What is a critical speed in rotor dynamics, and how is it addressed during overhaul?

• A. It refers to stall speed of the engine; addressed by throttle adjustment
• B. A rotor natural frequency near operational speeds; addressed by balancing, adjusting stiffness, redesign, or shimming to shift critical speeds away
• C. It is the speed of the cooling fan; addressed by replacing bearings
• D. It's the maximum RPM limit set by the controller

Answer: (B)

A critical speed in rotor dynamics is the rotor’s natural bending frequency that lies near the engine’s operating speeds, so when the rotor passes through that speed, resonance can occur and vibration grows. During overhaul, you address it by moving those natural frequencies away from the operating range or damping the response, using methods such as precise dynamic balancing to reduce excitation, adjusting stiffness or mass distribution (through shimming, machining, or bearing/support changes), or redesigning components to alter mode shapes and stiffness so the critical speeds no longer coincide with normal operation. This is not about stall speed, cooling fan speed, or a controller’s maximum RPM; it’s about the rotor resonating at a speed the machine actually runs at.