Im pretty sure after i split the wing i selected the edges along the split and extruded them inward once or twice to create a bevel if the centertering has not given you good result try not cetnering the extrude tool and slowly pick one of the arrows and move inward one of the arrows should extrude all teh edges uniformly it chages with each shape sometime.. That tools like to do weird things sometimes..

The last option if the above done work is just select edges along a straight line extrude them out just a bit and repeat until you creat the bevel that way. Once you hae the bevel nice move the vertices around the outside where the edges are not merged and sew them up.The lowerthe top half just a bit to have it not see through.

Let me know if that helps.