Don,

I'm using the 134 mm blades. GY501 gyro and a 9253 rudder servo. It feels pretty much like my X-cells on 105 mm blades.

I note that v-east has big tail blades available, and also RC Market in Belgium.

I have not tried 110 mm blades, and don't really want to. I've tried blades that are too small before and it look ugly with the tail wagging all over the place.

Eric,

The tail rotor will precess while it's yawing the helicopter. It has torque applied in the vertical axis, resulting in a roll-axis torque. If there was no vertical axis torque applied to the tail rotor, the helicopter would translate sideways without yawing.

I didn't consider the affects of the moment of intertia of the tubine shaft since the HP5 runs at more or less constant RPM.