Nice post Colin - I think by definition of an RCU thread, we will not be able to reach theorhetical agreement , which makes the field tests quite satisfactory for me to stick with for me.

Ryan - I only performed the experiment as I noted above. Between each plane the longitudinal placement didn't seem to matter as the overall effect was the same. The fin on the Abbra happens to be about 6" further back than the Aggressor. From the tip of the spinner to the center of the fin is 38" for the Abbra and 32" for the Aggressor (just measured). I did not change the location of the fin on each plane to test the performance difference. Like I said, I was satisfied with the performance gain and did not add the top part to the fin. For me this is not a full blown testing program to try different combinations as I only have so much time to devote to this.

Spiral slipstream diagrams.... Reaching back through Program Management, US Army brainwashing, kegs of beer, and my share of bad grades, I offer this opinion: A typical text book diagram is used to illustrate the point of a lesson, or is a momentary snap-shot of the flow, and does not necessarily depict the actual flow. Prop diameter, pitch, RPMs, number of blades all affect the shape of this spiral flow (or the pulse of it not withstanding free stream affects on the plane and prop-generated spiral). Once the prop starts turning it generates a cointinuous spiral woosh down the plane. There are not little tiny individual sprials travelling down the plane (like some of the sprirals miss the fin and others hit it) - it is a continuous sprial air flow. How tight it starts (or finishes at the tail of the plane), and what if any frequency change takes place as it interacts with freestream air, is totally unknown for these planes (if someone has this information please post it).

I sort of liken the situation to winglets. A wing tip generates a vortex that spirals from the bottom of the wing tip up, around, over, and splashes back down on the top surface of the wing (adds drag?). If you wanted to completely stop this phenomina the winglet would be huge (something like 1/5 the wingspan (help here) - anyway the winglet gets incredibly large to completely stop this from occuring). But we see smaller versions of the winglet in service on all modern newly build airliners because perhaps in reduced size they still affect the whole equation. These small winglest are not stopping 100% of this phenomina, but they are adding some positive benefit beyond marketing.

So, I just think that the flow is interrupted, then straightened by free stream airflow, prior to going over the tail. I couldn't begin to tell you though where the optimum place to do it is. Also, I don't think you can fix it another way by integrating this very small fin area into the overall fuselage profile and say, "now the fuselage has the same area as with the fin therefore it flies just as if there was a fin like before." It doesn't work that way. A fin can be an efficient surface to do this. If you just made the fuselage a teeny bit bigger to account for the fin, nothing really changed.
Thanks - now ready for the practicing engineers, folks with good grades, and trial-&-errors to offer some insight.
Jim W.
PS - did CPLR move the fin to change the effect, or move it to get it off the removable canopy and better fixed to the plane? Its nice to have the WC stories come out.