Originally Posted by
ShyTorque
I've looked in my three "Prouty" helicopter aerodynamics manuals.
He does mention C of G a few times, but nowhere does he answer the OP's question directly. My own take is that it is probably more about the aerodynamics of the fuselage as presented to the oncoming airflow, i.e. mainly parasite drag related and the least "draggy" aircraft nose up/down attitude will vary from helicopter type to type.
The stabilizer is ment to push the tail down in most helos, to achieve a less nose down attitude.
Aft CG set the attitude less nose down to begin with and reduce the negative lift(downforce) comming from the stab.
Most helos should gain from this by reduced induced drag on the stabilizator and as the negative lift reduces the need for vertical positive lift from the main rotor reduces it can be tilted more forward thus giving more forward thrust from the same power.