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John Travell                 http://www.travell.uk.net/


For caption on SSF (single-skin foil) photos

The following is an extract from a writeup I sent to Andrew Beattie to go with a photo of my single skin foil, and reveals the antiquity of my efforts, as it was sent Feb 1996!

The kite design was based partly upon having heard of (but never seen!) single skin paragliders, along with some drawings of highly undercambered airfoil shapes documented in a model aircraft journal.
Correctly or otherwise, I reasoned that the flight envelope of a buggy engine requires high lift at low speed and high angle of attack. This seemed to me to resemble the landing configuration of a modern aircraft wing, with slats on the leading edge and trailing flaps.

My kite has these slats and flaps, with an extremely high aspect ratio main panel between them. The foil shape is maintained by conventional ribs, with the slats and flaps attached by extensions of them. The bridling is very complex, combining a single point radial outer section each side, joined by an arch centre section, with longitudinal semi-arch bridling for the brake lines. This feature eliminates the trailing edge curl seen in so many other 4-line foils, and enables the entirely novel experience of flying a soft foil smoothly backwards across the window without the trailing edge collapsing.
It flies extremely well at low speed, take off is effortless in winds where a similar sized quadrafoil needs a very firm tug to lift it at all.
The shortcoming in the design is the way the outer front corners tend to close up and flap around at high speed.
John Travell

XA Bridle Glossary entry

Joe, found an email describing my 'XA' bridle, and why I gave it that name.

Cross-Arch (XA) bridle.
Single Skin Foil I built the kite a few years ago, but have not really been satisfied with the bridle. While it contains ideas that have proven valuable, in that brake line effort is distributed along each rib by what amounts to half of an arch bridle running front-to-back, other aspects are less desirable. Mainly, I am dissatisfied by the amount of distortion I get in the canopy during turns.

The following bridle (when constructed) is intended to eliminate these problems. Because I see it as a hybrid combination of the cross and arch bridles, I have chosen to call it an XA bridle. (who knows, in years to come this just might be my tiny claim to fame! :-)
NOTE: I do NOT know yet if this bridle will work, it is (as of writing this) still just an idea in the early stages of development.
In the image, the darker blue and yellow represent the canopy and ribs of my existing single skin foil, the grey line a bottom skin that virtually every other soft foil would have, the red lines the primary bridle, and the green and pale blue the XA bridle this page is all about. I have tried to choose colours that appear as different shades in a grey-scale printout.

Adapting this idea to kites with a differing number of bridled ribs is left to the individual choosing to try it.
I have a spreadsheet (MS Excel) that calculates the angles the secondary lines adopt, but it does not yet give me lengths of the components in the secondary bridle (I still have to work out the math, and that is 30-year rusty!).

I am also developing a new primary bridle for this kite, one that is intended to allow a degree of control of the angle-of-attack and camber shape. This image represents a preliminary sketch to illustrate the concept rather than the result of any analysis or development. Note the six attachment points to the canopy, this is solely because there are six points on each rib of my single skin canopy. On an inflated canopy I would omit the front line and only use the rear 5 lines.-- John Travell


SSF (single-skin foil)           A skin has two surfaces: top and bottom; aerodynamics on both surfaces of a single skin is important.
Webmaster working: photo set. Historical notes.   Photos are not up yet. (not intended for downloading, as files have not been reduced yet: http://www.energykitesystems.net/JohnTravell/documents-export-2012-02-22.zip    )