Why does Vy decrease with altitude in the H125/AS350B3 helicopter
Vy (best rate of climb, and autorotation speed) in the H125 / AS350B3/e is 65KIAS at sea level, decreasing by 1kt for every 1,000ft per the flight manual. All similar types of helicopters that I am aware of, only specify a fixed value for Vy.
Why is that?
I am finding plenty of explanations why this is the case in fixed wing aircraft, but they don't seem to make sense (to me) when talking about a helicopter.