"Bob Kuykendall" wrote ...

Interesting. When I run the moment of inertia for 2" tubing of .058"
wall, I get 0.1667 in^4. Using that number and a yield strength of 35
ksi I get a yield moment of 5833 in/lbs. Do those numbers agree with
yours? Of course, those figures disregard cripling or buckling, which
I've not seen mentioned in this thread.

For the spar alone those numbers are right and result in the poor spar
performance without the inserts. The maximum bending moment happens at the
strut attach point. This is where the insert sits and that increases the
moment of inertia to 0.3155. This results in the improved load handling
ability.

In the interest of laziness, I didn't look at spar buckling nor did I look
at negative loading. There is a potential for column buckling of the spar
between the root and the strut attach point. As the wing is lifted, the
strut is placed in tension. This places the inboard portion of the spar in
compression. The combination of the compression load and the lift load
could potenially cause buckling. Maybe I'll look at that sometime.

I suspect that this whole thing will come down to a somewhat subjective
matter of distributions and deflections. The distribution of loads
between the forward and aft spars will make a big difference, and I
think that the wing deflection will start to look scary before the spar
tubes reach yield. But those are just more non-engineer's guesses, and
there's been plenty too much of those already.

Reading what little has been said about the load testing, I suspect there's
a problem in the way the wing was held. It almost sounds like they didn't
have the rear lift strut attached

Taking this out on a tangent, one thing about little airplanes like
this that I don't understand is why so many of them use tubular spars.
It seems to me that you can get so much better strength/weight and
stiffness/weight using a built-up I-beam or C-section spar. Yeah, it's
a bit more trouble. But the result is either better strength and
stiffness for the same weight, or the same strength for less weight.
But again, that's just my non-engineer wing developer perspective.

I think it's to keep the labor costs down. You might ask Chuck.

Rich