Caging.
John (your #7913),
I didn't think of you people who Go Down to the Sea in Ships (not this child!), I'm afraid. (My hat is off to you!)
We were taught always to cage our gyros whenever we were going to do aerobatics (or, in our case, dives) and always when shutting down. This was to avoid damage to the rotor bearings, I was told. Knowing nothing of helicopter operation, I suppose that was the same problem.
When did the caging knobs disappear, and what did the aviators do then, poor things ?
Danny.