I know there are several theories to explain why right-thrust is necessary but here are my observations.

1. most of the effect is due to torque and P-factor.
2. very little of the effect is due to the amount of fin area above/below the centerline.

I base these observations on the fact that my Primus-clone profile (which has almost an identical (and symetrical) amount of fin above/below the centerline) has just as much tendency to turn left under power as something like a Mojo or Katana (both of which have tall fins).

I also note that the amount of right-thrust required to get a profile flying "straight" with a 4-stroke turning a big prop with lots of torque is more than a 2-stroke turning a smaller/finer-pitched prop requires.

What's more, so long as I laterally balance my models, they seem to hover just fine without right-thrust or right-rudder - showing no tendency to fall off to the left (but plenty of tendency to roll due to torque).

Also, when you hover a plane truly vertical, the effect of the torque is *very* pronounced - ie: a torque roll that can be quite fierce.

Now any right-thrust will oppose the torque and P-effect when the aircraft is upright but it will become left-thrust and therefore contribute to the tendency to turn left when the craft is inverted (same goes for rudder-offset).