Refer to figure.
The "Never-exceed speed", denoted as V_NE, 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.

V_NE 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 ASI reads incorrectly.

Things that impose a V_NE in helicopters include:

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