AC - that makes perfect sense to me. That explains why I get out of puff when I'm exerting myself at high altitude; it's all that thick 'treacley' air slowing me down.
Maybe that's also why Ethiopian long-distance runners are so good? Suddenly they're down from the mountains and in the thin air: less resistance = faster speeds.
And, as if even more evidence was needed, that explains why asteroids burn up as they hit the thick air next to outer space. You hardly ever see an asteroid burning up in the lower atmosphere - there's no resistance as the air's too thin.
13,000' in an EC120 for me, no dramas and the 60kt tailwind was useful. The one that made me decide enough was enough was 10,000 in an R44 - although it was still climbing nicely and had power in reserve, it just didn't feel comfortable...
