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Why does the VNE decrease with increasing pressure altitude?

  • A

    The groundspeed (GS) decreases.

  • B

    The rotor thrust exceeds the permissible structure limitations.

  • C

    The true airspeed (TAS) increases.

  • D

    The tail rotor effectiveness does not provide directional stability.

The "Never exceed speed", denoted as VNE, may not be exceeded under any circumstances, even in a dive.

This speed is almost always designated by a heavy red line on the aircraft’s speed indicator and if exceeded could result in poor handling qualities and even damage the helicopter.

VNE decreases with altitude for two reasons:

  • The thinner air means that the rotor blades need a higher angle of attack to get the same lift.
  • The VNE is an IAS. Since VNE as TAS increases with altitude, the VNE as IAS must decrease with altitude to avoid the following effects.

Things that impose a VNE in helicopters include:

  • Retreating Blade Stall
  • Airflow Reversal over the root of the retreating blade
  • Cyclic Range
  • Compressibility effect

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