Handling Qualities and First Impressions

[Chris CDN]

Why does the aircraft care about TAS? Your VNE is derived from IAS, it is usually an airspeed limit set for structural limitations directly related to load factor and ultimately dynamic pressure. IAS has a relationship directly proportional to dynamic pressure (keeping density as a constant, standard sea level density). Here is the relationship used to derive your true airspeed and indicated airspeed from your dynamic pressure which is derived off of the total pressure port and static pressure port in the pitot static system. For true airspeed you will need a static temp sensor as well to derive your air density.

Pdynamic = PTotal - Pstatic =.5*airdensitySSL*IAS^2 = .5*airdensity*TAS^2

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.

[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.

Reference FARS
http://ecfr.gpoaccess.gov/cgi/t/text...1.3.14.7.360.3

I appologize for the geeky math and references since I am a helicopter flight test and design engineer. Doh! =)