I understand that rib 12 becomes curved when the factory bends the rib but I didn’t like it because it puts too much tension on the skin. I’ve asked TAF and other Sling 2 builders (the Sling 4 doesn’t has this rib), they said it’s no problem. Yet I looked for a solution.
I could not use the fluting pliers because that would make the rib even more curved. So I looked for a solution to stretch the rib and I found 2 solutions:
1: metal shrinker / stretcher; see Sheet Metal Shaping: Easy Natural Metal Curves in Shrinker and Stretcher
2: bead roller with flat dies; see Metal Shaping with Lazze: Stretching Aluminum in a Bead Roller
Unfortunately, the tool leaves very ugly marks, which are too deep to polish. Because the tool creates these marks and at the same stretches the aluminium there is a high risk of cracks in the future:
I will order new ribs and try option 2 (bead roller), although it’s not a high stress area according TAF