Make a plane (Plane 2) perpendicular to "Plane 1". Draw a line (could be reference geometry) on Plane 2" that is also perpendicular to "Plane 1" (and on "Plane 2". Draw the OML (Outer Mold Line) curve on "Plane 2" exactly how you want the outside shape of the duct to be. Rotate the OML curve about the first line (reference geometry). Should work.

If you need the duct to be curved, you'll need to start with a curve and not a line.

Hope this helps. "Blue on Top" Ron