$17.$ True or False: The natural frequency of a rotor is always constant, regardless of operating conditions.

$18.$ True or False: Counterweights in an engine's design are only there to make the engine heavier and more stable.

$19.$ True or False: The operational speed of a rotor should ideally be set at or near the critical speed for optimal performance.

$20.$ True or False: Every rotating component in mechanical systems must be dynamically balanced to ensure smooth operation.