Here is an example of some simplified stress calculations for a model glider wing (courtesy of Dr. Mark Drela):

http://www.charlesriverrc.org/articl...par_sizing.txt

The numbers in the example are for a winched launched 2 m glider.

The shear load (at the wing root) is estimated to be 80 lb. The tension and compression in the spars (as a result of the bending load at the root) is estimated to be a whooping 1733 lb. Yikes!

The spars in the example are carbon wrapped balsa. There is another article somewhere on that site concerning their construction and testing.

There is a note at the end of the article about wooden I-beams as well.