Still they can beat e.g. a 32 feet catamaran, M 32, when used with a foil board: http://sailinganarchy.com/2015/03/11/and-the-winner-is-2/
Was originally written with the focus on skate sailing. But is now more general.
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 - which equals higher speed
Bow or Supported Leading Edge, SLE, kite, left. Foil kite, top. Leading Edge Inflated, LEI, kite, right. (From Wikipedia)
LEI, Leading Edge Inflated, or SLE, Supported Leading Edge, are semicircular when viewed from the front.
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 designing kites that were easier to make.
I hope the future will bring us a much better traction kite (and Para glider). And the world really needs one for all sort of sailing (foil board, buggy, ski, ice...) especially speed sailing and breaking of world sail speed records.
To get a L/D of above ten I think the following is required:
Preferably it should be reefable, on land or best while sailing
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.
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 section 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 sail/kite area is restricted you could get much larger driving forces than your competitors who use airfoils.
The cylinder need to be rotated. Can be wind driven. When used as a kite one could consider electric motors fed via two conducting lines which if connected to solar panels would give an all natural energy system.
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Modified Aug. 2016