Not a RL pilot either, but as an aircraft technician, I can tell you that at higher speeds, the rudder in a helo does lose the ability to TURN the aircraft. At higher speeds, it is designed to allow coordinated turns , just like an aircraft, only at low speed does it allow the aircraft to spin on its axis.
You have to remember that at higher speeds, it has to overcome all the wind resistance on its fuselage PLUS the effect of TORQUE from the main rotor head. BTW, there is a reason that the tail rotor is actually called the "ANTI TORQUE ROTOR". and not the RUDDER ROTOR.
