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Bow...

Bow kites are great! Power on / off when you need it... And they turn quick. Plus you can make an 8 metre BOW with a projected area equivalent to an 11 metre C kite.

The Theory

The bow design was developed to try to make more efficient use of the canopy of inflatable kites. 'Traditional' ILE's have a canopy curve such that only about 65% of the canopy area contributes to upward lift - the rest is lost in the tips. The flatter shape of the Bow means that a massive 85% of the canopy is now generating lift, meaning that we can use a smaller, lighter, faster turning kite to generate the same power...

Another advantage of the bow is that it will depower to a much greater degree than a C shape kite, which means it can be used in a wider wind range.

The canopy curve is now a long way from the shape that the kite will naturally take up in the air (the traditional C shape) so the kite now needs some external support to maintain the shape.

Foils (apart from ARCs) have always used bridles to maintain their shape, and we do the same with a Bow, but only on the leading edge. Bridling the whole canopy would result in a very lumpy aerofoil section...

If we tried to do this just by modifying the canopy curve of a C shape kite we would have a problem because the unsupported trailing edge would deform back to the C shape, while the supported leading edge stayed flat, and the AOA at the tips would increase... we would have a mess.

One solution (there may be others, but I don't know of any) is to sweep back the wingtips, making the trailing edge concave. I must confess I'm not entirely sure why this works, although it puts the outer bridle lines further to the rear of the kite, and 'feels' right. i suspect the trick is to get the amount of sweep just right...

I don't know how much is 'just right' so i kind of guessed based on photographs of commercial bows and, well, just guessing really. Scientific, eh?

Enough Theory, onto the Practise...

I love 'how to' sites with pictures detailing 'how I built a nuclear reactor in my basement' or 'how to change the oil filter on a Ford Cortina' and I'd like to write a site like that one day, but I just don't think I'm organised enough to take all those photographs and write about each step in detail. Still, I'll try to make this page something of a 'How I built a bow kite in my kitchen and it flew really well'...

Design

I designed the kite with Surfplan, you can download the sle here. It has an area of about 8m2 because I need a strong wind kite, and the projected area is roughly the same as that of a C shape 11m kite.

I'm going to make this one out of polytarp, blue and orange because those are the colours they have at my local hardware shop. I would use white if they had it, because it makes it SO much easier to mark the pieces and sew them together when you can see through the cloth...

I'm using polytarp because this is an experimental design and polytarp is cheap. Also because the ripstop nylon I've been using so far has been a disappointment. Over time it has stretched badly and lost its shape to the point where the kites are no good for surfing any more ;-( Both the ZP8.5 and the ZP15 (to a lesser extent) tend to luff at the tips when the wind is strong. The problem is worse when they get wet - then they are nearly unflyable. Not good for a surf kite!

The polytarp might have the same problems of course, but I'll only have spent about 20 euros on it, and if the design works maybe I'll make another one with good polyester fabric.

Building

30/01/2006 - I printed out the plans on 5 million sheets of A4 and stuck them all together. The shops were closed so I couldn't get any polytarp, but I had a bit left over, so I cut out what I could from that, and sewed together 3 of the ribs. I'll buy some more material tomorrow.

I'm using a triple zig zag for all the seams on this kite because my machine is a real pig when it comes to straight stitches - the stitches always come out ridiculously small no matter how I adjust the machine. If anyone knows why this is please tell me!

In any case I find it simpler to just overlap the panels rather than doing flat-felled seams.

07/03/2006 - I've made some progress with the kite!

So far I've cut out the panels for three of the struts, and most of the leading edge, and sewed them together.

I've also cut out a couple of the canopy panels. Blue polytarp is a bit of a pain to cut out because it's not see through, so it's hard to trace the panel shapes from the patterns.

I go over the lines of the printed patterns with a thick black marker pen, and then I can just about see the line through the polytarp to trace the panel shape.

A light table would make things much easier I suspect, and so would white polytarp, but the shop I buy from only has orange and blue...

The photo shows some of the completed struts and the plans, on the kitchen table. This was taken with my old rubbish camera - hence the image quality. I have recently bought a new shiny digital camera with advanced features such as a FLASH and AUTO FOCUS. Wow. Subsequent pictures will be crystal clear ;-)

It Flies!

The bow flies well. Better than my ZP8.5. It is stable, powerful, comes to the edge of the wind window and depowers well.

Sadly the polytarp is not up to the job, and has started to come apart at the seams, but I may rebuild the kite using the fabric from the now retired ZP15 and ZP8.5.

Sarah, my wife, had her first kite lesson with the BOW, and took this photo:

Design Details

I cut off the wingtips, as with the ZP15, and added battens instead. This made the tips less prone to folding, but they still folded a bit.

Then I added webbing half way along the wingtips, and made a 'V' bridle for the rear lines, with a pulley. This stopped the tips folding altogether. Yay!

On the LE. i tried the 'conventional' bridle arrangement first, as shown in the images at the top of the page. This didn't work well, and I don't think the Surfplan bridle lengths were accurate... I tried a 'Cartwheel' arrangement next with a single front line going to a ring some 7 - 8 metres from the kite, and lines to the leading edge radially from there... This worked much better.

Conclusions

I think I overdid the 'flatness' of this kite a little, because judging from the shape in the air the sweep at the wingtips doesn't quite make the trailing edge follow the arc of the leading edge. If I were to do it again I would make the canopy curve slightly more. I would also experiment with the bridling, as I think there is room for improvement...

Generally a success.

For those who are interested there is a large (3.25 mb) and not very exciting quicktime movie of the kite flying here

Contact me! redhot@freeuk.com