Ultrafoil 15 mods for stability part 1

Shortening the middle bridle line had an adverse effect on the stability of the Ufoil. it actually made things worse.  A short discussion with Ivo van Olmen pointed towards excessive internal pressure so I decided to vent the crazy weaving kite.

On paper the new holes on a top view with the kite in something like it’s ‘natural’ AoA:


Steve Sutton’s 1973 patent shows how the vents are opposed between top and bottom skins.Sutton-Wing-2

Page from Steve Sutton's Flowform patent 1975 plan 2

For the flowform kite the hole distribution follows the same pattern: outer cells vented to rear on top and to the front on the underside:


Ray Merry has used a similar distribution but with a narrow slot as a vent, I hope to increase the effect by adding holes.

I began with the rear top vents:


The hole is 85mm (1 pint beer glass) diameter.

The first 2 holes have helped a bit. Some improved stability is noticeable without any loss of power:


It’s still out flown by the Didak Explorer in terms of down wind line path, the weaving persists but is less violent even though peak flow was recorded at 20.2mph.


Next I shall increase the venting of the centre cells. Inflation seems to work fine with the Ray Merry slots so I’ll leave well alone on the lower skin.

I tried similar mods with the Didak Flowform4 with only limited success. With nothing to loose as the Ultraform kite is far too unsteady for me in Bft 4, I take my cue from Steve Sutton’s FF30: fairly big holes centred on the the long axis of the cell and staggered to avoid direct flow from top to bottom. Increasing the tail vent opening would be the last  step if the top skin vents don’t help.


The idea is to achieve inflation of the wing with balanced pressure.

Let’s hope the Ultrafoil can be cured of its weaving, it’s really well made and has a nice slack line characteristic. I’m  hoping to have 2 Bft 4 flowforms to work with, perhaps I’m expecting too much, and of course I run the risk of ruining what lift there is in the Ufoil but I can’t leave it as is.

One thing is certain Ivo van Olmen has produced a winner with the Explorer!




About billboyheritagesurvey

Heritage worker
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