Spar gluing

"Stealth Pilot" wrote in message ...
On 19 Nov 2003 10:01:51 -0800, (Lou Parker) wrote:

Anyone know of any downfalls to using liquid nails to glue spars together?
And yes, I expect the usual ignorant smart asses, who really don't have an answer.
Lou

maybe I'll be the only one to give you a serious answer.

I already pointed out that it doesn't have much sheer strength. Old liquid nails
joints also are real brittle. It only takes a few sharp blows to break the bond.
I've demolished a lot of stuff in my house that was put together with it.

Stealth Pilot wrote in message . ..
On 19 Nov 2003 10:01:51 -0800, (Lou Parker) wrote:

Anyone know of any downfalls to using liquid nails to glue spars together?
And yes, I expect the usual ignorant smart asses, who really don't have an answer.
Lou

maybe I'll be the only one to give you a serious answer.

aircraft wood glues have a number of requirements on them that the
usual off the shelf glues cant satisfy.

wings out in the sun get quite hot and they may be in the air under
load in this condition. this means that thermo softening glues, such
as liquid nails, pva etc cant be used because they WILL fail under
load when hot.

wood will expand and shrink slightly with different moisture
conditions, sometimes one side of the joint can be moist and one side
relatively dry. this may seem insignificant but in actual fact is one
situation which anything but the best of joints will be destroyed by
over time. the differential wood expansion sets up large sheer
stresses across the glue joint. if you have a weak glue you may just
end up with only half of it remaining and half of it showing cracking
from the stresses. for this reason varnishing to water proof the wood
is essential for any glue, as is making the glue joint with wood that
has the same moisture content.

aeroplanes (wonderful word that) last quite a while so it is essential
that the glue last longer than the life of the aircraft. Acid
catalysed phenolics (like Selleys 308 in australia ) use formic acid
applied to one side of the joint to set off the resin mix applied to
the other side of the joint (it sets off when they are put together)
this acid catalyst has been found to continue working as an acid after
the cure and has led to wood failures beside the glue joint. people
have died in structural breakups of aircraft because of it.

currently there are two basic wood glues that have stood the test of
time in aircraft structures. they are Resorcinol Formaldehyde which
was developed all the way back in the 1930's and wood compatible epoxy
resin. (not all epoxys are suitable)

It doesnt cost anything more to use a wood glue that will last the
distance so I would implore you to use a proper, tested by time, wood
glue.

Both of the glues I mentioned as suitable have little factors in their
use which need to be understood. I'll leave it to you to research
those.

Btw I asked Australia's CSIRO about modern glue research a few years
ago for my work with the SAAA. "yeah we're doing lots of it but under
contract so I cant show you the results of the tests. I will tell you
that Queensland Hoop Pine is exceptionally easy to glue (it's not much
different from spruce) and in all of our tests to destruction
Resorcinol Formaldehyde glues have outlasted the wood."

I should mention casein glue in passing. it works but is eaten by
bacteria if it ever gets moist an is unreliable over the life of the
vaerage aircraft.

Hope that helps.
simple as it is it is critical that you get glueing right.
Stealth Pilot



Thank you. It looks like we have a couple of winners. Just for your
piece of mind, I have used nothing but T-88 on my project, but once in
a while a question like this comes my way.
Thanks
Lou

Earlier, Stealth Pilot wrote:

Acid catalysed phenolics (like Selleys
308 in australia ) use formic acid ...
...this acid catalyst has been found
to continue working as an acid after
the cure and has led to wood failures
beside the glue joint. people have
died in structural breakups of
aircraft because of it.

The issue of acid-catalyzed urea-formaldehyde glues in Australia is
explored somewhat in the article I cited earlier in this thread. Not
being a wood enthusiast I don't really care one way or the other, but
I think that folks who do care should know that there is more than a
single side to this particular story.

Bob K.
http://www.hpaircraft.com/hp-24

Stealth Pilot wrote in message . ..
On 19 Nov 2003 10:01:51 -0800, (Lou Parker) wrote:

Got any info on urethane glues like Gorilla glue?

They're supposed to be good for outdoor wood projects.

Also a note on varnishing:

All film finishes for wood, paint, varnish, laquer, shellac (I think
that is all) will allow water vapor diffusion so that changes in
humidity will still cause the wood to expand and contract. What
these finishes do for the structure is to SLOW the rate of
diffusion so that any moisture gradient within the wood is
minimized. This in turn minimizes warping, but not the
overall dimensional change.

The expansion and contraction of wood with moisture content is
anisotropic. Wood expands and contracts the most along the
grain boundaries. This will be accross the fact of most boards
that you get by sawing the log through and through, that is
in the direction that was tangent to the circumfrence of the
tree. Wood expands and contracts less accross the grain, that
is in the direction that was radial in the tree trunk. It
hardly expands and contracts at all in length.

I have read that Doug Fir is uniquely stable among woods in
that it expands and contracts equally in both the radial and
circumferential directions and so it is the only wood commonly
sold as dimensional lumber while still green (others will usually
be kiln dried to 12 % MC) However, I have seen Doug Fir boards
cup and uncup with variations in humidity, indicating to me
that this is not true.

--

FF

"Fred the Red Shirt" wrote

I have read that Doug Fir is uniquely stable among woods in
that it expands and contracts equally in both the radial and
circumferential directions and so it is the only wood commonly
sold as dimensional lumber while still green (others will usually
be kiln dried to 12 % MC) However, I have seen Doug Fir boards
cup and uncup with variations in humidity, indicating to me
that this is not true.

--

FF

Indeed, it is not true.

See http://www.woodweb.com/Resources/woo...dbook/Ch01.pdf for all you
ever wanted to know about wood, and more.
--
Jim in NC