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A better traction kite would beat all skate sails, at least in light wind.

What is aerodynamically wrong with current traction kites and Para gliders?

Was originally written with the focus on skate sailing. But is now more general.

Copyright © 1996-2009 Anders Ansar. You are welcome to use my pictures and words as long as you include a link to my pages.  


Non Ansar Copyright 

Pathfinder have some features which may help to make a traction kite better than those we have now. It has a foil section with an upswept trailing edge to make the tail less craft stable. See more below.

Flexifoil Nexus traction kite looks a bit like Pathfinder. But Nexus didn't have a upswept trailing edge as far as I understand.

 

Traction kites have three major advantages when it comes to sailing fast compared to sail boats and the stand inside wing skate sail

For skate sailing I estimate that the L/D ratio of the traction kite must be above ten, to give a total L/D ratio, including sailors air drag (in streamlining) and skate drag, better than six, which we have measured for stand inside wing skate sails in light winds.

 

Current traction kites are far from perfect when it comes to produce a good Lift to Drag ratio

Well, at the time of writing, July 2009, the kite surfer is the fastest in the world, sailing downwind, some 40 degrees from the wind, when L/D is not so important. 
A parachute has a L/D ratio near 0 - you can not sail very much off straight down wind with it. But still in a say 70 knot wind you can sail 50 knot down wind.

Semi circular kites, when viewed from the front, e g Wipika, Naish and Arc 

Relatively easy to make. Nice smooth surfaces.

Para foil type kites

Difficult to build because of many fabric parts to design, cut and stitch. Also many lines to design, cut and attach. I never made one - instead spent time on designs that were easier to make. 


C-quad

Easy  to make. From the gifted and non tiring Peter Lynn. Maybe a bit out of fashion when this text is modified 2009.

Traction kite

The picture shows one of my traction kites being launched by a helper on ice. The kite is connected with two lines to the sailor, who is to the left of the kite. I have built, apart from many small prototypes, some five larger kites of this type. The largest with a span of 11 meters (33 ft) when flat.
  The construction is inspired by a design by the French brothers Bruno and Dominique Legaignoux, which they market as Wipika. They are the fathers of kite surfing! I saw Bruno sail on water skies in at the Sail Speed Week in Brest, France, around 1984. But instead of having the leading edge inflated (complicated design and I fear punctures) I use for example an aluminum tube (with the option of changing to lighter carbon fiber if or when the kite works fine).
  I have not yet been able to make these kites practical. At sizes above some 5 sq. m they become too heavy per sq. m because the weight of the leading edge tube increases quicker than the sail area when size increases. Leading to too heavy kites which fall down in the lulls. 

I hope the future will bring us a much better traction kite (or Para glider). And the world really needs one for all sort of sailing (buggy sailing, ski sailing...) especially speed sailing and breaking of world sail speed records.

To get a L/D of above ten I think the following is required:

For this traction kite to be practical the following is required: I also hope it will have an easy to make design. Apart from that in then can be relatively inexpensive I place emphasis on this because I want to make my traction kites myself. The Para foil is too complicated to make: too much cloth to cut and sew and too many lines to cut and attach. Especially as you need at least some three kites of different sizes for racing in light to strong winds. The C-Quad is a great step forward as it is much simpler to make than a Para foil.

These requirements should give a competitive traction kite for skate sailing, a kite with a L/D ratio above ten. Man powered airplanes have a L/D ratio of around 20 - 50. Somewhere there is the upper limit at present.



 
 

Non Ansar Copyright

Where to look for inspiration? I think the lightweight flying wing Pathfinder, http://www.dfrc.nasa.gov/Projects/Erast/pathfinder.html , is a great example. Light construction, large AR and L/D. Having no stabilizer, stability comes from upsweep at the rear part of the wing section.

Large turning circle

Pathfinder has a large span which means it has to be turned slowly or the inner wing tip will get too low wind speed and drop. This phenomenon will be a real problem  for large span traction kites as to be able to make a slow turn, with big radius, you need very long lines. (The turn rate of the Pathfinder, span 18 m, is some 3 deg./sec. Which means 60 seconds for a 180 deg. turn. The speed is some 5 m/s. Translated to traction kiting this means that if you started a turn at ground level the kite will be at the height 200 m when the turn is finished. To manage this you need long lines, over some 400 m.

Maybe the solution for traction kites with large span is not to turn them, instead just stop it and then start moving it in the opposite direction. This requires an airfoil which works both ways - an arc?

The beauty of traction kite design is that even with small material resources you can achieve world class results.


In order to promote the design of better traction kites The Sky Sail Society was formed in Stockholm 1987. Three of us  have put in considerable effort for some years to construct a good traction kite, but we were not successful.

  One way to compare the efficiency of different types of traction kites is to compare their lift to drag ratio, L/D, which is a measure of how much faster they are able to fly than the speed of the wind, when they are flown low back and forth by a stationary person. I suggest Methods for measuring: Lift to drag ratio of traction kites and sailing crafts. Pull and Cl of traction kites., which is next in the Skate sail index. If traction kite designers agree on a common method of measuring L/D we can compare different designs by correspondence. And maybe will arrive quicker at at faster designs.


I sat down the other day and tried to come up with the ultimate traction kite construction that has an aerodynamically smooth surface and that is made of single skin for lightness and contains no stiff parts (for low  weight and damage resistance).

And I came up with a design! If you rotate a vertical cylinder in wind it gives a side force, in the same way as an airfoil. This is called the Magnus effect and these rotating cylinders are sometimes called Flettner Rotors after Mr. Flettner who built a ship with such rotors.

If you make an inflated cylinder you have the construction I was looking for.

There are a serious drawback: The lift to drag ratio of these cylinders are low, specially when you include the energy need to rotate the cylinder. The total L to D ratio is in the order of two only.

It should also be mentioned that these rotating cylinder have very large coefficients of lift, up to around 9, which is about 6 times larger than for an airfoil. This could be useful in some applications, e.g. when sail7kite area is restricted you could get much larger driving forces than your competitors who use airfoils.

The cylinder need to be rotated. When used as a kite one could consider electric motors fed via two conducting lines which if connected to solar panels would give a all natural energy system.

Checking this side in 2009 it occurs to me that black inflated cylinders heated by sunlight rise in the air. You have a kite that don't fall down in the lulls - in effect a negative weight kite. You don't need to spend energy, create drag, on keeping the kite aloft as with most other designs.


The best tactical compass for sail boats? You see the wind shifts directly on the compass! No figures to read, write or compare. Can it get simpler?  The position of the white pointer directly shows where the direction of the wind is between best lift and worst header, in oscillating wind shifts. Picture on to the right is an animation. 

 

A concept for the world's biggest, longest, widest kite?

This kite is 22 by .5 m. You can sail straight up or down wind with it!

Next in the Skate sail index is: Methods for measuring: Lift to drag ratio of traction kites and sailing crafts. Pull and Cl of traction kites.

Back to Skate sailing index.

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