Ultrafoil 15 Bft 5 stabilty

Update. Ultrafoil 15 crash.

My confidence in the Ultrafoil has been building and I think I have found it’s upper wind speed limit. It does well at the lower end of Bft 4  but can be a real handful as it approaches Bft 5. This is disappointing. I have been concerned about its weaving characteristic and been prepared to put up with this as pay off for the slack line benefit BUT although it did not invert it sways soo much around the down wind line in 20mph+ I have reverted to the Didak Explorer 16 for the punchy end of Bft 4 as, despite the much stronger pull, the Explorer is much more stable.

The rocking visible in this animation is a clue to the high speed instability of this kite, as the wind-speed increases the rocking develops into big sweeps across the wind window. The initiation of the oscillation is at some point between 15 and 20mph.

Flying the Explorer and the Ultrafoil side by side off ground stakes about 30m apart, difference between them is very clear. So much so that as the wind picked up the Ultrafoil began to weave in bigger and bigger arcs and overlapped with the Explorer wrapping itself around the line and collapsing. The Explorer bobbed about a bit and began its rocking moves but never moved off the down wind line much. The difference in line tension between them was huge the Ultrafoil being hand haul-able and the Explorer not.

Looking at the outer keels in this sequence it may be possible to induce some improved channelling of the airflow by relaxing the outer bridle lines as Christian Becot did for the Sutton Flowform, he introduced something like a dihedral by doing this.

Becot-Ultrafoil_01This is a simple and reversible mod so that will be my first intervention.

I shall also experiment with more tail, drogue and, if all else fails, Becot style venting the side keels but it has let me down in a big way- a kite crash was an unplanned lesson on a KAP workshop!

The failure modes of the 2 kites are very different. The Explorer heels toward the edge of the wind window while the Ultrafoil oscillates across it.


Once again I find the advertised wind range ridiculously optimistic.


About billboyheritagesurvey

Heritage worker
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