There seems to be plenty of scope for more efficient, faster pulling, traction kites

What is aerodynamically far from optimal with current, 2015, traction kites and Para gliders?

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

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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 up swept trailing edge to make the tail less craft stable. See more below.



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I think the Flexifoil Nexus traction kite was a great step forward as it, among other things, see more under heading below, got away from the performance, L/D, killing semi circular shape of LEI kites. But it was not well received by the sailors. Maybe it works better, is more stable, if it is given an up swept trailing edge.

After recently observing kite surfers at Mui Ne beach in Vietnam for several days it became apparent that they were not interested in higher speeds, which may be achieved with a kite with better L/D ratio, as there was no speed competitions. They only sailed back and forth with some long or high jumps.

For course racing I think a modified Nexus type kite can beat all other kites.

 

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

  • The pull of the kite can be connected low on a craft, at the skates when skate sailing, making it possible to resist some three times larger sail force than when the force is applied at shoulder height to a sailor leaning  30 degrees to windward . This gives a three times larger driving force - resulting in a much higher speed, maybe plus 50% . Kite surfers are not yet exploiting this major possible advantage of kite sailing. 
  • The sail boat sail or stand inside wing sail has less aerodynamic drag (induced) when its height, aspect ratio, AR, is increased. But this is offset by its center of effort getting higher, resulting in a lower force. With a traction kite you don't have this offset - you can design it with AR for best lift to drag, L/D. This gives higher speed, say some 10 %.
  • As wind speed (usually) increases with height sailing or skate sailing with a traction kite gives an advantage compared to sailors with their sails close to the surface. Over water the wind speed is typically 30 % larger at 33 feet, 10 meter. Speed increase in the order of 10 %. 

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.

  • 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 become useful pull. We have measured the L/D drag ratio for this type of kite to around 6, see  https://sites.google.com/site/iwmeasurelifttodrag/   I guess that the ratio should increase a lot, say 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 designing kites 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 drag. 

  • The bridling consists of many lines that produces drag.

  • Their air inflation needs energy, which creates drag, mainly because of leakages, I think. 

  • They are far from flat which decreases efficiency.






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 marketed 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 (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:

  •  An aspect ratio above about seven.
  •  Smooth surfaces.
  •  A relatively flat form when viewed from the front, not semi circular as the LEI and SLE  kites.

For this traction kite to be practical the following is required:
  • Plenty of depower.
  • Low weight per sq. m, (apart from very strong wind sailing, e g record breaking) say less than 150 g per sq. m, also for large sizes like 15 sq. m. In this respect the Para foil is great as it needs no stiff parts, especially those with single skin.
  • Good handling characteristics (read: doesn't fall down too often) stable and quick response.
  • Almost unbreakable.
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.



  
 

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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 up sweep 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 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.

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.


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Modified March 2015