compound curves in plywood

OK, since there have been a LOT of questions about boatbuilding
technigues, I'll try and take them on.

* Angle of wood - old "wood and canvas" canooes DID have often have a
diagonal layer. "Strip Plank" canoes genearally do not, as they are
fiberglassed inside and out.
* Strip plank, general comment - this is a very labor intensive, and
relatively heavy way to build a canoe. The curvature in the bilge
areas is comparable to the Baby Bowlus, but the longitudinal curvaure
is greater, plus the substantial taper at both ends with drive you
bat**** trying to wrap the strips around that shape.
* Stitch and Glue is a GREAT idea for this project. Lightweight ply is
spiled over molds, and either tacked in place to the molds, or pulled
off the molds and stiched to the piece next to it, then taped along the
seams with fiberglass. It is even possible to "torture" a fair amount
of compound curve into the "gores" by cunning means describe in the
Gougeon book I mentioned previously. Lighter, quicker and cheaper than
the other methods, but not quite perfectly fair in all directions like
the molded alternatives.

I will NEVER build another strip plank boat, but I enjoy stich and
glue. YMMV....

"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?"

Yes! The cedar strips are simply space, rather than strength. The rigidity
comes from the 1/4" thickness of the cedar strips, plus the thickness of the
multi-layer epoxy coverings. My kayak has 3 layers of cloth on the
exterior, plus epoxy resin..The cedar strip is used for beauty and the
"plastic" ability to conform to compound curves. The strength comes for the
thickness of the cedar times the layers of epoxy - really like a foam
aircraft. Although additional strength could be gained by cross laminating
2 layers of cedar strips, it would also add to the weight - and the change
of delamination between layers may increase. Just as in foam airplanes,
bulkheads of other means of transferring stress from a single point to a
much larger area are necessary.

I have built a stitch-n-glue boat, also, but could never get the beautiful
compound curves that I could get from the cedar strips. There is a limit to
the bending.

I do think, though, that it would not be difficult to simply make a mold of
the shape desired and take very thin skins and make your own plywood,
conforming directly to the desired shape, much like the paper mache ugly
animals we made in grade school. Compound curves allow great strength with
thin materials.

Colin

On Mon, 11 Apr 2005 23:47:53 -0400, "Morgans"
wrote:



is not intrisically beautiful - wood is.


How about a normal composite layup with a mahogany veneer on the next
to last layer covered by fiberglas for pretty.

Ed Sullivan

"Morgans"
:


"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......


Well, in this case, there's no engine, of course, unless you count the
mounting of the pilot as a gravity engine.

The loads for the gear, strut points, wing attach and boom are all
distributed through formers, which are in turn held in place by the skin.
The alternative is 1/16 ply wrapped between each adjacent set of formers
and scarfed to it's neighbor, which doesn't appear to be any stronger than
a planked setup which basically amounts to a whole lot of stringers. They'd
have to be kept thin to keep the weight down, of course.

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

nafod40 :

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


OK, now i'm totally confused! Most canoes i've seen seem to have their wood
running at about 45 deg. This looks pretty much just like strip planking
any boat!
I used to build models like this, using strips of balsa from nose to tail.
I've never seen a full sized aircraft using anything like this. Most seem
to use either sheets planked on formers, like many thirties glider like the
Minamoa or other european high performance saillanes of the thirties as
well as the Albatross of WW1, or the Lockheed method. (And BTW, the guy who
says the dutch built stressed ply airplanes for the Germans is a bit off
course, Tony Fokker only built those things under duress, he hated them and
didn't design them, rather, he and his chief dsigner, Rheinhold Platz,
developed the steel tube fuse we all came to know and love, but I digress)
There were a few inthe late thirties, notably the Timm aircraft and one
odball little twin, the Langley which used something called the "Vidal"
method of contstruction which apparently involved about 5,000 sq feet of
veneer and god knows how much resin to make one airplane. Pretty litle
thing

http://www.aerofiles.com/langley-twin.jpg
http://www.aerofiles.com/langley-twins.jpg

As you can see, the unpainted versions clearly show a 45 on all the wood
visible. Presumably, the underlying layers al go 90 deg to each other.
You gotta love the way the did the cowls out of wood as well!

http://www.aerofiles.com/langley-twins.jpg There's little doubt but that
this airplane would be nigh on impossible to build at home, but surely one
of these must be able to produce the simple teardrop that is the Baby's
pod. By the way, on the original, the teardrop is pure. That is, the fillet
between the teardrop pod and the wing was not all one piece. That fairing
is added on afterwards and is non-structural, which,it seems to me, would
simplify molding the pod no end.

"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.