Power-off Vne

March 7, 2013 by Tim McAdams

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.

5 Responses to “Power-off Vne”

  1. Dale Long Says:

    Quick review. Thank you for your work.

  2. Jim Dulin Says:

    Tim,-You made a good point on not trying to go too fast during autorotation. Another way to increase rotor rpm in autorotation is to turn steeply. The vertical fin works the same, except backwards, in American helicopters. I think turning to increase rotor rpm would work the same in European helicopters as well. Keep in mind, there are no real aerodynamics in helicopters. They just beat themselves into the air, somehow.-Jim

  3. Alan Barnes Says:

    Of course turning would increase the rotor rpm – it doesn’t necessarily have to be a steep turn. And it makes no difference whether the rotor turns counterclockwise or clockwise (generally, north american vs european).

  4. Alan D. Resnicke Says:

    Alan et al. – Turning (clockwise) saved my bacon in a UH-1N (Bell 212) when we experienced (at night, over the desert) a main driveshaft failure, followed immediately by tail driveshaft failure and one engine overspeeding (due to shrapnel hit to fuel control). My engine tach and rotor needles were all over the dial as I entered a straight-ahead auto. The rotor rpm dropped enough that we were experiencing some pretty severe low-freq vibrations. Analysis later informed me that the rotor may have drooped as low as 87%! My stick buddy took the controls and rolled us into a bank, bringing the rotor right back to normal (as in, near overspeed that had to be controlled with collective). In memory of Capt. John “Augie” Augustine and TSgt Robert Lovell.

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