I've never liked powered ultralights that use the US part 103 definition of
ultralight. The FAA limited the empty weight to far too light a weight.
They could have added a hundred pounds to the empty weight and kept the
other limitations as they are. This would have produced a real viable
airplane class. As it is, the structures are designed without a lot of
redundancy. In the airplane in question here, the design is fine when it's
new but if there's much hangar rash on the leading edge of the wing the load
that the wing can take is greatly reduced.

I read all of the screaming and shouting about this plane's wing and I
finally decided to take a quick and dirty look at the stresses on the front
spar. I choose the front spar arguing that it's going to take most of the
loads anyway. Using a single spar unravels a knotty little problem of
resolving the reactions from the fore and aft lift struts and the two flying
wires. This was only a quick look after all. I just wanted to see if the
wing was in the ball park. Please understand that the real airplane needs
both the fore and aft spars and the fore and aft lift struts in order to
react to the wings chordwise torsion. For convenience, I used a level
spanwise wing loading. This is a conservative approach. We speak of
conservative and non-conservative. A conservative error is one which leads
to an over designed structure. A non-conservative error is one which leads
to an inadequate structure.

I used the drawings that are available on the web. They include the spar
inserts. I have a slight problem with these inserts. Since they aren't a
tight fit, its possible for the edge of the insert to cause a stress riser
on the inside of the spar tube. It's something to keep an eye on.

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.

Per MIL-HDBK-5 the tensile yield strength 6061-T6 tubing is 35K PSI

Rich