Hey Dick,

I'm sure some of the spiraling airflow could be blocked by the vertical fin, resulting in more force UP on the left stab than there is force DOWN on the right stab, resulting in a pitch down moment. I wouldn't think there was much force from that though, although maybe enough for a click or two of trim like you said. Obviously in your tests, you were careful to place the CG where it needed to be, but most people are fairly oblivious to this fact... people just put all the components below the wing because it's the norm and it looks a little better from the top. Throw in landing gear and it's pretty difficult to get the CG high enough. A few clicks like you experienced is nothing compared to some planes I've flown that require up elevator all the time in a hover.

Another technical consideration would be downwash from the wing. The downwash in level flight is probably inducing an angle of attack on the stab (a negative AoA = positive pitching moment), but then in a hover the wing isn't creating downwash like it is in level flight. Therefore, if you trim for level flight, it may be like being trimmed down elevator in a hover. More to think about... bottom line is you want to get as close as possible, because these little nuances have much less affect than having the CG an inch or two below the thrustline

Baron