Hello, I am looking to flatten this unhinged stringer is not possible neither to convert it into sheet metal nor to do the flattening because the function is grayed out.
Personally, I would do a surface scan of the profile, so as to have the shape of the stringer without volume, so as not to risk interfering with the future sheet metal function. Then create two 3D sketches recovering the inner edges of the stringer (Convert entities), and finally the Transition Ply sheet metal function using these two sketches.
The document is attached in 2021 version. A note: if the smoothing trajectory is a helix, the unfolded shape should be a straight bar, which is not the case with the 3D spline...