HELICOPTER VIBRATION REDUCTION BY ROTOR BLADE MODAL SHAPING

The general vibration design criteria currently used in helicopter rotor blade design is placement of natural frequencies removed from integer frequencies of rotor rotation. In this paper, a theoretical approach is presented which demonstrates that design consideration of blade mode shapes can be as important as consideration of natural frequencies for vibration transmitted to the helicopter fuselage. A Modal Shaping Parameter is derived from the theoretical approach that is a measure of blade modal vibration susceptibility. A baseline blade design is then revised in accordance with minimizing the value of the Modal Shaping Parameter. A comparison of hub vibration predicted by a fully coupled aeroelastic analysis for the baseline and revised blade design shows a 70 percent reduction in the contribution of the flatwise modes to vertical hub vibration as well as a 20-30 percent reduction in blade fatigue loading. It is demonstrated that the large reductions in vibration resulted directly from desensitizing certain blade modes to aerodynamic loading as predicted by the modal shaping approach.