1-Per-Rev:- Usually referring to rotor vibration cycles that occur once for every complete revolution of the rotor. 1-Per Rev rotor vibrations are caused by mass or aerodynamic imbalance of the rotor – the Center of Mass and aerodynamic Centers not concentric to the spinning axis of the rotor.. Rotor vibrations may be felt in either he cyclic control stick or in the airframe. 1 Per Rev vibrations can be verified if they continue in a zero airspeed vertical flat decent – no forward airspeed to create confusing 2-Per Rev vibrations. See also “Center of Mass,” “Aerodynamic Center,” “Rotor Shake,” “Tracking,” and “2-Per Rev.”

2-Per Rev:- Usually referring to rotor vibration cycles that occur twice for every complete revolution for a 2-bladed rotor system. Some 2-Per Rev rotor vibrations are inevitable to some extent in semi-rigid 2-blade rotor systems at forward airspeeds due to varying total rotor drag as the blades move from lateral alignment to longitudinal alignment. Other 2-Per Rev rotor vibrations can be generated by a mis-match of the teeter height and the coned CG of the rotor, and by looseness or “slop” in the teeter pivot and support. 2-Per Rev rotor vibrations or shake is usually felt more in the cyclic control stick rather than in airframe vibrations. 2- Per Rev vibrations can be verified if they decrease or disappear at very low or zero forward airspeeds. See also “Coning Angle,” “Rotor Shake,” and “1-Per Rev.”

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