Helicopter Vibration Reduction through Cyclic Variations in Rotor Blade Root Stiffness

This study analytically examines the influence of cyclic variations in flap-, lag-, and torsion-stiffness of the blade root region (at harmonics of the rotational speed), for reduction of vibratory hub loads of a helicopter in forward flight. The results indicate that considerable reduction in hub vibrations is possible using small-to-moderate amplitude cyclic variations in stiffness (no greater than 15% of the baseline stiffness value). Torsion stiffness variations produced moderate reductions in vertical hub force, lag stiffness variations produced substantial reductions in all hub forces and the hub torque, and flap stiffness variations produced very significant reductions in all hub forces and the in-plane hub moments. Cyclic stiffness variations of larger amplitude would be required for comparable reductions in vibration at higher advance ratios, while smaller amplitudes are required at lower advance ratios. For amplitudes of cyclic stiffness variation that are too large, the hub vibrations were actually seen to increase. Vibration reductions are due to a decrease in the inertial contribution to the hub loads, a change in relative phase of various contributors, or a combination of the above effects.