Vibration Analysis of Rotor Blades with Pendulum Absorbers

Abstract

A comprehensive vibration analysis of rotor blades with spherical pendulum absorbers is presented. These absorbers are mounted on the blade to attenuate the loads that pass through the rotor shaft at a frequency that is an integer multiple of the blade passage frequency. The spherical pendulum absorber is considered here because such an absorber may be more versatile in controlling the out-of-plane, in-plane, and pitching vibratory loads. A transmission matrix formulation is given to determine the natural vibrational characteristics and a direct transmission matrix method is given to determine the forced response of the rotor blades with pendulum absorbers. The pendulum absorber alters significantly the initial blade frequencies that are adjacent to the uncoupled frequencies of the pendulum. In the case of spherical pendulum, two natural modes of vibration are injected between the displaced initial modes of the blade. The pendulum hinge offsets significantly affect the torsional frequencies of the blade. The effect of root collective pitch, in the normal range, on the natural frequencies is small. With the use of a spherical pendulum, it is possible to achieve significant reductions in all five root forces of the blade simultaneously. If the pendulum is tuned such that its uncoupled natural frequency coincides with the excitation frequency, the pendulum introduces constrained deflections at its point of connection with the blade; this may not help in attenuating the root forces of the blade but may actually amplify them.