FAA Helicopter Flying Handbook Practice Test

Vertical vibration describes motion up and down along the vertical axis of the rotor system. This is the term used when the displacement is in the vertical direction, as opposed to lateral (side-to-side), torsional (twisting about the axis), or radial (inward or outward along the blade’s radius) vibrations. In rotor dynamics, identifying the direction of vibration helps pinpoint the likely sources, such as imbalance (which often shows up as vertical motion) or vertical stiffness/clearance issues, and it distinguishes this mode from other types of movement.