Hi simba - yes I think the way OSTexo describes is your best bet. Offset the curve that you have on one side of the blue wing, then delete the wing and extrude out a new one from the new offset curve.

It will be difficult to try and offset the surface itself due to the very tight bend in it on the trailing edge. Surface offsetting will tend to get confused and generate a self-intersecting result if you try to use an offset distance that's greater than the radius of curvature in a particular area of the surface.

Curve offsetting though can trim things like that away so when possible it can be good to do a curve offset of such things instead.

- Michael