"The up-down (flapping) motion of a propeller blade has a resonance exactly at the rpm of the blade." so has static imbalance.
They certainly do not operate helicopter rotors, or any other rotating mass for that matter, anywhere near its resonant (critical) frequency (speed), it would shake itself to pieces! Rotors operate either well below their critical speed, a rigid rotor, or above their first critical speed, flexible rotor. If the rotor is flexible, as was the case of a large steam turbine I used to operate, one has to accelerate it rapidly through its first critical or it would jump off its mountings!