REDUCTION OF HELICOPTER VIBRATION THROUGH CONTROL OF HUB-IMPEDANCE.

During the initial flights of a 206L Bell Helicopter with an experimental four-bladed, soft-in-plane rotor system, a high four per-rev (4P) vibration was encountered in transition. Incorporation of bifilar pendulum absorbers significantly reduced the vibration. The absorbers were then removed and the pylon was mounted on four elastometric supports that provided a favorable impedance at the hub. This configuration reduced the vertical hub-shear by 75 percent and the cabin vibration was as low as obtained with the bifilar absorbers. In support of these tests, a simple two-degree-of-freedom mathematical model was developed that coupled the rotating and nonrotating systems. A semiempirical formula was derived and employed for representing the wake-induced aerodynamic forces in the transitional flight regime. The model indicates that the 4P hub impedance is the single most dominating factor governing the value of 4P vertical hub shear for a given rotor at a specified thrust level. Flight-test results for eight different pylon configurations reasonably confirm this prediction.