ORIGINAL: Shoe
I have heard the same as Dick Hanson, that in some cases you can actually see a drag REDUCTION with a properly truncated trailing edge. The idea is that the low pressure in the separation bubble behind the trailing edge will reduce the adverse pressure gradient seen by the boundary layer it approaches the trailing edge, thus keeping it attached. Unfortunately, I don't know of any "rules of thumb" or analysis tools that might help here.
I would expect that if you didn't have a boundary layer that is at or close to separation, truncating the trailing edge would result in a drag penalty.
.
Unfortunately. a major portion of the aerodynamic anecdotal information is "seen", and not measured.
There is little genuine quantifiable data that can be depended on to give consistent answers when applied to a model's performance.
Usually we just add more power to make things "better".
Where the performance specs are tight and restrictive, the use of "seat-of-the-pants"/"feel and seem" take second place to genuine information which is generated under test conditions which pick out the good and bad points of the object under test.
Competiition under such specifications weeds out the TLARs and even SWAGs.