fainjon

I am building a wing for a glider/trainer and wanted to put a carbon fiber tube to take any torsion. i was wondering where the center of the torsion moment is on the airfoil. also is the center of gravity on the thickest part of the foil and should the spars be on the thickest part of the foil. help me out here.

Thanks

Tall Paul

For a tube supported wing, the primary tube is usually placed at 25% of the m.a.c.
This may NOT be the thickest point, but it is where the airfoil pitching reacts.
To prevent torsion, the tube all by itself won't do this.
A second point of attachment is generally added near the trailing edge of the wing in the form of a dowel which fixes the wing's angle on the fuselage, and prevents any torsion around the primary spar.
The attached shows two joiners for the load, and the aft joiner to prevent twisting/torsion.

fainjon

That is a reasonable answer.

BMatthews

But in addition to all that you should be aware that it's much more important to put any wing joiners AT the spar so that the loads are transffered INTO the spar in the most direct manner.

If you use a tube at the 25% point and the spar is at the tickest point back around 30% then the ribs need to carry the load. Not a good situation.

Standard practice is to rely on the spar, ribs, leading and trailing edges in concert with D tube sheeting and the covering to resist torsional loads. It's worked well in the past for many designs. You may be worrying about something that is not an issue.