The purpose of a yaw damper is to (1) prevent Dutch roll and (2) coordinate turns.
The purpose of a roll dampers is to (1) damp/remove Dutch roll, (2) provide roll damping in turbulence, and (3) provide spiral stability.
The yaw damper’s main purpose is to prevent Dutch roll. When the fin area is insufficient to provide a natural oscillatory stability, the effective fin area must be increased in some other way. This is accomplished on power-operated rudders with the yaw damper.
A yaw damper is a gyro system that is sensitive to changes in yaw, and it feeds a signal into the rudder, which applies opposite rudder to the yaw before the roll occurs, thus preventing Dutch roll. The roll damper works through the aileron controls and can be used to:
1. Supplement a yaw damper for Dutch roll control by damping out the roll once it has been established (especially common on oscillatory unstable aircraft).
2. Purely for roll damping in turbulence.
3. Control spiral stability.
A roll damper normally is associated with providing spiral stability.
Parallel yaw dampers
Parallel yaw dampers (early type) apply rudder control through the same control run as the pilot, and their activity is reflected in the rudder bar activity (moves the rudder pedals). While this provides a visual indication of the yaw dampers’ serviceability, it does increase the rudder loads experienced by the pilot. To prevent this making matters worse in the event of an engine out failure on takeoff or a crosswind landing, the damper can be switched off for takeoffs and landings. Since this damper in effect parallels the pilot’s actions, it became known as the parallel yaw damper.
Series yaw dampers
The series yaw damper is a development of the parallel yaw damper and is found commonly in modern jet aircraft. This system is attached to the rudder control circuit at the back of the aircraft, and as such, it does not move the pilot’s rudder pedals when it moves the rudder. This means that the rudder foot forces are not increased and therefore allows the series yaw damper to be used for engine-out takeoffs and landings.