Helicopters have a power-on never exceed airspeed (Vne) that can be an aerodynamic limitation, a structural issue or based on the onset of retreating blade stall. Some also have a power-off airspeed limitation which will be shown on the airspeed indicator as a red/white hatched line or sometimes a blue line.
During autorotation at high airspeeds it may not be possible to maintain sufficient main rotor RPM even with full down collective. In an autorotative descent the incoming airflow goes up through the disk to maintain rotor RPM. As a helicopter’s speed increases the airflow becomes more horizontal causing the main rotor rpm to decay. As such, a power-off never exceed speed would prevent the main rotor RPM from dropping too low at high speeds.
However, a power-off never exceed speed could also be based on the vertical fin, as is the case with Eurocopter’s AS350 helicopter. The AS350’s rotor system spins clockwise (when viewed from above) – opposite of most helicopters. Therefore, the tail rotor produces thrust that pushes the tail to the left to counter the torque and hold the fuselage straight. To help reduce the power required by the tail rotor the upper part of the vertical fin is angled 6 degrees to the right to also apply a left force on the tail. The higher the airspeed, the more effective the vertical fin becomes. In autorotation the pilot can neutralize tail rotor thrust with the pedals, however, the vertical fin continues to push the nose right. Moreover, transmission drag wants to turn the fuselage in the same direction as the rotor system causing the nose to go to the right as well. At high airspeeds, the amount of left pedal needed to maintain trim increases and the power-off never exceed airspeed (125 knots vs. 155 knots power-on) insures adequate left pedal to maintain yaw control.