker). As I analysed this profile in XFLR5 i also knew why in hindsight (Pitching moment etc.)
Then I analysed some 10 more profiles used in popular freeride and race kites and those DIY profiles that were discussed at this forum. I quickly could understand the impacts especially of the shape of the camberline on stability. When doing stability analysis I mainly watched out for the location and strength of the resultant lift force on the profile, and watched out to choose an airfoil where it is not behind 30% of chord for as many AoA and relevant Re Numbers as possible as I wanted a stable kite. I then designed an airfoil using the spline function which would fit my demands and interpolated it with Airfoil sections used in modern depowerable kites to see if it was getting better or worse on a stability point of view. I came out with an airfoil which was oriented on some modern race kite profiles but a bit thicker, a bit less camber especially in the back 50% of chord and more reflex also. I got tolerable losses in L/D while gaining a lot of stability at least if XFLR doesn´t lie and I tested based on realistic values and numerical stability.
For the tips i went thinner and with even more reflex. My design ambition is not to make the best race kite ever nor to reinvent the wheel but just to get an ok kite that I can have fun with on the beach, snow and water all day and hopefully can understand more of what modern kite designers are doing and why they do certain parameters.

So in theory I´m quite confident with this profile now but before I will put it all together on my 11.5m² AR 5.3 Kite (Blue/Green) which I will do in late March/April, I will test the profile I´m using at first in a very simplistic kite which is in rectangular shape (like those old quadrifoils). I will build this kite in the following weeks and thought I´d share the process with you.
Why such an oldschool design? Because its fast for manufacturing! It will only have 4 different Panels (Same Rib shape, same top skin, same bottom skin same D-Ribs all over the kite-being rectangular and constant canopy curve)

Some Specs on this kite:
Closed cell, Depowerable
Area: 5.6m²
Proj. Area in %: 81%
AR: 6
Cells: 33
Span: 5877mm
Chord: 978mm
Structure: Dribs full cell, primary bridle every third cell, spanwise straps.
Material: Ribs, Dribs, straps: Porcher Skytex 41g
Lower skin: Porcher Skytex 27g-32g ish from an old reserve parachute (never used)
Upper skin: Porcher Skytex 41g
Bridle: Liros DSL

When designing this kite I realized following benefits which are useful for fast manufacturing/research of design parameters:

-due to rectangular shape and constant canopy curve I can make the whole kite from 4 wooden cutting patterns. I put 8-10 layers of ripstop material on a glass plate. Then I put the wooden pattern on top and secure it with some clamps. I now can use the hot knife cutting for example 8 cells in one batch. This makes for very fast and precise manufacturing. Due to small holes in the pattern I can also cut some marks for bridle locations, D-Ribs and vents as well as alignment holes into all pieces of fabric really efficient.
Due to this method I could cut out all 32 Ribs in under an hour today!! (for a normal bi-elliptic 30 cell kite this took me up to 8h incl.paper-pattern printing, gluing and cutting)
It´s maybe even faster than using a pen plotter.

Pictures, D-Ribs, Upper and lower skin will follow Saturday.

-Spanwise internal straps can be made out of one long strap going through the whole span of the kite. I can therefore just mark this strap at a constant rate for a homogeneous strap tension.

-Because I´m using the center profile in all the kite for simplicity, I probably have to tune it towards the tips when flight testing. The bridle layout lets me do this really easy because I can already adjust the AoA and reflex of the tips in the secondary bridle at a constant value without touching the mixer or affecting the centre profiles. To avoid tip tucking this will do. When having a non constant chord at the tips a smooth AoA or reflex alternation by hand on the beach is really hard to do because each bridled cell would have to be fixed at a time, at least if it should be a clean transition. At best this will be done before hand in the bridle design on the computer, but not for me as being inexperienced with this new profile and using this quick simplistic kite as a practical study.

I was quite stunned how far I came today now being 4h into the manufacturing phase today. I still would have been into cutting paper patterns with a more complex design whereas today I spliced and sewed two dyneema mixers at different purchase rates, cut out all wooden patterns and already cut out all 32 ribs.
It could well be that I start to sew on my next free day, saturday.
I forgot to take pictures today they will follow.

Best regards,
Niklas
sewing @Schietwedderunlimited Boatcovers

[kitexpert]

Ok, lot of stuff to comment. First of all I warn you, trying to make a bigger useful foil kite is not an easy task. Smaller sizes hide design flaws because they are small, they are naturally fast and used in higher winds.

IMO you are rushing now too much, you should take a lot of more time with designing many different kites, say at least 20, preferably 50. You should study different shapes, different AR's, different cell counts, different airfoils and how to morph them in different parts of the wing, different cell shapes, different bridles, different canopy curves, different air intake locations, different inner structures etc, etc.

And you should study carefully how existing kites are designed. Then you would develop your own view of kites and get clearer view what kind of kite you want to design.

I see from your pictures you are already using Surfplan. That is good, it is a very good design program which gives quite a lot possibilities to do many kind of kites.

You are overestimating labor and difficulty with the patterns, it should not guide kite designs. For 5.6m2 kite SP uses less than 100 A4 pages, to tape them together is about one evening work at most. Then patterns are done. That work is not significant at all compared to total construction time which can nearly 100 hours if kite has as many cells as 33.

If you can use bigger printer A3 or some custom paper size work is still reduced a lot. I once used an ink jet printer which allowed to use very long sheets of paper (from roll), then patterns had perhaps 8 sheets of paper and joining them was a breeze.

Cells and ribs can be marked with ordinary pencil on top of the patterns because ripstop is transparent enough. Cutting with scissors is easy and sufficient, there is no worry about fraying edges. This work takes several hours if kite has 33 cells, but it is not that much either.

There is some flaws with rectangular shape if AR is that high, actually they can be quite severe. It is quite possible you are not getting any useful information from it for the next kites.

Anyway I wish all the best.

[foilholio]

Great post OP. Having gone through your wall of text I now know how some must feel reading some of my posts. I have wondered about various changes to camber at sections of the airfoil. Thanks for introducing XFLR5 I will have a look at it and hopefully it can enlighten me more. Although increasing reflex can worsen L/D, don't forget bridles can be set to then pull it out to some degree.

The rectangular design is great for simplicity and will allow for even AOA change across the span. I wouldn't be discouraged to try something original. The kite industry has people working in it's highest level as so called "designers" that merely copy others designs and make alterations to hide said copying. Truly innovative things do not occur in such situations. As you can see with a more original design company Flysurfer releasing in 2008 what is today still one of the most advanced kites, the Pyscho4, and then the design stagnation of the inflatable world, which laughably has exalted aerodynamic benefits to thicker inflatable tubes lol.

[Hansen Design]

Hello Schietwedder:
Very cool to see your approach to embark on this challenging project, both cutting/sewing as well as design - especially with custom profiles. Looking forward to seeing your next construct in the air!
Cheers!
- Bill

[kitexpert]

The rectangular design is great for simplicity and will allow for even AOA change across the span. I wouldn't be discouraged to try something original.

You think I've not done high AR rectangular design? Unfortunately I have and I learned my lesson from it. Rectangular design works ok if kite is small and has low AR. Your thought for even AoA change across the span is mildly said optimistic

I'm not saying it is completely hopeless try because there might be some things which are better than in my try over 15 years ago. I used it for snowkiting once or twice but in general it sucked.

Perhaps it is true that everyone who designs/makes kites have to do his own mistakes and learn from them. It is a hard way.

Mostly I like how Schietwedder approaches kite making process. For example air foil studies are quite similar I used to do.

[foilholio]

I think you are in some deep logical fallacy stuff there. One just because you think your are an expert and 2 because you tried it before. 15 years is a long time. There are a few rectangular like designs with good success.

[kitexpert]

I know what I'm saying. I've never seen good high AR rectangular design but I've had one bad and seen many similar bad looking kites in the pictures.

Let's hope Schietwedder has some success with his kite. If he is going to continue kite making he has to make many more kites anyway, so failure with this one is not that horrible. His design has quite high cell count which is always good thing, especially for that shape.

[Schietwedder]

Hi all,
Back in my small workshop on Saturday I made some pictures of the process.
I have now cut all 250 Parts roughly around 10h into the process.
I decided to go for leading edge reinforcements from a very stiff dacron/dyneema hybrid composite which i use for aero tramps for foiling Catamarans normally. Its comperatively light and very stiff, I dont like the Mylar Laminate used in many kites before leading edge battens. I will of course do leading edge battens in my future kites but for this one i hope it will do hopefully.
With Reflex I went for a bit more than i will actually have in flying mode as its always easy to pull reflex out with the briddle/mixer settings but harder to put more in I think as its always a combination of B and C. I dont care about rinkles in the cloth when "bending" the ribs with the mixer especially in the upper skin as the boundary layer is thick anyways in the back of the kite. (Around 3-5cm acc. to XFLR5.)
What I'm concerned about more and what I dont like to see in many foil kites on the market, at least in underpowered and midrange conditions is that the trailing edge is often pulled straight down by tight brakes and this of course will result in a late kutta condition behind the sail or even some flow seperation and drag to a certain degree. This can be helpful though for preventing the kite from collapsing at the edge of the wind window and depo2wering it a bit in very gusty or strong wind conditions. I thought about putting pockets for Nylon rods on the ribs from the trailing edge to lets say to C lines or a bit beyond to bend the profile a bit more consistant in the back with non linear mixer settings. In real life the mixer is never pulling a linear rate even if its supposed to in its initial design. I know it is quite a deep topic with a lot more to it what I not might be aware of but I will just design the kite in a way that i can quickl put this rib battens in and out on the beach to see if theres a positive/negative effect or if its just unnessesairy weight.

Ok some comments to more theoretical comments on this design:
I am aware of the risks of this concept but as its really easy for me to make this kite I will just try it than spending even more time in front of the Computer and consodering thinks which i dont know their exact impact and corelation with the experience and research on kite design I got at hand.
Im always a fan of just doing things practically and was quite succesful with this methorld in other aspects of engineering and sailmaking.
The nice thing about kite making is that the manufacturing process takes a long time with a lot of repeats on every phase so you can touch all different parts see them in many different perspectives and most importantly think over all desicions you took and see how they affect other parts so as I'm sewing a certain kite I already know a lot of things I would do differently in my next one or whats even a good design choice. Some thougts are wrong when flight testing afterwards of course but in some you get good confirmation so I see no point in doing 20 different designs in the computer with unknown outcome really when I dont have a full CFD Ranse Analysis tool coupled with FEM and VPP Programs and even then theres far too many unknowns to make a realistic prediction in the first shot.

For the Green/Blue AR5.2 Kite from above which i will do after I put a lot more analysis into. I measured ceveral succesful foilkites (Chrono, Soul, Jojo Racemasters etc.) in all aspects from strap tension to skin tension, profile shape to the more basic parameters like AR Area canopy curve etc.
I then did a Excelsheet of 50 Foilkites or so where i put in all different relations (AR to cell count/area/canopy curve etc and many many more to see if theres popular links or even go to settings when choosing a certain AR Area and Cell count. I found out a lot there and could always rate some of my design choices if I'm sticking to the values often used for my type of kite and how important the different parameters might be for a good kite depending on how much the Numbers spread on modern designs. So actually I think i did my homework to allow for some practical work again and then of course a lot of time on the beach testing. I'm not so worried about certain parameters I went to as Ive got confirmation from values used a lot in the industry. When it comes to the profile shape though it gets a lot more complex and this simple tool will not help me unfortunately beyond profile thickness and position.
If this rectangular design works ok i can always decrease AR or adding more round tips and improve it a bit or see the impact of those parameters on this special and easy concept.

[Schietwedder]

Here some pictures

[kitexpert]

Sounds good and reasonable.

To make a lot of designs helps you to master design program and it will develop your ideas and views on kites. But of course on some point designs must be made reality, unfortunately that part is quite arduous if you work alone. At hindsight my development would have been faster if I had been more conservative and also more long term development oriented. But I wanted to make all kinds of kites and kites for certain interest which I had that time and it was just too much work to make experiments.

I like how your airfoil looks like. We will see if it helps to avoid some monkey business. If you don't know what I mean you perhaps will, hopefully you don't have to

It is good point that you can alter your design afterwards. My path to high AR rectangular kite was I added cells in the middle of low-mid AR kite. If I remember correct I finally rounded corners by redoing wingtip 2-3 cells, but it didn't help that much, if at all.