I am late to the party on this thread but here goes.. My issue is the published CG location in the manual. It claims 89mm, and my response is not possible. Here is why!
First a brief explanation of the setup. An OS 46 power plant and battery behind tank. I normally put the plane on a CG machine, balancing the plane at the published CG point. I then use weights to make it slightly nose heavy say, 15-20 deg nose down for good stability. To achieve this condition I added 11-7oz. weights to the hind end. I am quite anal about adjusting the clevis on the elevator so the elev. lines up with the top surface of the horiz. stabilizer.
This being said I went flying yesterday and expected, if anything, given my setup, the plane would tend, if anything, to go nose down in flight. Instead, quite to my astonishment, it wanted to go into orbit which was totally unexpected. This was not a slight trim issue. The logical conclusion that I arrived at was, that if at 89 mm it is slightly nose heavy on the CG machine, but in flight it behaves as though it is noticeably tail heavy, then the "real/actual" CG must be less than 89 mm giving the rear weight a longer moment arm, making it tail heavy; in other words the true CG is less than the published number of 89mm. Being suspicious of the published CG number, I removed 3 weight segments from the tail for the next flight, as in the absence of better info, this will be a trial and error process. I invite any comments from other pilots who fly this model who have had similar experiences, or someone that can tell me where my thought process is flawed. Thanks!