Coriolis force tends to make a rotor blade want to speed up and slow down
it's rotation around the mast. If the blade is rigidly attached to the mast
but isn't strong enough, it could develop stresses large enough to break
the blade. This is exactly what happened to Juan de l'Cierva's early rotor
systems. His solution was a lead lag hinge.
A lead lag hinge simply is a hinge which allows the blade to pivot slightly
forward and backward. Here is a diagram of a lead lag hinge:
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