Pitch Control at the Blade Root

In order to achieve equal lift on the advancing and retreating side of the rotor in spite of the unsymmetric flow velocity distribution, the common helicopter concept makes use of a varying angle of attack. This is introduced with the necessary cycle duration of one revolution by the swash plate mechanism. The idea is to actively control the blade pitch, and cancel or reduce the appearing vibrations by superposition of an adequate signal. 

As most of the characteristic perturbations occur with the blade passage frequency and its higher harmonics, the simplest approach is to employ such signals to achieve cancellation. With sensors at relevant points of the airframe, the vibratory load factors are measured and then processed by a control system. The necessary motion is produced by stationary hydraulic actuators, inducing a vertical displacement of the swash plate and thus a collective actuation of the blades. Such an actuation mechanism may also modify the inclination of the swash plate, but it still is not possible to respond to events at an individual blade. 

To improve this situation, the blade root actuation mechanism was advanced by inserting hydraulic actuators between the swash plate and blade roots in 

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