A servo’s job is to convert the angular movement of a servo arm to the linear movement of a control surface. This is done by attaching linkages, called control rods to the servo arm and the associated control surface. When the servo head rotates, it pushes the control rod back and forth. The rod is linked to a control surface, and can move it up or down as the servo rotates.
Three wires control a RC helicopter servos: two to provide the DC electricity that the motor needs, and one that sends the signal, controlling the servo. The signal wire works by sending the servo a series of pulses, which are interpreted by it’s internal circuitry. By varying the timing of each pulse, the servo knows exactly which position to move to.