For caption on
SSF (single-skin foil) photos
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.
XA Bridle Glossary entry
Joe, found an email describing my 'XA' bridle, and why I gave it that
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
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
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