On Mon, 18 Aug 2003 16:36:46 GMT, Richard Lamb
wrote:

One thing is the level of craftsmanship required.
Polyurethane glues need exceptionally tight joints to
develop any strength. Where there is any air space
the glue will "foam" up. The foam has very little
strength...

T-88 is more of a gap filling glue, and much more
consistant.

Richard

Not only that T-88 cures slowly. It's best for gluing wood. You lay
some on a scarf joint and it will migrate into the wood over a period
of hours before it cures. This gives you a real strong bond once it
cures. Experiment with it. I've made but joints with it using spruce
and Canadian white pine, let them cure for two days, and tested them.
I've put them in a vise and smashed them with a hammer, the wood
almost always fails somewhere other than the glue joint. In other
words, the T-88 joint is stronger than the surrounding wood.

Of course that's a problem in some cases because the added strength of
the glue joint actually concentrates the stresses at that strong
point. Most people don't understand that but here's a simple example:

Take a yard stick and clamp one end of it to a table. Put a 50 pound
weight on the other end and watch what happenes. The whole yard stick
will bend like a willow and support that load.

Now go out to about 30 inches on the yard stick and put a support at
that point near where you are going to place the load. Put the load
on it and the stick will snap at the support point. What you did was
concentrate all the stress at the point where you beefed it up. You
need the whole stick to take the load, not just one tiny little point.
The problem wood builders have is that many of them don't follow the
plans and they beef up a certain area using T-88. All they do is
concentrate the stresses at those strong points and can actually cause
a failure because of it.

Same thing with T-88 itself. The stuff is so good, the scarf joint
won't be the point of failure but the loads will concentrate there and
cause the wood just outside the scarf joint to fail.

Be careful. Follow the plans. I saw a guy add extra drag braces to
his Minimax wings once. Later on they caused him problems because of
this. The wings didn't flex enough to take the turbulence loads.
They were too rigid. He had to cut them out and destroy a perfect
fabric job to do it.

Badwater Bill