Kellermann

Jack
to understand this, one must be aware of the way the fullsize was built. Actually, it´s a triangular truss welded tube structure, over which 1/2" plywood formers are placed and over these, wooden longerons.

Let´s take a look at the 4 top longerons. The outer 2, which define the fuselage width, end at the rudder post. The inner 2, "die" earlier, right in front of the stab attachment. There is no such thing like a fillet, only a curved tube soldered to the rudder and the top (non-aparent)middle tube. The fabric covering, when shrunk and tautened, makes for that undefineable compound curve that can only be replicated if done exactly the same way. I am including 2 pictures of Aeronca 7AC Fuselages, which are basically the same for Citabrias and Decathlons, as shown by the 2 smaller pictures taken at the ACA factory.

It is important to keep in mind that the full-size is covered in 2 pieces of fabric, reaching all the way from the middle of the bottom to the midle of the top, and the fin is integral with these side pieces, and the fabric is stitched to the fin diagonal braces. As the lowest diagonal brace on the fullsize is about 40% of the fin height, that´s the point where actually the flare begins. The halves are stitched together at the middle, both top and bottom, and covered with tape. Dope, primer and paint make for an almost invisible joint line. This is hard to do on a model but it is possible. I was able to replicate it with reasonably good results by covering the bottom and sides separately, and then the top in 2 halves with integral fin, joining in the middle. IMPORTANT: these sections were not ironed on, but CA´d onto the edges and the middle joint as well. Otherwise, when stretching with the heat gun it would come off the edges (which doesn´t happen on the full-size because the fabric is one single piece each side...)

The correct stab outline (top view) will show a non-parallel gap, i.e., the inboard edges of the stabs will be slanted forward, to accomodate fabric shape and keeping a parallel gap between the stab and the fin.

Most kit makers (including myself) make concessions in that area in order to ease the building process. In most kits, there is a 60-size-trainer-style butt attachment of the rudder fin over a flat top surface (in some cases with a small balsa or plastic fillet) and a direct attachment of the stab halves to the flat fin sides.

I do it a bit different. I also do sheet the section between the 2 inner longerons way down to the end of them, under the fin. I keep the parallel stab-fin gap (3mm) which looks scale, and 2mm in front of the front stab attachment tube I glue a small triangle of balsa for the covering to attach to allow for the fabric (or plastic film) to make that compound curve (can be seen on pix 5 and 10). It is not actually necessary for the curve itself, but to create an edge where I glue the covering onto, otherwise when shrinking the fabric it will again come off the edge and enlarge the hole for the stab tube way too much. When covering the fin, I make a small cut under the front hole to allow for the in-between-stab-tubes-section to be ironed onto the flat fin surface while the front part conforms to the fuse/fin transition and form that beautiful curve.
Maybe hard to understand as a text but I am including some photos to show how it´s done.

Any further questions, please don´t hesitate to ask...hope this has been of help.

Marcus