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subsonic jets. This oscillatory condition which is called “dutch roll”due to low drag and high speed aerodynamic design (wing sweep back angle is about 35). Figure 34-1 illustrates the “dutch roll”of an airplane.
The airplane moving to the right of the flight path is caused by gust wind, the tail moves right, this condition is called “right sideslip ”.when the airplane sideslip right, the right wing increases lift and drag, the airplane rolls left and yaws right, bank moves to the left of the flight path. Once the “left sideslip ”occurs, the tail moves left of the flight path, the left wing increases lift and drag, the airplane rolls right and yaws left and then bank moves to the right of the flight path. The airplane is sometimes sideslipping left and sometimes sideslipping right, which results in the dutch roll. Dutch roll is sufficiently objectional without damping so that most subsonic aircraft are speed and altitude limited if their yaw dampers are inoperative.

Fig. 34-1 dutch roll

[2] The yaw damper minimizes dutch roll by controlling rudder displacement to oppose the yaw rate of the airplane. An electromechanical rate gyro in the yaw damper coupler sense yaw rate of the airplane and generates a command to oppose the yaw. The yaw damper actuator components of the rudder power control unit moves the rudder  3 degrees in response to this command signal.A position signal from the rudder power control unit provides feedback to the yaw damper indicator. The yaw damper indicator shows rudder movement due to yaw damper inputs. The rudder pedals do not move when the yaw damper moves the rudder because the yaw damper actuator is a series hydraulic actuator.

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 34-2   yaw damper

[3] The yaw damper engage switch sends a signal to the integrated flight system accessory unit (IFSAU) to engage the yaw damper. The IFSAU contains relays and time-delay relays for engaging the system. A signal from the energized relays activates the yaw damper actuator. Other signals inform the yaw damper coupler that the system is engaged and turn off the yaw damper warning annunciator on the overhead panel. The yaw damper coupler contains BITE circuitry for troubleshooting and system verification. Switches and indicators on the front of the unit operate the BITE system. During yaw damper BITE, the autopilot light on the autoflight status annunciator (ASA) illuminates red. Some BITE tests move the rudder and check the complete system. Other tests check the internal operation of the coupler.

[4] The yaw damper coupler is a 3/8 ATR rack assembly containing solid state subassemblies for yaw computation, BITE, and rate sensing, the coupler weighs approximately 10.2 pounds (4.5kg). The coupler detects yaw oscillations (dutch roll) and generates rudder commands to cancel the oscillation. Because the yaw damper coupler contains static-sensitive circuitry and an internal rate gyro, certain handling precautions are necessary. Be careful not to touch the connector pins on the back when removing and installing the coupler. Wait 3 minutes after removing power from the coupler when removing the yaw damper coupler from the airplane. This allows the gyro to “spin down”, avoiding damage.
　
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