Rotor Vibration Reduction Using an Embedded Spanwise Absorber

This paper examines the effectiveness of an embedded spanwise Coriolis absorber in the rotor blade, in reducing in-plane vibratory hub loads. Simulations based on a light, 4-bladed, hingeless rotor helicopter similar to the BO-105 showed that in high-speed flight (140 kts) over 85% reductions in both 4/rev longitudinal and lateral hub shears could be achieved using an absorber mass 3% of the blade mass situated at 60% span, oscillating at 3/rev with an amplitude of about 0.03 ft. The phase of the 3/rev absorber motion is critically important to realizing hub vibration reduction, and the absorber tuning frequency is set to a value close to, but not exactly at 3/rev, to achieve correct phasing. The absorber reduced the 4/rev in-plane hub forces by reducing the magnitude of the 3/rev blade root drag shear while simultaneously reversing the phase of the 3/rev blade root radial shear. The reductions in vibratory hub forces observed at 140 kts high speed flight condition were largely preserved as the flight speed was reduced, without any need for retuning of the absorber.