A Brief Survey of Characteristic Vibration Sources in Helicopters

Extract

Rotor Unbalance Rotor unbalance is a once per rev impulse This is a case of clear separation of impulse and response Unbalance is a rotating force Its origin can be mass unbalance of blades or geometric dissymmetry in their distribution which can be caused by (a) unequal pitch, (b) errors in the geometry of articulation, (c) unequal damper settings Unequal pitch can occur in spite of equal pitch settmg if the elastic twists are unequal due to unequal pitching moments In practice these are mainly aerodynamic moments which can be corrected with tabs or avoided by superprecision in blade making

Response depends on rotor mountings Rotor r p m must be removed from the natural frequency of the hub on its mountings Subcntical and supercritical mountings are practical Supercritical mountings due to the isolation of the rotor system are mostly introduced for other reasons (discussed below)