Low frequency vibrations usually manifest as a ____ vibration per revolution and emanate from the main rotor system.

• A. Zero
• B. One
• C. Two
• D. Three

Answer: (B)

Low-frequency rotor vibrations show up as a single vibration each time the rotor makes one full turn. This is because the main rotor system imposes a recurring load and structural response once per revolution, driven by unbalance, drive-train dynamics, and hub coning as the rotor spins. In contrast, higher-frequency vibrations, like those at blade-pass frequencies (2 per revolution, 3 per revolution, etc.), come from phenomena tied to the number of blades and blade passing events, and they occur more often within each turn. So the typical low-frequency signature from the main rotor system is one vibration per revolution.