VNE of light helicopter, altitude reduction ?

[daniel-fr06]

What is the factor to determine the VNE of a light helicopter?
Why is the Robinson R22 VNE reduced with altitude ?
Thx

[Shawn Coyle]

The Vne reduces with altitude as a true airspeed effect (primarily) in nearly all helicopters. It does not follow an absolute true airspeed schedule, but it's close.

[Genghis the Engineer]

Does that mean that the deciding factor is usually advancing blade compressibility?

G

[rotorfossil]

Retreating blade has to run a higher A of A due to reduced density, therefore retreating blade stall tends to be the deciding factor. Compressibility on the advancing blade, at least initially, causes increased vibration as it flies out of track. Retreating blade stall has a sudden onset of pitch up and roll which makes the effects worse.

[heole]

Hi Sawn.

Indeed!, You a right!

I have seen various other factors for VNE limits, such as critical loads caused by blade stall, Mach tuck, , Tail rotor flapping, dynamic instability, Compressibility, Airframe vibrations or critical loads on stabilisers, excessive feed back, Control margins and sometime regulations.
Leo