From: highflyer <highflyer@alt.net>
Newsgroups: rec.aviation.homebuilt,rec.aviation.restoration
Subject: Re: Wrinkle-free skinning in plywood - any suggestions?
Date: Tue, 05 Jan 1999 08:59:03 -0600

Alex Yeilding wrote:
>
> jimshorts@my-dejanews.com wrote:
>
> >Dan, you might also try a unidirectional plywood--all the grain is
> >running the same direction rather than being laminated in alternating
> >pattern.
>
> Jim, I have NO expertise in the areas of aircraft restoration or
> aeronautical engineering. However, based on my knowledge of wood from
> woodworking, this would scare the daylights out of me. Presumably a
> competent engineer specified the types, thicknesses, number of plies
> and ply directions for the needed strength in his or her design.  To
> use your own unidirectional veneers could seriously change the
> strength characteristics.
>
> > You can also do gain surface strength by doing multiple thinner layups
> >with glue and build up to what you want.
>
>
> Well, that makes me feel a LITTLE better. But I still think you need a
> bigger engineering study and more production equipment than warranted
> for any gain you might get.  Assuming your layup has the same number
> of plies, in the same wood(s), and in the same order and directions as
> the specified plywood, it does seem that you could get a smoother
> shape that would be retained better. But how are you going to match
> the original plywood for bond strength between the plies? Are you
> going to build a big veneer press with molds to match every curvature
> on your fuse and wing? Or are you going to use the plane itself as the
> mold and clamp down with straps to hold the glue-up? How are you going
> to put pressure for the glue bond where there is no structural member
> underneath (i.e., between ribs) without creating depressions in the
> surface?
>
> As I said at the beginning of this note, while I know a little about
> wood, I know nothing about building or restoring  wooden planes. So if
> my concerns are stupid, go easy on me, and educate me!
>

Alex,

Your concerns are NOT stupid, and they are right on the money.
Using the fancy "bendable" plywood with all the plies oriented in
the same direction would instantly render the aircraft unairworthy.

While it is quite possible to make you own plywood by building up
plys yourself, it is never as good a plywood at the purchased
variety, which is laminated with high quality glues in heated
veneer presses.

There is a technique where you can build up curved shapes using
multiple layers of veneer that works reasonably well.  It has been
used for many years for boat building, particularly for building
canoes and small craft.  The Brits call it "cold moulding."

You build a firm form of the shape you desire to achieve.  This is
usually built like a carvel planked boat, with moulds and edge
fitted planking of substantial thickness for solidity.  This "mould"
is sanded to final shape and varnished and waxed so it glows.

Then you take small pieces of veneer, three or four inchs wide and
several feet long, and staple them to the mould, usually at some
diagonal angle to the centerline of the mould.  Then the second
layer is coated with glue and stapled across the first layer.
This layering process is repeated, carefully fitting the edges
of the veneers so they meet tightly and do not overlap.  When
sufficient "plies" are built up, the staples are cut off where
they can be reached and the finished product is removed from the
mould.

This process was used to build the fuselage halves for the "Mosquito"
bomber in WWII.  There they laid the outer skin into a female mould
and then lined that with a layer of balsa wood, before laminating
an inner wooden skin on top of the balsa wood.  Great composite
structure.  The process has been used with balsa or spruce core
and glass skins inside and outside to build canoes called "strippers"
and some sailplanes.

The Lockheed Vega, flown by Wiley Post, used cold moulded wood fuselage
construction.  They made the mould out of concrete.  Then laid up
the fuselage halves inside a concrete female mould.

HF

From: highflyer <highflyer@alt.net>
Newsgroups: rec.aviation.homebuilt,rec.aviation.restoration
Subject: Re: Wrinkle-free skinning in plywood - any suggestions?
Date: Tue, 05 Jan 1999 09:02:08 -0600

Cy Galley wrote:
>
> Aircraft plywood is made in TWO versions.  90 degree plys and 45 degree
> plys.  If you want it to stay flat or slight bends use the 90 degree.  If
> you want to bend it, you use the 45 degree plys. Of course for the tighter
> bends, moisture helps


Actually, the 45 degree plywood is made for building up I beam or
box wing spars.  The ideal grain direction is at 45 degrees to the
centerline of the spar for all plies in the shear web.  In both
normal plywood and 45 degree plywood, the plies are at right angles
to each other.  The 45 degree plywood has the plies at 45 degrees
to the long edge so you don't have to cut it from the diagonal of
the sheet and scarf the pieces to make up a shear web.

I may be wrong.  I just cut it on the bias and scarf them together
myself.

HF

From: highflyer <highflyer@alt.net>
Newsgroups: rec.aviation.homebuilt,rec.aviation.restoration
Subject: Re: Wrinkle-free skinning in plywood - any suggestions?
Date: Tue, 05 Jan 1999 08:47:55 -0600

Rick Hengeveld wrote:
>
> D.N. wrote:
> >
> > Hi All,
> >
> > I am in the process of restoring a vintage English aircraft that is sheeted
> > in plywood, and I would like to hear of techniques to obtain a wrinkle free
> > surface.
> >
> > All of my surfaces are to be sheeted in Birch, thickness ranges from 1/32"
> > to 3/32"
> >
> > Does anyone have any suggestions?  Any and all are appreciated.
> >
> > Thank you in advance,
> >
> > Dan
> Can't say much about wrinkles,,But an old R/C builders trick to shapeing
> thin ply around structures is to use ammonina based glass cleaner
> sprayed on the top side of the ply. After a few mins. the ply becomes
> much easier to work around corners.

A little ammonia in water in a spray bottle is all you need to soften
the plywood enough for forming.  You can even get some mild compound
curves that way.  For example, the wing tips on a Howard DGA are ply
covered compound curves and the ammonia and water trick allows you to
form the ply to the shape smoothly.

Unfortunately, that is NOT the wrinkling problem he asked about.  It
is an unfortunate fact that plywood is made out of WOOD.  Natures
composite.  Wood changes its dimensions quite noticeably when the
moisture level of the wood changes.  The more moisture in the wood,
the fatter the cells get and the wood grows.  When it drys out again,
it shrinks.  This is referred to in all the woodworker journals as
"wood movement" and must be considered in the design and construction
of wooden objects.  Failure to do so will result in broken glue joints
and wrinkled surfaces or cracks.

Even sealing the plywood with epoxy will not STOP this moisture
change from occurring.  However, a good thorough finish will greatly
slow the moisture change.  Since the environment is constantly
changeing as well, and that is what causes the change in the wood,
slowing the moisture migration significantly results in averageing
out the changes in the woods moisture content and consequently
stabilizing the dimensions somewhat.

The best solution to a smooth and durable wooden airplane is to
stabilize the moisture content of all of the wooden pieces used
in the airplane somewhere near the average humidity value for the
intended environment.  Assemble the aircraft as close to that value
as you can manage.  Seal or varnish with a GOOD grade of varnish,
every part as it is assembled.  Even varnish the insides of bolt
holes.  ESPECIALLY varnish the insides of bolt holes.  Ferrous metals
in contact with wood and moisture will cause the wood to rot and the
metal to corrode.  This can be prevented with a good barrier film
between them.  The old timers used to bed their fittings in wet
paint to keep moisture out from between the fitting and the wood.

When you varnish the insides you have to be extremely careful NOT to
varnish anywhere that glue is eventually going to go.  When you glue,
to the wood left bare in the varnishing operation, make sure you cover
all of the exposed wood with a good film of glue.  The glue will seal
the wood as well as the varnish does.

If you really want to learn about aircraft woodwork,  the EAA does
publish a number of books about Wood in aircraft construction.

HF