Yagu SLE

Yagu SLE is a modification to the original Yagu, adding a SLE (supported leading edge) style bridle to the design. This bridle is one way to correct a kite with stalling center section and luffing wing tips. This article also talks about SLE bridling effects and autozenith.

SLE Bridle

To the right see a rear view of Yagu flying with a SLE bridle. Below is a front view plan of bridle layout and measurements.

The bridle plan shows the rear tip line connecting with the front primary line as setup for two line flight. These two lines can also be separated for flying on four lines. A 5m line extension on each side is used for single line flight.

Two lines, one on each side of the canopy, each 1/3 up the leading edge, constitutes the SLE part of this bridle. The simplest possible SLE bridle would have one line in the middle of the leading edge. However, the canopy distortion produced would be even more severe than what we see here with two lines.

Flat SLE

Yagu SLE flies slightly more "open" or "flat" (wingtips further apart) than Yagu without the SLE bridle. There is a relationship between flatness a SLE and how much bridle force is required to keep the center section of a kite flying at the correct AOA in relationship to the tips.

Recall the unbridled Yagu where the center stalled and the wingtips luffed. Making the LE of Yagu convex instead of straight would exasperate this behavior. More bridle force would be required in the center section and the kite would be even more flat when the bridle was tuned for correct AOA across the LE.

More bridle force would increase the point loads on the canopy and create even more canopy deformation. Going down the "more flat" route would require more bridles to compensate. Inflatable SLE's get away with fewer bridles as the mechanical strength of the inflated tube on the LE prevents local deformation at the canopy/bridle interface.

The picture above and the first picture on this page show canopy deformation where the bridle is attached to the LE. Instead of following a straight line, the spanwise sew lines between the tubes dive down just before reaching the bridle attachment point. Ideal would have been a straight line or gradual bend from the LE to the TE. Having a rib connecting the topskin to the bottom skin would have created an "I-Beam" structure and might have prevented a kink in this location.

Autozenith

The unbridled Yagu uses small bike lights on the rear pigtails to induce autozenith when flying single line. Yagu SLE uses a pair of Danish 5kr coins to serve the same function. The Danish mint thinks that small denomination coins should double as kite stabilizers and conveniently include a hole in the center. I attach one coin to each rear tip inside the kite.

"Can you really bribe a kite into stability?" I hear you ponder. You bet! In fact kites to prefer "hard currency" and won't respond to paper money, no matter how large the denomination. Here's how it works:

The drawing above is a kite pilot view, where the LE is facing you. The kite is leaning over to its left side, but the LE is still facing the wind flow.

When Yagu moves towards one side of the window, the weight at the rear wingtips cause them to sag. This sag means a change in tip AOA and net lift generated in the opposite direction. The force involved is proportional to displacement to each side of the window.

The exact same lean + gravity autozenith effect is present in other kite styles as well. E.g. a classic Eddy kite is zenith seeking because the bridle center is in front of the weight center.

Peter Lynn has a more in depth look at kite stability in his January 2009 newsletter.

Performance

Flight day at the beach saw wind at 6-7 m/s. The best and worst thing about winter is that I have the beach all to myself. My helpers (having more common sense than their dad) politely decline involvement in below freezing kite tests. No helpers means only pictures and movie of kite in single line mode. The 'white sand' you see in the picture is actually snow.

Using a SLE bridle to adjust the AOA across the entire kite results in better performance. The flying angle indicates an L/D improvement over Yagu sans bridle. Not bad for an easy to build design, but there is still room for improvement. Reducing drag would improve performance. Sources of drag include:

• bumps instead of flat top skin
• coarse AOA adjustment
• abrupt trailing edge
• abrupt leading edge

The coarse AOA adjustment comes from having only 2 lines on the LE used as SLE. Two effects are seen: There is a step instead of a smooth transition from LE to TE at the bridle attachment point. There are differences between the AOA of the canopy at the bridle attachment point and cells further away. Both of these can be mitigated by more SLE bridle lines.

Both the bumps on the top skin and abrupt trailing edge can contribute to drag. However, both these areas are located where we expect there to be lower pressure and significant turbulence already.

The most fruitful avenue for drag reduction is likely LE changes. Here the pressure is highest, and the change from front to top is quite abrupt. Looking at the videos of the flight reminds me of the "wiggles" seen when a sheet is held flat to the wind and it alternately dumps air from one side to the other. I have an idea of how this can be improved... :-) Stay tuned.

Motion pictures speak louder than words. Here is a video of the first flight. It's an 11mb download. A smaller, lower quality version of the video is available on YouTube.

Related pages:
Yagu
Lift to Drag Ratio