ORIGINAL: BMatthews
That's a pretty high speed and a rather thick airfoil on quite a big wing. Is this intended for a 1/4 scale unlimited racer by any chance? Because that's about the only way you'll see that sort of speed from a model of that sort of size.
I would also suggest that even if the spreadsheet says .035 is enough that you double it at least over the first portion to .063 or so. The reason being that if you can hold the tube in a perfect circle shape it may well hold that much load. But if the bending forces cause a small dimple at a wing joint that dimple can lead to a catastrophic failure at that point. Going for a thicker wall will not only make the tube stronger but will support the tube's shape far better at these shear plane joints.
Thanks for the input. The calcs say an 2" diameter tube .035" wall at 35# gross takeoff weight at 20's gives a 9.5 safety factor. (below 1.0 will result in failure). An .049" wall will give a 13.0 safety factor. So.....Theoretically....a 2" tube at .049" wall will fail at 260 G's at gross take off weight.
Yes an .049" wall would be best with very little weight penalty and great peace of mind.
For comparison, a BVM KingCat at 35 pounds gross take off weight can do 225 MPH and uses a 1 1/4" diameter by .035 wall aluminum main spar tube.
Dave