As a candidate at Ft. Wolters, I took a Bell OH-13 to 12,000 feet, doors off in the summer, and was so scared as the aircraft shrank that I stopped climbing purely from fright, gently autorotated back down. I felt as if I was on a tight-rope, and could flip over if anything upset me! The open door out my left shoulder was awesome.
Since then, no problem, been to 20000 several times.