| Bob it sounds as if the skinning did not have sufficent glued area contact onto the wing ribs or has lifted away for some reason,try first pinning the entire wing flat on a perfectly flat surface,use heavy weights such as those old flat irons or lead blocks etc against the centre section and wingtip,then use an heat gun and ply heat too and fro along the length of the wing,this should do the trick,if this does not work then a drastic measure would be to cut lengthwise into the sheeting and somehow try to get adhesive underneath to stick to the ribs holding the seams down with masking tape.
The balsa could have been damp before hand possibly causing this dilemma,try using a porcupine roller or tap gently with a new wire brush against the sheeting ( used on iron on film covering to get rid of dampness in the wood structure ) and run this down the sheeting then use a domestic iron onto the sheet and try to tighten it up a bit.
Let us know how you get on,it should tighten up given a little bit of assistance,once tightened leave under the clamps for a few days to allow the wood to settle avoiding the dreaded warps.
__________________ 'And there I was oil on my goggles from a broken pipe,then I looked at the altimeter,all I could see was the makers name !' www.wonwinglo.scale-models.net/ | 
Linear Mode
