compound curves in plywood

"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

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.

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....

COLIN LAMB wrote:

Question: 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?

Response: Some of the cedar strip boats that I built were made from 1" x 6"
cedar planks, which means they were about 3/4" thick. Then I ripped those
planks on a saw (table saw, band saw or radial arm saw - whatever is handy).
This will give you long strips that are about 3/4" wide and 1/4" thick.
Cedar is used because it bends easily, looks good and is light. Stations
are built conforming to the curved shape you want. Each station is about
12" to 16" apart. Wax paper or tape are put over the plywood forms. The
first strip is installed, and following strips are glued adjacent to the
first step. The strips are stapled onto each form. Alignment between
stations can be assured by clamps over the strips you are gluing. The glue
does not matter, becase no strength comes from it. You can butt joint each
piece. Although some perfectionists angle the edges of the strips going
around a curve, it is not necessary. If there is a slight opening, epoxy
will seep in and make the structure stronger. The cedar simply replaces
foam, and the strength comes from the layers of cloth and epoxy resin.
Carbon fiber is stronger and lighter, but would cover up the beautiful cedar
strip finish, but could be used on the inside. Obviously, the application
and strength of the epoxy will control the strength of the overall
structure, but it can be quite high. The strength requirements for the
fuselage of something like a Bowlus would be quite low - but there would
obviously need to be some method of attaching the wings to the fuselage and
spreading out the stress of the lifting point.

However, the pod itself could be created similar to a cedar strip kayak. My
17 foot cedar strip kayak weighs about 50 pounds and I made no effort to
save weight.

One other process, which could be used is a process developed for the HK-1
Hughes "Spruce Goose" It is built of laminated birch. As I understand the
process, the it was plywood made in the shape required for the aircraft and
known as Duramold. The aircraft companies made a number of small boats
using the same process to perfect it. The Evergreen museum no doubt has
some information on the process.

The inventive can replace the Molt Taylor Aerocar of yesteryear with a
Glider/kayak. When making an outlanding, simply land by a lake ro river,
remove and store the wings and tail, and paddle home.

Colin N12HS


Sounds like some of the early Curtis flying boats. I think they'd be fun.

Dan, U.S. Air Force, retired

"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

Rich S. wrote:

How many builders align their wheels using a micrometer?


I do. I place the micrometer on the wheels, then get out the wheel
alignment tool (sledgehammer) and adjust...

"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.

"nafod40" wrote in message
...
Rich S. wrote:

How many builders align their wheels using a micrometer?


I do. I place the micrometer on the wheels, then get out the wheel
alignment tool (sledgehammer) and adjust...

Remember - the micrometer has all those number thingys on the handle. The
C-clamp doesn't!

Rich S.
&-)

"COLIN LAMB"
thlink.net:

"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.


Ouch! 1/4 inch? All covered with resin this would be fairly heavy,
methinks. What does one canoe weigh? The fuselage would probably be roughly
double that, plus formers., hardware, ....

"COLIN LAMB"
thlink.net:

Question: 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?

Response: Some of the cedar strip boats that I built were made from
1" x 6" cedar planks, which means they were about 3/4" thick. Then I
ripped those planks on a saw (table saw, band saw or radial arm saw -
whatever is handy). This will give you long strips that are about 3/4"
wide and 1/4" thick. Cedar is used because it bends easily, looks good
and is light. Stations are built conforming to the curved shape you
want. Each station is about 12" to 16" apart. Wax paper or tape are
put over the plywood forms. The first strip is installed, and
following strips are glued adjacent to the first step. The strips are
stapled onto each form. Alignment between stations can be assured by
clamps over the strips you are gluing. The glue does not matter,
becase no strength comes from it. You can butt joint each piece.
Although some perfectionists angle the edges of the strips going
around a curve, it is not necessary. If there is a slight opening,
epoxy will seep in and make the structure stronger. The cedar simply
replaces foam, and the strength comes from the layers of cloth and
epoxy resin. Carbon fiber is stronger and lighter, but would cover up
the beautiful cedar strip finish, but could be used on the inside.
Obviously, the application and strength of the epoxy will control the
strength of the overall structure, but it can be quite high. The
strength requirements for the fuselage of something like a Bowlus
would be quite low - but there would obviously need to be some method
of attaching the wings to the fuselage and spreading out the stress of
the lifting point.

However, the pod itself could be created similar to a cedar strip
kayak. My 17 foot cedar strip kayak weighs about 50 pounds and I made
no effort to save weight.

I'm pretty sure that the process would be to heavy for the strength it
would provide, unless it were modified to provide several very thin
crossing layers, all of which woul have to be tightly glued to each other
to have any real strenght.
But an idea has just popped into my so-called mind..
If I were to make three mold of each side freehand over a male mold, each
90 deg to the last, and then take them and stack them over the same mold
and vacuum bag the whole lot together....

Hmm.


One other process, which could be used is a process developed for the
HK-1 Hughes "Spruce Goose" It is built of laminated birch. As I
understand the process, the it was plywood made in the shape required
for the aircraft and known as Duramold. The aircraft companies made a
number of small boats using the same process to perfect it. The
Evergreen museum no doubt has some information on the process.

This isn't to far off the original method of manufacture for the Baby as i
understand it.


The inventive can replace the Molt Taylor Aerocar of yesteryear with a
Glider/kayak. When making an outlanding, simply land by a lake ro
river, remove and store the wings and tail, and paddle home.


Would have come in handy at least once in my gliding career when I left a
waterline on a 2-33!