A servoactuator unit to operate flight-control mechanisms, in particular for control of the position of the oscillating plate which adjusts the cyclic pitch and collective pitch of helicopter rotors, in which two pairs of hydraulic actuators are used, connected together in parallel, arranged with their axes parallel and close together like the diagonals of a quadrilateral, their rods connected together at their end and acting on the said oscillating plate and the cylinders also being connected together at the back. Each pair of actuators is powered, through a servovalve, by an independent hydraulic circuit so as to enable operation even in the event of one of the pairs breaking down, with symmetrical stress being placed on the pair of actuators during operation.
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