Earlier, Richard Isakson wrote:

With my quick and dirty assumptions, I found that the spar would yield at
4.4 g's at 600 pounds gross weight. That is looking at bend moment stresses
only. A betters analysis would raise that number. This includes the
inserts. Without the inserts the spar yield at 2.3 g's at 600 pounds and
2.8 g's at 500 pounds.

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.

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.

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.

Thanks, and best regards to all

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