compound curves in plywood

My question is, would it be possible to laminate your own shell using
vacuum bagging? Seems to me even the wettest veneers would split if you
tried to force them into or around a male or female mold. to date, the
only
sort of manipulation I've done with ply is to wrap 1/16 leading edge
pieces
over a hot pipe. I just don't see how Hawley Bowlus got the compound
curves..


They used a boatbuilding and canoemaking technique that used bands of thin
veneer laminated over a male mould. The process is called "Cold Moulding"
and there are a couple of books about it in the "boatbuilding" section of
the library.

Essentially you build up a male plug with plywood formers covered with
fairly closely spaced strips of lath. The veneer thickness is selected to
give you an odd number of laminations, 3, 5, 7, etc. The bottom layer goes
on about forty five degrees to the lathing. Staple it down here and there
to hold it in place. It is wise to use heavy cardboard tags under the
staples because you are going to pull them out when you put the next layer
in place.

Brush the first layer with glue and cross it with the second layer at
something approximating 90 degrees. Aerolite glue works well. Brush the
cream component onto the first layer and then brush the hardener onto the
bottom of the second layer. Pull the staples as you go and restaple through
both layers.

The third layer goes on like the second, but aligned with the airflow so the
finished grain will be pretty! :-) Pull staples and staple it down as you
go.

After all the layers are down pull you vinyl over the whole banana and turn
on the vacuum pump to pull it all tight and hold it down.

If you want more than three layers of veneer repeat layers 1 and 2,
finishing with the top layer aligned with the airflow.

After the glue is hardened, remove the bag and sand the whole thing with a
random orbital sander to contour.

Standard industrial veneer for veneering furniture, etc. is 1/28 inches
thick. That is just under a millimeter for you continental folks. A three
layer veneer would be close to 3mm thick or a stout 1/8 inch. Be gentle
sanding because it is really easy to sand completely through a veneer layer
and get real embarassed!

Any stringers or frames are glued inside after the skin is finished. This
CAN be done without expensive or bulky tooling. The Dutch made fighters
using this technology in WWI.

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )

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

Bob Kuykendall wrote:
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.

Well, they had some duPont money at least ;-)

Once you get the first one ship-set spiled out, you can use a router or
a shaper to chew out as many as you want, likety-split. And they had
the prototype to use as a full-size "tailer's dummy" to get a first
approximation of the shape of the veneers for the molded version -
neither advantage will apply to a one-off.

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

COLIN LAMB wrote:

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

Dang your brave I wouldn't want to do that in my airplaneG!!

"COLIN LAMB" wrote

I have built a number of cedar strip canoes and kayaks, and the process is
simple and beautiful.

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.

One problem is that for an airplane, there are bunches of point loads, like
the spar, engine, and landing gear. There are also some wicked bending
moments involved. If you canoe breaks, you go swimming. If you airplane
breaks, you......
--
Jim in NC

"One problem is that for an airplane, there are bunches of point loads, like
the spar, engine, and landing gear. There are also some wicked bending
moments involved. If you canoe breaks, you go swimming. If you airplane
breaks, you......"

No question the point loads are a big issue. However, it is the same issue
for those who build epoxy airplanes - because that is what a cedar strip
boat is really, an epoxy boat with a cedar core, instead of foam. All of
the cautions against cedar strip aircraft would apply to foam/epoxy
airplanes - yet there are a number of successful creations flying around.
The main difference is that you have to paint a foam/epoxy job because foam
is not intrisically beautiful - wood is.

I am not sure I advocate building a twin engined cedar strip fighter, but
the original query was regarding a Bowlus. It has no engine and the
strength demands are not nearly as great as a 10 G fighter.

Colin

"COLIN LAMB" wrote


No question the point loads are a big issue. However, it is the same
issue
for those who build epoxy airplanes - because that is what a cedar strip
boat is really, an epoxy boat with a cedar core, instead of foam. All of
the cautions against cedar strip aircraft would apply to foam/epoxy
airplanes - yet there are a number of successful creations flying around.
The main difference is that you have to paint a foam/epoxy job because
foam
is not intrisically beautiful - wood is.

My concern is the fact that there is only one layer, or ply of wood. It
will have way less than 1/4 the strength than a craft with 2 plies of wood,
unless many more plies of fiberglass and epoxy are used. There is also no
resistance to twisting loads, without much more fiberglass. Another layer
of wood at between 90 and 45 degrees would make it much stronger in that
regard, also

The point loads in fiberglass airplanes are carried through strong
bulkheads, bonded very well to the body. I don't think what your describing
has that.

If the use of a single layer of wood is used to be the core, instead of
foam, and the same thickness of fiberglass is applied, as it would be in the
fiberglass and foam plane, there should not be a problem. Is this the
intent?
--
Jim in NC