Funny you should ask. I just completed a Goldberg Sukhoi and added a wing tube. It came out great. I put in slight dihedral. My goal was to have the top of the wing perfectly flat, and all the taper of the wing on the bottom. To do that, when I cut the wing tube holes in the ribs, I maintained a constant distance from the top spar. It wasn't that difficult to do. The sukhoi has a similar wing span (72.5"). I purchased a 3/4" x 30" carbon tube from TNT, that worked perfectly. See the pics, they expain it better than I can.