building a `foil: constructional notes


I've just finished building a `Speedfoil', which is a kite based on a
Flexifoil.. Although I haven't had chance to fly it in earnest yet, I
thought I'd pass on my constructional experiences.  Prior to building
the Speedfoil, I've made a LiteFlite clone & a 5-bladed Flaix, but
this was the biggest kite challenge I've tackled to date.

These notes are fairly detailed, as I know the basics of sewing,
constructional techniques etc., but have found it useful to learn
from other peoples detailed experiences.

The Speedfoil plans come from the (Dutch) book `Stuntvliegers: Bouwen
en Besturen' by Servaas van der Horst & Nop Velthuizen. Since I
bought my copy it has appeared in a translated version, but I can't
remember the exact title. The Speedfoil is described as a `nephew' of
the Flexifoil.  It uses the same tapered leading edge spar, and as
far as I can tell, the main difference is the front-to-back depth,
and hence aspect ratio.  The Speedfoil is around 50cm deep, narrower
than a Flexi.  I think this is mainly so that you can cut both top
and bottom out of ripstop 104cm wide. The rib sections are also
spaced more closely on the speedfoil.

The authors claim an unofficial world speed record with a variant of
the speedfoil. This variant has ribs spaced 10cm apart, instead of
12cm as on the Speedfoil.

1) Materials
        - Ripstop - no problem
        - Flexifoil spar - likewise.
        - Dacron reinforcing tape. I found the wide (8cm) tape
        difficult to get hold of, and in the end used two widths
        of 5cm tape, overlapped and sewn. Is there another source
        of this stuff apart from kite suppliers?
        - Gauze. I ended up using a remnant of the strongest net
        curtain material I could find. I think it's terylene (?).
        It's been suggested to me that the gauze used for windows
        on tents would be best, but I haven't yet found a camping
        supplier who sells it.
        - webbing. Normally this is easy to find from camping or
        mountaineering shops. However I needed narrow 10m wide stuff.
        I'd almost given up on this, and was going to hot-cut some
        wider webbing down, when I found that my camera strap was
        made of exactly the right thing! I guess I shall have to
        haunt camera shops now.

2) marking and cutting out.
        The pattern for the `profiles' (ribs) is given as tabulated
points, from which it is easy to make a pattern. Rather extravagently
I used a program available by anonymous ftp called PLOTFOIL. This
will take such a table and generate a PostScript output file.  This
is a bit OTT I realise, but I had a copy of this program (useful for
generating rib sizes for model aircraft) anyway. It also has a useful
option which allowed me to add a seam allowance of 10mm to the
pattern.  The plans suggest a 7mm seam allowance; I used 10mm because
the baseplate of my sewing m/c is marked in 5mm steps. I've since
realised that there _is_ a 7mm mark, and would use that next time.

I tried hot-cutting ripstop for the first time on the ribs, and it
worked pretty well. I used an old soldering _gun_ with a cutting tip,
and cut round the pattern, which I had glued onto a sheet of
corrugated cardboard. The cardboard wouldn't survive this treatment
indefinitely, but was fine for the 16 ribs I needed.  I used an old
mirror which I'd recently taken out of a wardrobe to cut on. My main
complaint was that the gun takes a while to heat up between uses, and
the fumes given off - neither were too unexpected. I may use an old
soldering iron with a dimmer unit next time, but will carry on

I found marking out the top & bottom `decks' of the kite fairly
awkward, mainly because I don't have a big enough table to work on.
The plans suggest marking the ripstop with pencil - a soft grade
(`B') works pretty well.  I actually marked where the _edges_ of the
ribs would go, & not the sewing lines. I think this was a mistake.
Looking back, this aspect of the marking out is more important than I

3) Sewing.
        - Firstly, the Dacron tape reinforcements are sewn onto the
ends of the lower surface. A tunnel is then folded in the leading
edge, and one side of the gauze sewn on. The Webbing, together with
a 10mm aluminium ring, is sewn onto one surface of the tunnel. This is
then folded and sewn once more, forming the final reinforced tunnel.
The upper surface is then sewn onto the other edge of the gauze,
forming a narrow strip of exposed gauze of width 16mm.

Most of this is pretty straightforward. I found sewing the thin
webbing a bit tricky, and sewing it onto the tunnel trickier - at
this point there are several layers of dacron, and the thick creases
make it easy for the material to `jump'. I may have been using a
needle which was a little too large. I was using a no. 11 needle.
Next time I might make some sort of jig to
hold the webbing in place.  I also had to use the narrow zipper
presser foot when sewing the final tunnel seam.

For sewing, I used standard polyester thread. I coated the beginning
and end of most seams with glue (`Bostik'), after sewing backwards
over the last few stitches of a seam, _and_ tying off the ends of the
thread. I'm not sure if all this is necessary, or if I'm being

In view of the trouble I later had when sewing gauze, it is worth
trying to arrange matters so that the gauze is the _top_ material
being sewn.  (ie. closest to the presser foot). This, I think, is the
`natural' way to work anyway.

- The upper surfaces of the ribs are sewn in place.  The plan makes
great play of the neccesity of accurately placing the ribs. I marked
on each rib the point which corresponds to the junction between the
gauze and the lower surface. After some trial and error, I found that
the best method was to baste each rib at this point, and then (using
the narrow zipper presser foot) sew, very slowly at first, (since the
leading edge of the kite is the most curved, and the contiurs change
quickly), holding both pieces of ripstop by hand and adjusting the
alignment as necessary.

This part I found the most difficult. The surfaces of the kite are sharply
curved, and it feels as though you have to keep an eye on four things
at once. I found that the dog teeth on my machine kept catching on the
gauze, making for very untidy sewing. I have heard that one way to sew
gauze is to put tissue paper under it, and pull this away afterwards.
In retrospect, I also feel that marking the actual seams, rather than
marking the seam allowances and follown those, might have been easier
in this case.

- The ribs are sewn onto the lower surfaces of the kite
        Perhaps surprisingly, I found this rather easier than sewing
the upper surfaces. Although things start getting a bit cramped, the
curvature of the ribs is less now, and there is no gauze to get in
the way. Perhaps I was getting more experienced! As the plans
mention, the last rib sewn is the most constricted, and you have to
almost turn the `rib box' inside out to sew the first few cm.
Overall, things are easier here now. I took care to make sure that
the top and bottom seams of each rib just meet at the trailing edge,
to make a nice streamlined profile.

After sewing I went over all of the rib seams, tidying them up, tying
them off and gluing them.

- The trailing edge of the kite is sewn and trimmed.
        Its here I notice my inaccuracies! I drew a line across the
end of each rib seam, and sewed along this line. It varied by a
couple of mm at most between each rib - a little more than I would
have liked. I also now realised that it is important to make sure
that the markings of the top & bottom of each rib seam meet at the
trailing edge - otherwise you will be unable to sew the trailing edge
without puckering one surface or other.  Next time I think I will
complete the marking for the ribs after sewing the gauze & upper &
lower surfaces together.

I hot-cut the trailing edge a few mm behind the seam, to keep it nice
& streamlined.

4) Flying
        - I've only been out with the kite for half an hour or so - 
the weather hasn't been good, but I was itching to test out my
creation.  Not having had any experience with a `real' Flexi, I can't
compare the two, but I note the very high angle the kite can fly at,
the graceful way the leading spar bends, and the nice `swish' it
makes - I'm used to kites which make a louder noise than this! A
friend saw me trying it out and wants one already.

I hope to build a bigger foil sometime, depending on if I can get hold
of some rod to extend the leading spar.

- Comments & Correspondence welcome 

- Thanks to Nique Croonenberg and Tania Campbell for assistance with
translating the original plans from the Dutch, and to Paul Crowley
(puc@aber.ac.uk) for his translation, addresses, and encouragement.

        john nicoll (jkn@ohm.york.ac.uk)

john nicoll (jkn@ohm.york.ac.uk)

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