Canopy Building and the Learning Process
Written by Bob Walters builder of a Duster sailplane
This article tells how not to make
a canopy. It is written that way to discourage you from
trying. People have asked
me to let them in on the secret of Al Nelson’s and my success.
We have developed many secrets, but not much success. Al and I got together on this canopy project
and have spent several hundred frustrating hours and several hundred
dollars just to get a nice canopy.
Here is how you, too, can fail:
Start
by deciding on the profile shape and make a canopy frame mockup
out of plywood right on the fuselage.
Install at least one false canopy bow near the high point.
Carefully shape the wood to define the shape you ant, which
will depend on the amount of headroom you want, etc.
Now, fill in this mocked-up “plug” with urethane foam. Fitting the foam is very, very difficult.
We used 4-inch thick, 2-lbs./cu. ft. density foam.
There must be no gaps where the foam meets the wood.
Don’t try to get by using filler.
The fit has to be perfect.
Now, take your Sur-Form tool and shape the foam to the
desired contour. Here is where your standard, Mark I, ham-fisted
hacker will screw up first. Al
Nelson is one of the two or three best known surfboard shapers
in the world. He has been
shaping foam since the early 1950’s and he says that the shaping
of the foam plug is next to impossible for the average guy.
One pass too many with the Sur-Foam or sandpaper and the
plug is ruined. You cannot, repeat, cannot repair a low spot
with filler because the filler sets up harder than the foam around
it and you can’t trim it without damaging the foam further. After the plug is shaped as perfectly as possible,
lay on several layers of glass cloth and resin. Now remove the plug from the fuselage and add
several inches to the edges to form a skirt.
Sand everything until you have approached optical perfection. No hills, no valleys. Smooth.
Next,
coat the plug with polyvinyl alcohol (PVA) mold release and lay
up a female mold over it. Use
glass cloth followed with mat and/or woven roving to get 3/16”
or more thickness. Reinforce the edges with laminated wood clamping
strips and glass them on securely.
Now pop the mold off the plug and dress the inside with
#600 wet-or-dry sandpaper. Drill
eight or ten 1/16” air vent holes in the high point of the mold.
Get some billiard felt and soak it in hot water and stretch
it over the plug until it is dry.
Then take it off the plug and glue it to the inside of
the mold. Seal the edges
with tape. This felt costs
about $9.00 per yard. Get
the cost picture so far? If
you use cheap felt you get terrible mold mark-off...guess how
we found out.
Now,
get every book your library has on acrylic forming and start reading. We built two ovens out of scrap wood and asbestos
paper. One is tall and
thin to hold the flat sheet vertically, like the books say. The other looks like a doghouse and is designed
to hold the mold. Make
a paper pattern and cut a sheet of 1/10th” thick acrylic
to fit the mold. Use a
saber saw and a special abrasive blade or a fine-toothed coping
saw. This stuff is incredibly
expensive and cracks easily...guess hoe we found out.
Hang
the flat sheet inside the tall oven, using C-clamps and wooden
clamping blocks on the edge which will be trimmed off later.
Weak clamping here, or not enough clamps, will permit the
sheet to sag as it gets hot. Wanna
know how we found out? We
found out in a basement filled with flaming plexiglass!
Put
a two-burner Coleman stove in the bottom of the oven and cover
the burners with metal deflectors to keep the fire off the plastic.
Make the oven so the gas tank is outside, and
keep a wet rag on it. The
only thing we haven’t done so far is blow up Al’s basement!
Stick
a probe-type candy thermometer in the oven and heat the plexi
to “soft ball” (about 240 F).
Now, remove the plexi, using cotton gloves, quickly bend
a simple curve in it, and put it into the inverted mold.
Clamp the edges to the mold, using many C-clamps and pressure
strips inside and out while the plastic is still hot.
When it cools, unclamp it and seal the edges to the mold
with high-temperature masking tape. (If you use regular tape you will fail because
the adhesive gives up at about 250 F...guess how we found out). A perfect seal between the glass and the mold
is mandatory. So tape carefully. The high-temperature tape is expensive but is
good up to about 350 F). Reapply
the clamping strips and clamps over the tape seal.
Turn the mold over and support it on blocks so that the
two ends are at the same height to best trap the heat under it.
Make some sort of plenum chamber over the air holes so
that you can attach your vacuum cleaner to it.
We used an old funnel.
Now drop the doghouse oven over the mold.
The air connector for the vacuum cleaner should stick out
the top. Cut a peep hole at each end and slip the stove
under the edge. Make sure
your heat deflector works well enough so that you don’t burn the
edges close to the stove...guess how we found out.
Heat
the whole thing to about 250 F and remove the oven to tighten
the clamps while the plastic is hot.
This is a very important step.
Now, set up the oven again.
Turn on the stove and heat the oven to about 320 F.
If you have trouble getting the temperature this high,
throw a couple of wool blankets over the oven.
Keep pumping the gas tank and don’t forget to keep it cool.
The plastic should sag at about 270 F, but don’t worry.
Turn on your vacuum cleaner at 315-320 F and pull the plastic
up to the mold surface. You
should need only partial vacuum.
If
you haven’t used enough C-clamps, at this point the edges of the
plastic will slip out from under them and ruin the canopy...guess
how we found out!
When
the plastic has stretched to fill the mold, turn off the heat. While holding partial vacuum, direct the vacuum
exhaust into the oven to cool it.
The thick mold will retain a lot of heat. Now remove the ruined sheet of plastic and save
it for laughs. Keep trying,
though, and continue ironing out the details.
Eventually you’ll get a good canopy.
Trim the canopy, mount it without cracking it – and you’re
all done.
This
whole canopy-making operation sounds pretty easy (it does?-Ed.)
but in my own case it was by far the most difficult, expensive,
and frustrating part of building the sailplane.
In
retrospect, after having spent all that time and money learning
how to mold plastic, I find myself haunted by the thought that
it would have been for naught had the contours of the plug not
been almost optically perfect to begin with.
If you’re contemplating building a fancy canopy like this,
give the problem of making an essentially perfect plug a great
deal of thought.
A
fancy canopy won’t do a thing for performance.
It just looks pretty. The
construction method described here will work, but don’t expect
it to be easy.
I
would recommend that you stick to the stock Duster canopy (the
one shown in the plans). However,
I would also recommend that you hot-form it, even if it does have
only a single-plane curve. Make a plug with a simple curve out of sheet
metal. Heat your plastic
to about 240 F and wrap it over your plug.
You can use cheap felt for this lower temperature if you
wish. This will give you a stock canopy shape but
without the terrific stress buildup associated with cold-forming. It will cost only about $7.00 extra for the
felt and asbestos paper, and you should be able to build the canopy
in a very short time.
Bob Walters
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