Briefly, there is an inflatable bladder, extending from the head to the foot of the wing, placed between the panels approximately 30% behind the mast.
The bladder maintains the
thickness of the airfoil (I'd suggest some 15-20%), and naturally
makes the lee side cambered. When tacking, the sail cloth moves
slightly around the mast, in order to accommodate the difference in
panel lengths (actually widths). Handling is equally easy as of the
'conventional' one, even easier because of smaller forces.
To keep
the shape stable, the leech should be as tight as possible. For
high air speeds, both 'outhauls' should be carefully tuned. To
accommodate the wind gradient, the upper boom (if it exists) could
be allowed to 'swing' horizontally to a predetermined angle, or the
upper portion of the mast could rotate partially (look haw a guy
figured out the mast/gaff configuration on wingtttt.jpg). My kayak
sails are short and I didn't bother with the twist.
"For a 16 sqft wing, the cloth portion should be 10' high and 1'8"
(50cm) wide (on
average), i.e. aspect ratio 6:1 or slightly lower (shouldn't be
lower than 5:1). The mast
should be 1-1.5 foot longer (for bury, depending on the depth of
your hull, either 1 3/8" or 1 1/2" outer diameter (o.d.) aluminum
tube, preferably anodized (composite is certainly better). If you
want to make it collapsible, use 'spring buttons' to lock the
mast's sections, because upper and lower booms should be in the
same plane. I found booms 0.875" o.d. convenient - fit nicely into
1" o.d. supports (short mounts built into the mast).
The part "a" on that goes
around the mast is made of "Cordura" nylon - it is strong and
pliable. There are two side Dacron panels "b". You've already
determined the curvature of their front edges (that go onto the
part "a"). The foot and the head are roughly horizontal (feel free
to play with the shape). You can make the wing tapered toward the
top, with the foot wider than the head. The aft edge is 'hollow'
(concave) for better tensioning (I found 1% - 1.5% sufficient). I
usually join aft edges of panels "b" at the leech, which I make
from "luff" tape.
Also, the trailing portion of the sail can be made as a single ply
(part "c", up to 30% of the sail's width), which would make the
trailing edge angle smaller and the sail lighter. The tension of
the leech is very important and should be as high as possible. I
'pre-tension' the tape/rope, glue the cloth on it and then sew it.
Another way could be a wire threaded through a narrow pocket at the
leech. The upper 'clew' (top rear corner of the sail) is attached
to the upper jaw-slide (which can be horizontally adjusted on the
gaff for proper 'lateral' tension of the cloth). In a reefable
version of the wing, a hoist can be attached to the clew.
To keep the tension, I cut a "V" at the end of the lower boom. The
'main sheet' is first
threaded through the lower jaw-slide and then secured in the "V"
slot. If the leech is loose, the sail gets deformed under pressure
- at least with my modest design techniques.
If you want a totally collapsible wing, you will have to find an
inflatable tube to place
between the panels (this 'sausage' is essential). I've been told
that such tubes can be
found in pool supply stores, even online. I used inflatable
sponsons for folding kayaks
(Feathercraft custom-made them for me).
I was quite satisfied with partially foldable wings, where the
'sausage' was made from a
closed-cell foam. I usually
fix several pieces of
Velcro on inner sides of panels "b", and on the 'sausage'
correspondingly, as means of
attaching it. Inflatable bladders are a bit problematic - if
inflated fully, they tend to get round, which doesn't give a nice
shape to the wing. In order to get an oval (elliptic) shape, I
inflate a bladder partially (say 60%), but then it is too soft, and
if fixed at the top and the bottom only, tends to fall aft. It can
also be held in place by straps sewn between the panels, or a full
size pocket. Such a bladder also tends to be thicker in the middle
than at its ends, which makes the design more difficult (kind of
tri-dimensional). However, the thickness of the 'sausage' should be
15-20% of the sail's chord."
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