Vibration absorber for helicopter

A helicopter including a rotor system having a plurality of rotor blades arranged for rotation about a generally vertical axis and a translational vibration absorber arranged for rotation with the rotor for rotation about the axis of rotation, the absorber comprising a support attached to the rotor system, a mass portion encircling the axis of rotation, and a plurality of resilient arms extending in a spaced-apart sprial pattern between the support and the mass with a free end of each arm separately attached to said mass to carry said mass so that, during operation, the mass is rotated with the rotor system in a plane substantially perpendicular to the axis of rotation and is capable of substantially equal resilient deflection in any direction within its plane or rotation, the absorber being de-tuned torsionally at a selected rotor design speed and being tuned so as to have, at the selected speed, a translational or in-plane natural frequency corresponding to the frequencies at which vibration absorption is required, thereby reducing in-plane rotor induced vibrations.