Completely tubular the entire length, no.
In the Rhino model there are two closed squares identifying the start and stop of the nacelle body; I chose not to loft these two in the model to prevent unnecessary clutter. The two white airfoil outlines are the shape of the wing where the nacelle is to be positioned; again not lofted to avoid clutter. The aft nacelle square closed curve designates where the transition is to start to a shape that needs to be semi-rounded along the shared edge. Doesn’t HAVE to be round as seen in the photo, just representative, oval-ish would be acceptable. Photo provided merely to show what I am after - pictures are worth a million words. I am just trying to achieve a rounded shared edge except without the concave area present in the model where the transition starts. During the actual build process of the airplane model a balsa block would be roughly cut to the shape with hard edge shown in the Rhino model. Sanding the shared edge to render it more streamlined would come next.
I am not sure how to fix or the proper way to fix the concave area - if it can be fixed. The concave area is highlighted by red line in photo below. If if would follow more along the original outward bow of the shared edge curve that would be great.
The middle test is very close to what I am after, but without the concave area at the start near the aft square closed surface is desired. If I have to live with it I will. If it helps to know how the surfaces were created, the top surface was created lofting the two top curves. The side surface was created lofting a profile along two curves; in this case it is a vertical line.
Incidentally, in variations I tried of joining, not joining, trimming the two surfaces and other exploratory means of running the _VariableFilletSrf command, sometimes the shared curve edge would be available its entire length, sometimes only portions, sometimes only a very small portion would fillet. I presume because in those situations where the command failed the edges weren’t intersecting.
Appreciate the assistance.