Innovative Systems to Reduce One Per Rev Vibrations in Helicopters

Helicopter blade systems are constructed to tight tolerances, but slight manufacturing variations cause lift variations among blades. The resulting unbalanced lift forces cause undesirable 1/rev vibration in the airframe. Typical practice calls for three rotor adjustments to be made; blade pitch control rod, blade trailing edge trim tabs, and rotor head mass balancing. Pitch rod and tab adjustments are used as the primary in-flight adjustments, while hub weights are used primarily for ground balance. The tabs are used in conjunction with changes to pitch link length and balance weight locations to track and balance the rotor system, such that rotor-induced 1/rev vibration is minimized primarily to achieve a comfortable ride quality environment for pilots and crew. Automated pitch rod and mass balance actuation systems were developed, built, and tested on a full scale hover whirl stand. Test results indicate an automated adjustment approach can help to minimize helicopter 1/rev vibration with better efficiency but continued development is needed to make these actuation systems more robust and more weight efficient.