compound curves in plywood

As a naval architect, and at one time, a budding yacht designer, I've
got a lot of familiarity with "Cold Molding", which is the amateur's
version of the Lockheed process for making molded plywood shells.

The best book on the subject is "The Gougeon Brothers On Boat
Construction"
http://www.westsystem.com/webpages/p...m#publications
http://www.amazon.com/exec/obidos/AS...874798-1028916

It's available in many libraries near the water, so you don't have to
run off and buy it yet. And you shouldn't, for once you realize how
much work this method will be for a One-off, you may, you should, think
twice.

Due to the small sized, and compond curves on the Baby Bowlus fuselage,
cutting and trimming the veneers is going to be your key challenge.
Each piece of veneer will need to be fitted to it's mate PRIOR to
gluing it up. All this hand work is why I don't think vacuum bagging
is going to buy you much on this project. You would have to vacuum bag
EACH piece of veneer seperately, clean up the glue squeze - out at the
free edge, and then fit the next piece of veneer. At least using this
method, you won't have to pull all the damn staples!!

I can easily see spending upwards of a 1000 hours just on the fuselage
alone, between building the male form, spiling (fitting) the veneers,
glue up (you need 3 layers), fairing, etc.

Some respondents have mentioned steam bending plywood. I've tried
that, and it's a non-starter, for the most part:
http://users.lmi.net/~ryoung/Sonerai/MySonIIL.html Scroll down to find
my tale of woe.

Note the mention of special "bending plywood". I got the stuff from
Woodline - The Japanese Woodworker in Alameda, California, it was
Italian Poplar, and was extremely easy to bend. This may offer you a
way out of a lot of labor, albeit at a high price in matierals. You
may be able to use ONE layer of this stuff, cut into "gores" and faced
inside and out with the lightest weight fiberglass and epoxy you can
manage. There is a similar product out of Gabon made with some
tropical hardwood, and a Hoop Pine product from Oz.

http://www.marineply.com/stagflex.htm
http://www.australply.com.au/pr_bendy.html
http://www.tapeease.com/plywood_panels.htm
http://www.lumberproducts.com/Produc...ingplywood.htm

Good Luck,

"flybynightkarmarepair"
groups.com:

As a naval architect, and at one time, a budding yacht designer, I've
got a lot of familiarity with "Cold Molding", which is the amateur's
version of the Lockheed process for making molded plywood shells.

The best book on the subject is "The Gougeon Brothers On Boat
Construction"
http://www.westsystem.com/webpages/p...x.htm#publicat
ions
http://www.amazon.com/exec/obidos/AS...874798-1028916

It's available in many libraries near the water, so you don't have to
run off and buy it yet. And you shouldn't, for once you realize how
much work this method will be for a One-off, you may, you should,
think twice.

Due to the small sized, and compond curves on the Baby Bowlus
fuselage, cutting and trimming the veneers is going to be your key
challenge. Each piece of veneer will need to be fitted to it's mate
PRIOR to gluing it up. All this hand work is why I don't think vacuum
bagging is going to buy you much on this project. You would have to
vacuum bag EACH piece of veneer seperately, clean up the glue squeze -
out at the free edge, and then fit the next piece of veneer. At least
using this method, you won't have to pull all the damn staples!!



OK, I've never done anythng like that before (aside form planking models
when I was a kid) but it's close to what I reckoned was involved. I only
imagined that each srip might have to be trimmed to match it's neighbor,
and was hoping that it might not be so, and that someone here would tell me
some magic secret that would sail me right past that problem!
Oh well!

I can easily see spending upwards of a 1000 hours just on the fuselage
alone, between building the male form, spiling (fitting) the veneers,
glue up (you need 3 layers), fairing, etc.

Some respondents have mentioned steam bending plywood. I've tried
that, and it's a non-starter, for the most part:
http://users.lmi.net/~ryoung/Sonerai/MySonIIL.html Scroll down to
find my tale of woe.

Oh dear.

Note the mention of special "bending plywood". I got the stuff from
Woodline - The Japanese Woodworker in Alameda, California, it was
Italian Poplar, and was extremely easy to bend. This may offer you a
way out of a lot of labor, albeit at a high price in matierals. You
may be able to use ONE layer of this stuff, cut into "gores" and faced
inside and out with the lightest weight fiberglass and epoxy you can
manage. There is a similar product out of Gabon made with some
tropical hardwood, and a Hoop Pine product from Oz.

http://www.marineply.com/stagflex.htm
http://www.australply.com.au/pr_bendy.html
http://www.tapeease.com/plywood_panels.htm
http://www.lumberproducts.com/Produc...ingplywood.htm

Good Luck,


Thanks. I'm thinking if I get around to the Bowlus it'll have to be with
the scarfed pod,though!

Earlier, flybynightkarmarepair wrote:

Due to the small sized, and compond curves on the Baby Bowlus
fuselage,
cutting and trimming the veneers is going to be your key challenge.
Each piece of veneer will need to be fitted to it's mate PRIOR to
gluing it up...

I dunno about that, Ryan, I think you're making it a lot harder than it
was. Hawley and his crew weren't foolish, and they didn't have tons of
time to spare, and they stomped out dozens if not hundreds of Baby pods
in a relatively few short years. And they did it starting in the
Depression, so I think you'll find that they didn't have a lot of money
to throw at it.

I'm thinking that it doesn't necessarily take finely crafted and fitted
strips of veneer, and that the thing can be done with relatively wide
swaths of thin veneer if you've got enough pressure. Note that the Baby
pod has the very nose portion crafted in formed aluminum, and that the
compound curvature of the fuselage aft of that is relatively modest.

I agree that the pressures required to press the veneers into curvature
are probably greater than you can get with vacuum bagging, but at a
guess I'd say not hugely so. If I hankered for a Baby like the one I
watched rot away at Sky Sailing in the early 1980s, here's what I'd do:

I'd start by making a fuselage plug as I did for the HP-24 kit
sailplane (see http://www.hpaircraft.com/hp-24 specifically the 2001
Updates), or perhaps by planking a set of bulkheads. Then I'd dress it
with parting flange features and make right and left female molds of it
in the back yard using regular old concrete. After the concrete cures,
I'd pull the plug out, and then lay up Freeman tooling wax in the
female molds to the thickness of the expected plywood layup. And then,
I'd pour concrete into the tooling-waxed tubs to make male molds of the
inside of the fuselage.

To make the fuselage pod skin, I'd laminate relatively large swaths of
veneer with +/- 45 degree grain and press them between the male and
female mold parts. I'd press the male plug down using nuts on threaded
rods set into the flange of the female mold.

Sure, that plan is relatively time intensive, but the basic materials
are pretty cheap, and once you get the process figured out you can
bring your fellow Baby enthusiasts over to make their own pods before
you turn the tubs into planters or backyard barbecues. Just a thought.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24

Montblack wrote:

Speaking of marine grade plywood

I just finished up a stitch-n-glue (4 mm plywood with single layer
fiberglass inside and out) kayak yesterday, and took it out for it's
maiden run at the local lake. It's 17' long, single seater sea kayak.
Weighs about 35 LBs, but can take an ocean entry driving out through
heavy surf, so it's not a frail structure.

While paddling around, I got to thinking about this thread, and
imagining wings either side of me, tail feathers behind...

A bright finished plywood glider would be a true thing of beauty.

I have built a number of cedar strip canoes and kayaks, and the process is
simple and beautiful. Basically, thin cedar strips can be easily bent into
almost any shape, then glued together. Once the shape is completed, the
inside and outside is dovered with cloth and epoxy resin. In essence, the
cedar is simply the core (replacing foam). The strength is the inner and
outer layers of epoxy multiplied by the thickness of the cedar strips. The
epoxy is almost clear, so the beauty of the cedar shows through nicely. By
mixing up grains, patterns and colors, gorgeous boats result. They are very
light and strong. A cedar strip canoe properly built will weigh 50 pounds.

The same technique could be use for aircraft. I have taken my canoe through
rapids, over small water falls and survived collisions with rocks. The same
technique should work for a lightweight aircraft.

Colin

I just looked at the fueslage of the Bowlus and it would be easily
duplicated using cedar strip techniques. Assembly is very quick with almost
no tools required, except for a saw to cut the wood strips. I did use some
mahogany in one of my canoes, just to try it, and it works fine. Because
the inside of the fuselage does not require the transparancy that the
outside does, you could even use one of the lighter and stronger cloths
(such as carbon fiber). This would result in increased strength and lighter
weight.

Now I may have to build one. The tube from front to rear could be
constructed easily using carbon fiber cloth rolled into a tube, or a
commercial tube purchased.

Colin

One of the Emeraude builders has constructed his gull-wing canopy using the
strip method. He has also built canoes in the past. There are some photos at
the Yahoo Emerauder's site http://asia.groups.yahoo.com/group/Emerauders/,
but you do have to sign up to see them. Look under "Photos" ---
"Construction projects" --- "Canopy".

Rich S.

"COLIN LAMB"
thlink.net:

I just looked at the fueslage of the Bowlus and it would be easily
duplicated using cedar strip techniques. Assembly is very quick with
almost no tools required, except for a saw to cut the wood strips. I
did use some mahogany in one of my canoes, just to try it, and it
works fine. Because the inside of the fuselage does not require the
transparancy that the outside does, you could even use one of the
lighter and stronger cloths (such as carbon fiber). This would result
in increased strength and lighter weight.

Now I may have to build one. The tube from front to rear could be
constructed easily using carbon fiber cloth rolled into a tube, or a
commercial tube purchased.


I sourced a tube for it years ago. Not a problem. If you really want one of
these beauties, the plans are available from the Vintage Soaring
Association! to the best of my knowledge, noone has yet finished a replica,
though.
The rest of the glider is pretty simple. It has a D-tube leading edge which
takes up most of the loads and a diagonal sub spar. The stab is all flying
and also is a D-tube structure. The original kit had a variety of ali
castings for wing fixing and such, but they're easily replaced with either
machined replacements or welded up 4130. The shell version would be
stronger and lighter as far as I can see, not to mention prettier!
Does the cedar strip technique you mention use multiple layers of veneer?
Presumably you need a form to wrap this around. Does it have to be solid or
can you just use formers, say, every 9 inches or so? How do you clamp to
get a good bond between layers?

BA-100 wrote:
Does the cedar strip technique you mention use multiple layers of veneer?
Presumably you need a form to wrap this around. Does it have to be solid or
can you just use formers, say, every 9 inches or so? How do you clamp to
get a good bond between layers?

A different build method from cold moulding. Does not use veneers.
Here's a great site on the strip-built approach.

http://www.guillemot-kayaks.com/Buil...ilt/index.html

"BA-100" wrote in message ...
snip
Does the cedar strip technique you mention use multiple layers of veneer?
Presumably you need a form to wrap this around. Does it have to be solid
or
can you just use formers, say, every 9 inches or so? How do you clamp to
get a good bond between layers?

Years ago (1930's & 40's), my dad sponsored neighborhood kids for the Soap
Box Derby.

He had them use a form of this construction for the shell of their racers. A
couple of planks formed the bottom of the car and formers were fastened on -
the main ones being the "dash" to hold the steering column, and the seat
back. Then thin strips of cedar were tack-nailed on the plank and run up and
across the formers at a 45° angle (or so) and nailed to the plank on the
other side.

When one layer was complete, the wood was covered with strips of thin
canvas, saturated with glue. Then a second layer of wood was added, run in
the opposite direction.

The end result was a smooth, compound-curved, thin, light, streamlined shell
of a body.

Every racer who had my dad as a sponsor won the Pacific NW championship. But
that's another story. My dad discovered the secret to honestly and fairly
blow the doors off all the other racers. It involved a four-foot long
micrometer. . .

Rich S.