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.
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.
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
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.
They have plenty of excess drag because a
large part of the force developed (square to the kites
surface) is used to extend the kite to this form. It is only
the forces in the direction of the lines which becomes useful
pull. I guess that their L/D should increase 30 % if they were
near to flat.
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.
Their surface is very uneven because they are
made up of cells. This produces plenty of drag especially as
the flow over the kite is not parallel to the cells.
The trailing edge is quite blunt which produces
The bridling consists of many lines that
Their air inflation needs energy, which creates drag, mainly because of leakages, I think.
Easy to make. From the gifted and non tiring Peter Lynn. Maybe a bit out of fashion when this text is modified 2009.
They have excess drag because of the several
curved surfaces and the force developed (square to the
kites surface) is used to extend the individual arcs and the
kite to this form. It is only the forces in the direction of
the lines which becomes useful pull. I guess that their L/D
should increase 20 % if they were near to flat.
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.
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:
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.
|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.
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