Hi -Second sending, without attaché-
There is an old excellent article from Soaring (I send it for you) that
explains how to make an excelent acrylic windshield in two steps:
The first over a male mold of simple curve and the second in female mold for
the final compound curve.
The temperatures of softening are specific for each type of plastic. Some
must be heat 100° C during some minutes because they absorb humidity and
this it can cause globes in the surface.
I have made some tests with small pieces and it is not easy, but neither
impossible.
Have luck and communicate like it leaves everything -
Eduardo
La Pampa
Argentina
Stan Hal I
(This is a scanner copy an have many little errors.)
One article in particular caught my attention because not only was it fun to
read but it reinforced an opinion 1 have long held Itat insofar as he builder
is concerned, a homebuilt sailplane is fundamentally a collection of mixtakes
that soinehow all get righted in the end. The article was written by Bob
Wailers, DUSTER builder and former editor of the "Rag." As it must to all who
build sailplanes, Bob and his partner. AI Nelson. catise to the Time of the
Canopy. But let Bob tell it:
Canopy Building and the Learning Process
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 AI Nelson's and my success. We have developed many secrets, but not much
success. AI and 1 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 want, 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 diflicult. We used
4inch thick, 2-ibs.lcu.ft. density foam. There must be no gaps where the foam
meets the wood. Don't try to get by ~sing filler. The fit has to he perfect.
Now, take your Sur-Form tool and shape the foam to the desired contour. Here is
where your standard, Mark 1, hamfisted hacker will screw up first. AI 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.1he 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 wi;hout 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.
JUNE 1975
-. i .--- 1 . ~ 1-1-11
Next, coat the*plug with pojyvinyl alcohol (PVA) mold release and lay up a
female mold over it. Use glass cloth followed with mat andlor woven roving to
get 311W 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-ordry sandpaper. Drill eight or ten 1116" 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 phig 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 farTlf you use cheap felt
you get terrible mold mark-off . . . guess how we found out.
41
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 verticalli, 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 111 Oth" thick acrylic to fit the mold. Use a saber saw and a special
abrasive blade or a finetoothed coping saw. This stuff is inc~ooibiy expensive
and cracks easily . . . guess how 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 Sets hot. Wanna know how we
found out? We found out in a basement filled with flaming plexiglasal
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 AI's basementl
Stick a probe-type candy thermometer in thue 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
hightemperature 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
scat 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. Tum the mold over and support it
on blocks so that the two ends are at the same height to best trap the beat
inder 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.
Makesure your beat deflector works well enough
31
- '171
so that you don't burn the edges close to the stove . . . guess how we found
oul.
Heat the whole thing to about 250* 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 outl
When the plastic has stretched to fill.the mold, turn off the beat. While
holding partial vacuum, direct the vacuum exhaust into the oven to cool it. The
thick mold will retain a tot of heat. Now remove the rulnei 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 y6u'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, 1 find myself haunted by the thought that it would have been for
naught had the con
tours of the plug not been #Imost optically perfect to begin with. If you,re
contemplating building a fancy canopY,Iikc this, give the problern--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.
1 would recommend that you stick to the stock Duster canopy (the one shown !p
the plans). However. 1 would also recommend that you hot-form it, even if it
d0C4 114YO only a single-plane curve. Make a plug with a.simple curve out of
sheet metal. Heat your plastic to about 24001P 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 asociated 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.
