compound curves in plywood

"BA-100" wrote in message ...
"W P Dixon"
:

Luthiers commonly steam wood and form it over jigs, clamps hold it in
place until it drys out. May be the same kind of construction used to
build this sailplane. Just a thought and maybe something to check out.


Yes, I've done this in fact. Have a guitar and a violin under my belt, but
no compound curves. Most flattop guitars have a small radius which is
compound, but nothing like what I'm talking about (a complete teardrop)
This sort of technology realy died out when fibreglass hit the scene,
though i'm sure there are some masters still around.Most likely source of
info are boatbuilders, of course, but their curves wouldn't even appraoch
the radius I'm talking about.....

The key to bending wood with steam is how hot it is. Superheated steam is
best, which takes a re-heater after the water is boiled. Make sure the
plywood is made with water proof glue, which is anything of A grade or
better, or marked withEXT after the grade. It might be B grade, but I don't
remember, right off.

For testing, that type of plywood is fine, but for building a plane, use at
least marine grade, and best is aircraft grade.
--
Jim in NC

"Alan Baker" wrote

Is everyone forgetting the deHavilland Mosquito? Constructed almost
entirely of cold-moulded plywood.

Isn't cold molding done using layers of wood, with glue applied as the wood
is bent in the mold, with the glue setting up in the new shape?
--
Jim in NC

BA-100 wrote:

My question is, would it be possible to laminate your own shell using

vacuum bagging?
------------------------------------------------------

Yes. But with a bit of fore-thought. My first effort at vacuum
bagging leading-edge skins ended up crushing my mold :-)

---------------------------------------------------

Seems to me even the wettest veneers would split if you
tried to force them into or around a male or female mold.

--------------------------------------------------------

Forming a compound curve, the plywood is usually HOT as well as wet,
and is remarkably plastic.

A standard technique in the repair of such structures offers a bit of
insight into fabricating such a hull from scratch. The general
procedure is as follows:

A mold is taken from the opposite side of the structure. (The
assumption here is for a symmetrical form). The mold is usually made
of plaster, stiffened with burlap or jute fiber and is a few inches
larger than the section to be replaced.

The plaster mold is used create a pair of re-enforced concrete molds,
much thicker than the plaster mold and usually in a frame of welded
steel angle-iron that has been previously drilled for bolts.

The concrete is wet-cured for about a week then air-dried in a warm
hangar for about ten days.

The plywood or veneer is boiled or steamed then laid into the mold,
which has been pre-heated to about 170*. If veneer, the usually
adhesive is a urea-formaldehyde type such as Weldwood 'Plastic Resin'
or resorcinol (the concrete mold must be protected). The upper part of
the mold is put in place and drawn down with bolts. The closed mold is
then returned to the oven or hot box and allowed to cure for 24 hours.

The resulting part is then scarfed to match the area to be repaired.

---------------------------

The point often overlooked is that this method may also be used for
scratch-building the entire pod. This is because the pod is
symmetrical. Other than the nose, which is usually carved from balsa,
you need only five or six molds to duplicate a longitudinal SLICE from
the pod. The builder simply makes eight copies of each panel then
scarfs them onto a frame.

It should also be apparent that this method works even better using
composite materials and vacuum-bagging, since you need only a few
pounds per square inch to cause the fiber & resin to conform to the
shape of the female mold (the male part is not required). Prior to
WWII there was considerable effort devoted to composite structures
including a few that actually flew (circa 1940) but they tended to be
heavier than their aluminum counterparts. I know the glider people,
but here (ie, Sandy Eggo) and Santa Monica had considerable success
using linen fabric, balsa cores and urea-formaldehyde 'resins.'

-------------------------------------

Although the Loughead brothers are often cited as the first to use the
laminated ply fuselage, there were some earlier airframes that
pioneered the process. I can't recall the details but 'LMB' comes to
mind; some outfit on the east coast, shortly after WWI. (I'm old. I
forget things and fart a lot.) This eventually lead to the patented
Duramold process which Howard used on the HK-1.

-R.S.Hoover

In article ,
"Morgans" wrote:

"Alan Baker" wrote

Is everyone forgetting the deHavilland Mosquito? Constructed almost
entirely of cold-moulded plywood.

Isn't cold molding done using layers of wood, with glue applied as the wood
is bent in the mold, with the glue setting up in the new shape?

Yup. And the original post was talking about just that, ISTM.

"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,"

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

("Morgans" wrote)
For testing, that type of plywood is fine, but for building a plane, use
at least marine grade, and best is aircraft grade.


"...and best is aircraft grade"

I know about marine grade plywood, but aircraft grade?

Speaking of marine grade plywood (I paid $77 w/tax for a sheet of 3/4" last
year) is there such a thing as marine grade + green treated that they sell?
That's what I wanted, couldn't find any. Something about varnish + green
preservative doesn't do well together I was told. (???)

It was for a carpeted-over floor in a 16 ft boat. We ended up spar
varnishing our sheet a number of times.


Montblack

"Nafod40"
egroups.com:

BA-100 wrote:

My question is, would it be possible to laminate your own shell using

vacuum bagging?

I would do some googling on "cold molding" and boatbuilding. Very
common method for building hulls. It's also how the SPruce Goose and
Mosquito bombers were built. You'd find...

Yeah, knew that, though I'm not so sure it's suitable for this particular
airplane. Unfortunately, I've never seen one of the molded ones, only the
one in the NASM and that was built using scarfed rings. If I'm not
mistaken, the molded ones were made of single sheets of veneer made into a
ply in the mold, but I may be wrong about that.

http://www.nwmarinedesign.com/coldmold.htm

You can build up a structure as strong as fiberglass but lighter with
this technology.

Mmm, that's the idea! Plus it just looks beautiful. I don't think they were
made in quite the same way as the cold molded boats are though.

egroups.com:


BA-100 wrote:

My question is, would it be possible to laminate your own shell using

vacuum bagging?
------------------------------------------------------

Yes. But with a bit of fore-thought. My first effort at vacuum
bagging leading-edge skins ended up crushing my mold :-)

---------------------------------------------------

Seems to me even the wettest veneers would split if you
tried to force them into or around a male or female mold.

--------------------------------------------------------

Forming a compound curve, the plywood is usually HOT as well as wet,
and is remarkably plastic.

A standard technique in the repair of such structures offers a bit of
insight into fabricating such a hull from scratch. The general
procedure is as follows:

A mold is taken from the opposite side of the structure. (The
assumption here is for a symmetrical form). The mold is usually made
of plaster, stiffened with burlap or jute fiber and is a few inches
larger than the section to be replaced.

The plaster mold is used create a pair of re-enforced concrete molds,
much thicker than the plaster mold and usually in a frame of welded
steel angle-iron that has been previously drilled for bolts.

The concrete is wet-cured for about a week then air-dried in a warm
hangar for about ten days.

The plywood or veneer is boiled or steamed then laid into the mold,
which has been pre-heated to about 170*. If veneer, the usually
adhesive is a urea-formaldehyde type such as Weldwood 'Plastic Resin'
or resorcinol (the concrete mold must be protected). The upper part of
the mold is put in place and drawn down with bolts. The closed mold is
then returned to the oven or hot box and allowed to cure for 24 hours.

The resulting part is then scarfed to match the area to be repaired.

---------------------------

The point often overlooked is that this method may also be used for
scratch-building the entire pod. This is because the pod is
symmetrical. Other than the nose, which is usually carved from balsa,
you need only five or six molds to duplicate a longitudinal SLICE from
the pod. The builder simply makes eight copies of each panel then
scarfs them onto a frame.

It should also be apparent that this method works even better using
composite materials and vacuum-bagging, since you need only a few
pounds per square inch to cause the fiber & resin to conform to the
shape of the female mold (the male part is not required). Prior to
WWII there was considerable effort devoted to composite structures
including a few that actually flew (circa 1940) but they tended to be
heavier than their aluminum counterparts. I know the glider people,
but here (ie, Sandy Eggo) and Santa Monica had considerable success
using linen fabric, balsa cores and urea-formaldehyde 'resins.'



Yipes. Getting complicated. Here was my plan:
Get a couple of 4x8 sheets of the thinnest stuff I could get, and lay them
over a male mold soaking wet but cold with a bit of ammonia thrown in for
good measure. Get a good coating of glue on the sheets and slip the whole
lot into the vac-bag. I-m pretty sure the original came in two halves, but
now you have me thinking it might be do-able if it was quartered
longitudanally.
just to give you all another look at one of these contraptions,
http://www.museumofflight.org/collec...lay.html?ID=32
as you can see, this one is also made of segmented, scarfed cones, but most
were made of two halved sides.
(at least i think they were halved, I have an original assembly manual for
one and that's the way the kit came)
unrealistic?

"Montblack" wrote in message

Speaking of marine grade plywood (I paid $77 w/tax for a sheet of 3/4"
last
year) is there such a thing as marine grade + green treated that they
sell?
That's what I wanted, couldn't find any. Something about varnish + green
preservative doesn't do well together I was told. (???)

It was for a carpeted-over floor in a 16 ft boat. We ended up spar
varnishing our sheet a number of times.


Montblack

Yea, I know what you mean.

A friend wanted to do the same thing. We found a place that treats wood,
and got him to throw in some sheets of marine grade into his treating
thingy, the next time he was running a load. We then (after waiting for it
to dry out some) put it down and put a layer of fiberglass cloth and epoxy
over it all, before laying the carpet. It looks like new, about 5 years
later.
--
Jim in NC

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 )

"Morgans" wrote in message
...

"Alan Baker" wrote

Is everyone forgetting the deHavilland Mosquito? Constructed almost
entirely of cold-moulded plywood.

Isn't cold molding done using layers of wood, with glue applied as the
wood
is bent in the mold, with the glue setting up in the new shape?
--
Jim in NC


Exactly. Plywood is nothing but thin veneers of wood glued together face to
face with the grain running at different angles, generally ninety degrees,
but sometimes other angles for special applications. Plywood is normally
laminated on a flat plate making a flat sheet of wood that is very hard to
bend into anything other than a conical curve because the laminated layers
are glued together so they can't slide.

Cold Moulding is merely a low temperature process using ordinary waterproof
glues, for makeing you own plywood from thin veneers that is formed over
something other than a flat plate, so that it comes out with the shape of
the mould it was laminated in or on, as the case may be.

No reason in the world you can't make compound curved plywood by laminating
it up over a male plug and useing a vacuum bagging process to apply the
requisite clamping pressure. Ideally pick a good waterproof glue that gives
you a rather long working time and bonds well with a fairly low clamping
pressure. Say 5 to 10 pounds per square inch clamping pressure that you can
easily obtain with simple vacuum equipment. Working time of fifteen or
twenty minutes should allow time to build up thin laminations and get
clamping pressure onto them. This can be stretched a lot by using staples
with a pull pad under them to apply temporary clamping pressure as you build
up the lamination.

I have only done this using older glues like resorcinol and aerolite and
urea resin. I would think it would work very well with some of the more
modern epoxy based wood glues like T-88, etc. If it works for canoes, it
oughtta work for a Bowlus. :-)

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )