Making your own canopy

I know this has been beat to death over the years, but, I am ready to
proceed with canopy fabrication. I am fortunate enough to have a
canopy tool (female mold) but am unsure how to proceed. Use the
female to make a male plug and try stretch forming over the plug? If I
use this method, how do I compensate for the thickness of the plexi
when making the plug? Or I could use the female and attempt to vaccum
it down?
I have an oven started, a huge plywood shipping crate about 6'x4'x4'.
My plan is to heat the plexi and plug together in the crate and upon
reaching proper temp, slide it out and stretch it down.
I have already sent photos of the tool to a canopy house and they
estimate $3k to make one. I figure I can burn up a lot of plexi for
that.
Words of wisdom appreciated.
Thanks,
Clark

no wisdom offered, but please keep us informed of your progress...

Back in the '60's I worked with a couple of sailplane homebuilders trying to
make canopies. Yes, we filled a dumpster with trashed Plexiglas before we
climbed the learning curve.

We decided we didn't like the results from molds. The optics just weren't
great no matter how good the mold. Fortunately, there's another way.

If your canopy is close to a section of an ellipsoid, you can free blow the
canopy and get perfect optics. The way we proceeded was a 'cold bend-hot
blow-cold bend' process. The plexy was bent cold into a forming tool and
loosely clamped. The tool and plexy was then heated to forming temp in a
large plywood oven like you describe. The clamps were then tightened and
the canopy was blown to a curve slightly greater than the finished shape.
(Actually, we used a shop vac to suck the canopy into the desired shape - no
cold air introduced.)

When we had the 'suck' step where we wanted it, we shut off the heat and
kept the vacuum on until the plastic cooled. Then we trimmed the flashing
and cold bent the plastic the last bit to fit the frame. The folks using
this method have gone on to make a lot of canopies with perfect shape and
optics.

The trick to make this work is to plan on wasting a lot of plastic in the
trimming step. In other words, start with a much larger plastic bubble than
you really need. That way you can select the part of the bubble that best
fits the curve you want. When you do the last cold bend, the curve flattens
a bit which is why you want to 'over blow' it a little.

Bill Daniels


"c hinds" wrote in message
om...
I know this has been beat to death over the years, but, I am ready to
proceed with canopy fabrication. I am fortunate enough to have a
canopy tool (female mold) but am unsure how to proceed. Use the
female to make a male plug and try stretch forming over the plug? If I
use this method, how do I compensate for the thickness of the plexi
when making the plug? Or I could use the female and attempt to vaccum
it down?
I have an oven started, a huge plywood shipping crate about 6'x4'x4'.
My plan is to heat the plexi and plug together in the crate and upon
reaching proper temp, slide it out and stretch it down.
I have already sent photos of the tool to a canopy house and they
estimate $3k to make one. I figure I can burn up a lot of plexi for
that.
Words of wisdom appreciated.
Thanks,
Clark

I made the canopies for my Spencer Aircar, see the pros and cons on
doing it yourself at
http://dougnlina.home.mindspring.com/Dougs/chaptr19.htm . Generally,
simple curves or "bubbles" are easier than three dimensional curves,
but if you persist, you will succeed.



"Bill Daniels" wrote in message news:LUqnd.428441$D%.412718@attbi_s51...
Back in the '60's I worked with a couple of sailplane homebuilders trying to
make canopies. Yes, we filled a dumpster with trashed Plexiglas before we
climbed the learning curve.

We decided we didn't like the results from molds. The optics just weren't
great no matter how good the mold. Fortunately, there's another way.

If your canopy is close to a section of an ellipsoid, you can free blow the
canopy and get perfect optics. The way we proceeded was a 'cold bend-hot
blow-cold bend' process. The plexy was bent cold into a forming tool and
loosely clamped. The tool and plexy was then heated to forming temp in a
large plywood oven like you describe. The clamps were then tightened and
the canopy was blown to a curve slightly greater than the finished shape.
(Actually, we used a shop vac to suck the canopy into the desired shape - no
cold air introduced.)

When we had the 'suck' step where we wanted it, we shut off the heat and
kept the vacuum on until the plastic cooled. Then we trimmed the flashing
and cold bent the plastic the last bit to fit the frame. The folks using
this method have gone on to make a lot of canopies with perfect shape and
optics.

The trick to make this work is to plan on wasting a lot of plastic in the
trimming step. In other words, start with a much larger plastic bubble than
you really need. That way you can select the part of the bubble that best
fits the curve you want. When you do the last cold bend, the curve flattens
a bit which is why you want to 'over blow' it a little.

Bill Daniels


"c hinds" wrote in message
om...
I know this has been beat to death over the years, but, I am ready to
proceed with canopy fabrication. I am fortunate enough to have a
canopy tool (female mold) but am unsure how to proceed. Use the
female to make a male plug and try stretch forming over the plug? If I
use this method, how do I compensate for the thickness of the plexi
when making the plug? Or I could use the female and attempt to vaccum
it down?
I have an oven started, a huge plywood shipping crate about 6'x4'x4'.
My plan is to heat the plexi and plug together in the crate and upon
reaching proper temp, slide it out and stretch it down.
I have already sent photos of the tool to a canopy house and they
estimate $3k to make one. I figure I can burn up a lot of plexi for
that.
Words of wisdom appreciated.
Thanks,
Clark

Earlier, (c hinds) wrote:

I know this has been beat to death
over the years, but, I am ready to
proceed with canopy fabrication.
I am fortunate enough to have a
canopy tool (female mold) but am
unsure how to proceed.

My recommendation, based on my own direct experience, for whatever
it's worth:

Unless you are planning to make more than about ten units a year, I
believe that it just doesn't pay to try to climb the learning curve of
making your own acrylic or polycarb transparencies. Instead, I
recommend this procedu

Take your female mold, and use it to make a male plug. Make sure that
the male plug embodies at least 2" of perimeter outside the line of
the required finished transparency. Smooth and polish the plug the
level of smoothness commensurate with the optical clarity you want.

Now, this is the hard step: Take that plug and send it to a reputable
aircraft transparency company and tell them to make you one of those.
You will save a ton of time and trouble, and get a good (or at least
usable, depending on your plug) transparency the first time. It's what
I do, and it's always worked for me.

If you're near Southern California, I recommend Aircraft Windshields
Inc. in Los Alamitos. They've made transparencies for the new HP-24,
the HP-18 and HP-11, and other gliders, and I've always been happy
with their work.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com

Plus a Phoebus and....a... LO-150.
GA

I have also successfully free blown canopies using a method similar to that
described by Bill D.

I blew the canopy using compressed air and used a deflector plate to make
sure that the cold air did not directly blow on the canopy.

I started with the plexiglas clamped between two sheets of plywood .... the
lower one with a plan view of the canopy cut out of the middle. The
plexiglas and plywood were calmped together by a hundred or so screws around
the edge of the profile. The whole thing was then sat on top of the oven
(really an insulated plywood box with approx 5 kw of electric heaters in the
bottom (make sure you shield the plexiglas from direct radiant heat !!) .
The temperature in the box was monitored with thermocouples. When the
desired temp is reached open the valve on the compressed air and slowly blow
the canopy down into the box. The rest is as per Bill's description.

Actually the hardest bit was getting the plexiglas hot enough (we had to
insulate the box). Also you need a fan in the box to circulate the air and
keep the temperature constant or you WILL get a funny looking canopy !! I
know - I learnt the hardway.

The above process was developed after seen how one manufacture of
certificated aircraft was producing their canopies and by reference to an
article in the old Homebuilt Aircraft magazine.





"c hinds" wrote in message
om...
I know this has been beat to death over the years, but, I am ready to
proceed with canopy fabrication. I am fortunate enough to have a
canopy tool (female mold) but am unsure how to proceed. Use the
female to make a male plug and try stretch forming over the plug? If I
use this method, how do I compensate for the thickness of the plexi
when making the plug? Or I could use the female and attempt to vaccum
it down?
I have an oven started, a huge plywood shipping crate about 6'x4'x4'.
My plan is to heat the plexi and plug together in the crate and upon
reaching proper temp, slide it out and stretch it down.
I have already sent photos of the tool to a canopy house and they
estimate $3k to make one. I figure I can burn up a lot of plexi for
that.
Words of wisdom appreciated.
Thanks,
Clark