They are held rigid primarily by centripetal force and if you let the rotor RPM drop too much, the blades may experience structural failure due to excessive flapping.
Once upon a time us pongos were told all about "blade sailing", as an awful warning never to go under a rotating rotor unless/until signalled by the pilot who, presumably, would do so only when the rotor speed was enough to avoid decapitating the entire load? I always understood that he would keep a little upward load on it as well to make sure it would not flap downwards.
And even then we kept our heads down as we ran from the 2.00 o'clock position to the door, for no rational purpose. I've noticed that people still do that.
Back to the thread..........