Ok this has been bothering me, but I figured it out. You want to do a lofted sheetmetal bend. I made this one by making a parallel offset plane, with a bigger dia arc on the lower level, and a cmaller dia arc on the upper level. The click on 'lofted bend', pick the two arcs, and specify the thickness. That makes a sheetmetal cone that will flatten on command. You could also put one end of the arcs on a centerline with an angle dimension for more control.