DIY Foil Kites

[kitexpert]

Aerodynamics is not any mystery science. It has well known basics in physics and with flying human made objects it is based quite a lot in practical testing, for over a centurys period. It is nonsense to think that some revolutionary inventions for example for airfoils will come, because every reasonable and huge number less than reasonable shapes are tested years ago. This is true for airfoils working RE numbers kites have.

So real design challenges for kites are elsewhere. Latest big development is race foil kites, to have efficient high AR kites which are not too difficult to use. Airfoils they have are not anything very special, just suitable compromises between lift and stability. But if airfoil is not correct for high AR kite failure is guaranteed, so choice is critical.

thinner airfoil section ,on the tip as it is, performing better.

Say "thinner performs better" does not mean anything if not defined more precisely. In wingtips thin airfoil works because it makes less turbulence, same purpose is for having sharp wingtip edge and to have wingtips narrow. Usually wingtip chord is 35% to 45% of chord at the center.

[socommk23]

Thinner aerofoils at the tips will reduce lift, therefore drag. Just one way that is being used to improve foil kites. I dont believe there is too much more to come from aerofoil sections themselves. Improvements will come from the reduction of drag. Wingtips and trailing edges and cell shape ontop of bridle design and materials.
Winglets, washout, and profile changes to reduce tip vorticies, cleaner trailing edge/miniribs/no seam tape to clean up the trailing edge airflow.
Less bulge in each cell to clean up airflow over the canopy or just lots of narrower cells.
All being used but limited to the weight and strength of materials available.
Once a material half the weight of the lightest material currently available with the strength needed to be useable is available, then all the above can be used in the extreme with no weight penalty. But prices will become crazy.
Flysurfer seem to be leading the way on material front so far.
For self build, can you even get hold of the stuff flysurfer use?
How would a kite made purely of the lightest weight materials stand up to use and abuse?

[foilholio]

Lift is not a function of the thickness of the airfoil, that would be shape. You can't even achieve maximum lift by increasing the thickness. You do however achieve increased drag and stiffness. Stiffness is desirable especially for something that only has fabric and a slight amount of air pressure as it's whole construction method. Even for large expensive things made of aluminum some stiffness is desirable. Drag apparently is something kite designers confuse with lift or find desirable for other reasons. You can increase drag and not lift, they are not tied at the waist though they can appear that way.

[kitexpert]

Thinner aerofoils at the tips will reduce lift, therefore drag. Just one way that is being used to improve foil kites. I dont believe there is too much more to come from aerofoil sections themselves. Improvements will come from the reduction of drag. Wingtips and trailing edges and cell shape ontop of bridle design and materials.
Winglets, washout, and profile changes to reduce tip vorticies, cleaner trailing edge/miniribs/no seam tape to clean up the trailing edge airflow.
Less bulge in each cell to clean up airflow over the canopy or just lots of narrower cells.
All being used but limited to the weight and strength of materials available.
Once a material half the weight of the lightest material currently available with the strength needed to be useable is available, then all the above can be used in the extreme with no weight penalty. But prices will become crazy.
Flysurfer seem to be leading the way on material front so far.
For self build, can you even get hold of the stuff flysurfer use?
How would a kite made purely of the lightest weight materials stand up to use and abuse?

Exactly, there is no need to try create lift in wingtips, because they have a small area and their projected area is even smaller. Wingtips should be low in drag and stable.

You listed good stuff what can be improved. Like you wrote lot of it is already more or less done, so single improvements may not be very great. But if you can improve many things a small amount, added result is significant improvement.

Using exotic materials (dyneema fabrics etc) might be the next step. High end race foils are already so expensive that added cost may not be so difficult to bear. But because foil kites always have many kg's of air inside small saving in weight of the kite itself does not make it very much better.

I don't have any experience of fabrics lighter than Chikara, at least not significantly lighter. I've used some siliconized nylon which might have been some grams lighter, but it was of lower quality than Chikara is. At the moment I don't see restricting factor here, because I'm most interested in to design very efficient kite. Not simple ultra light wind kite like Pulsion which main feature is light weight fabric.

[kitexpert]

You can't even achieve maximum lift by increasing the thickness.

Increasing it from which value? You should really tie your thoughts more to reality, foilholio. Otherwise they don't mean anything.

Stiffness is desirable especially for something that only has fabric and a slight amount of air pressure as it's whole construction method.

Those parts of a foil kite which take loads in pull are very stiff. The bridle is stiff and ribs and D-ribs are stiff also. But it is true foil kite will become stiffer if airfoil thickness is increased. Also lift and drag usually increase.

Even though air pressure is low, it has a lot of area to work against. For example fully flat rectangular foil kite stands very well all spanwise compression kiter can produce with his weight (inertia). If kite has curved shape it produces lift in directions which support it against compression, extreme example is of course PL arcs. They don't need any bridle support their shape, just air pressure and lift.

You can increase drag and not lift

For sure. Better way to study airfoils is to look L/D ratio. Depowerable foil kite needs also stability for the airfoil it has.

[socommk23]

Turbulator blow holes is what we need next. A few inches from the trailing edge. That should improve aerodynamics . Well it works on high performance gliders

[foilholio]

When kitetwit learns what a bridle is he'll have progressed beyond kitedesigner or I'll reassess my stereotype of them.

[stefmoris]

Left half done!!




This is probably the most confusing step. If you look at my previous build picks from my last kites, you can see I turned the whole kite inside out to sew the final seam, but this time I just turned inside out only the part that needed sewing. You can see I tied a length of ripstop around the material where it exited the trailing edge to keep it bunched together and under control. This made everything so much more manageable to sew as the bulk of the weight was off the table. I'm not sure how the pros do it, but this is how I do it. -stef

[kitexpert]

Your method for the last seam seems to work, even if I don't see exactly how you do it.

Last seam is of course the trickiest one. Or more precisely, the last half of it. When starting sewing the final part whole kite starts to close itself inside of that cell, because like in any other cell seam allowances will finally go inside. While sewing they must be outside, so last cell will turn inside out.

There is nothing to "fear" when sewing the last seam. There will be some a bit difficult moments, but working slowly and pulling/adjusting the rest of the kite so that it distracts the work as less as possible it will go fine.

It is good to control the bulk of the kite somehow. If it is a big kite I have used a helper to hold it for the toughest part of the seam. I don't think it is necessary to tie the rest of the kite, it may limit the possibility to pull it and control the "fullness" of the cell at the point you are sewing.

The "birth" of the kite is when you pull it out from the cell. Then closing TE, adding the bridle and mixer - kite is ready for testing. I see Stef is not very far from this exciting moment. Good luck

[Blackrat]

wow thats looking great !