That's some pretty impressive math. I googled around a bit trying to find a better explanation than my own, but at the end of the day the reason I care about decreasing Vne at altitude is because the POH told me to. Your explanation makes sense to me, but doesn't change the fact that every single aircraft has decreasing Vne at altitude. If you figure out why, feel free to fill me in.
Originally Posted by buznee
oh thats what you meant. Doh! Yes I agree VNE decreases with airspeed.
This is primarily due to blade stall as you go higher in altitude. Thinner air means you need more pitch for the same thrust output. Higher pitch means higher blade loading which inturn translates to getting into blade stall at lower and lower airspeeds. I think the FAA regulations also has a minimum VNE (40 KCAS) as well which at the end caps the altitude capability of the aircraft. Just in case your curious, FAR Part 29 has all the certification requirements for rotorcraft.
I appologize for the geeky math and references since I am a helicopter flight test and design engineer. Doh! =)