Earlier, UltraJohn wrote:

My question, How much permanent
deflection in the wings after the test?

I think that that is one of the most central questions in this
exchange. If there was no permanant deflection, then the material never
reached its 35 ksi (some books like Machinery's Handbook say 40 ksi)
yield stress, and without deflection records we'll never really know
how close the test wing came to actually failing.

Overall, I'm satisfied with the descriptions that a fairly valid static
test was done. However, I know from watching videos of static tests and
from running deflection estimates for various wings that tests even to
just the design limit can result in some seriously spooky deflections.
And to add to that, the spar under consideration here is very shallow,
and that will also tend to give it a lot of deflection.

One thing I would have expected for a test like this is a comparison
between the wing under test and some sort of deflection reference like
a template. Since the spar in this case has a constant depth of 2", the
same template can be applied anywhere on the spar to check the
deflection and deduce the stress from that.

If I've run the numbers right, for a 2" tall aluminum (Young's modulus
of 10 meg) element, at 35 KSI in the extreme fibers the element will
have a curvature radius of about 20 feet. That may sound like a pretty
shallow curve, but over four feet it results in a bend of about 9.5
degrees and a vertical deflection of about 4".

But don't take my word for it, I'm _not_ an engineer...

Thanks, and best regards to all

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