Aviation Glossary - Rotor Shake

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Rotor Shake

The vibration of the rotor system felt in the controls and in the fuselage of a rotorcraft. The predominant vibrations are either “1 per rev” (at the same frequency as the rotor is turning) or 2-per rev (twice the freq of the rotor) in a two bladed rotor system. 1-per rev rotor shake can be the result of a mass and/or aerodynamic imbalance of the rotor. 1-per rev imbalance rotor shake can be thought of as the either the Mass Center or the Aerodynamic Center not being exactly concentric with the spinning axis of rotation of the rotor. Imperfect rotor blade tracking is a contributor to aerodynamic imbalance leading to 1-per rev rotor vibrations. 2-per rev shake can be the result of a teeter height that does not match the vertical CG of the coned rotor, mechanical “looseness” or “slop” in the rotor teeter joint, or (unavoidably) from the forward airspeed of a semi-rigid 2-blade rotor

source: Glossary of Gyroplane Terms

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