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