1.
General 1.
General
A. The Yaw Damper System provides airplane stabilization about the yaw axis. The Yaw Damper minimizes dutch roll during flight by providing rudder displacement proportional to, and opposing, the yaw rate of the airplane.
B. The Yaw Damper System is engaged by a switch on the flight control module through the AFC accessory unit (or IFSAU). When the system is engaged, a rate gyro in the coupler senses yaw rate of the airplane. The yaw rate signal is processed to generate a command to the rudder power control unit which moves the rudder. The signal is translated from electrical to mechanical in the rudder power control unit by a transfer valve. Position feedback is accomplished by an LVDT. A position indicator in the flight compartment indicates rudder movement due to yaw damper inputs. There is no feedback to the rudder pedals since a series hydraulic actuator is used in the power control unit.
C. The Yaw Damper System consists of a yaw damper engage switch and warning annunciator, a yaw damper coupler, a yaw damper engage solenoid, transfer valve and position transducer on the rudder power control unit, and a yaw damper rudder position indicator.
D. Yaw damper components are located in the flight compartment, in the electronic equipment area, and in the lower portion of the vertical stabilizer
(1) The yaw damper engage switch and warning annunciator are located on the flight control module which is mounted on the overhead panel.
(2) The Yaw Damper Coupler and AFC accessory unit (or IFSAU) are installed on the E1-3 rack in the electrical and electronic equipment compartment.
(3) The Yaw Damper engage solenoid, transfer valve, and position transducer are mounted on the rudder power control unit (PCU) which is located in the lower portion of the vertical stabilizer.
(4) The Yaw Damper rudder position indicator is installed on the center instrument panel.
F. The Yaw Damper power is supplied through two circuit breakers located on main load control center P18-1. One circuit breaker is for 115 volts ac and the other is for 28 volts dc. The B hydraulic system powers the Yaw Damper actuator portion of the rudder power control unit.
2. Yaw Damper Coupler
A. The Yaw Damper coupler provides rudder deflection commands to compensate for movement about the yaw axis.
B. The Yaw Damper coupler is a rack assembly containing solid state subassemblies for yaw computation, BITE, and power supply and a rate sensor with yaw BITE panel subassembly.
C. The front panel of the coupler contains two lights (FAIL, PASS), a GO pushbutton, a LAMP TEST pushbutton, a 2-position BITE toggle switch, and a seven-segment BITE test number display. BITE is armed with the toggle switch. The GO pushbutton is used to start each of the eight BITE tests.
3. Yaw Damper Engage Switch
A. The Yaw Damper engage switch is used to place the Yaw Damper system in the operating mode.
B. The Yaw Damper engage switch is a 2-position solenoid-held switch. The solenoid is controlled by the Yaw Damper engage logic and holds the switch in the ON position.
4. Warning Annunciator
A. The Yaw Damper warning annunciator indicates that the system is not engaged in the operating mode.
B. The warning annunciator is an amber annunciator that illuminates whenever the Yaw Damper is not engaged. The master caution lights and the flight control master caution annunciator lights, located on the glareshield, also illuminate when the warning annunciator is on.
5. Yaw Damper Rudder Position Indicator
A. The Yaw Damper rudder position indicator displays rudder movement induced by Yaw Damper coupler commands.
B. Whenever the Yaw Damper system is engaged, the Yaw Damper rudder position indicator reflects the movements of the rudder. The position transducer on the rudder power control unit senses rudder displacement from neutral and provides a signal to the indicator.
6. Rudder Position Transducers
A. A linear position transducer mounted on the rudder power control unit supplies a rudder position feedback signal to the Yaw Damper coupler. The transducer is a variable reluctance transformer which is excited by 26 volts ac. The magnitude of the output signal varies directly with the length of the input stroke and the phase of the signal changes with the direction of the stroke from the center null position.
7. Rudder Transfer and Solenoid Valves
A. The engage solenoid and the transfer valve control hydraulic flow for Yaw Damper control of the rudder power control unit.
B. The Yaw Damper engage solenoid is mounted on the rudder power control unit. It is a solenoid actuated valve which is energized when the Yaw Damper is engaged; it then completes hydraulic power to the transfer valve.
C. The transfer valve, also mounted on the rudder power control unit, converts the Yaw Damper electrical signals into hydraulic flow to move the rudder. The unit consists of a torque motor which moves a jet pipe assembly that regulates the hydraulic flow to the control valve.
8. Yaw Damper Operation
A. General
(1) The Yaw Damper may be turned on by placing the Yaw Damper switch to ON. Hydraulic power must be available and the "B" flight control switch ON. Two seconds after the yaw damper is ON the YAW DAMPER light extinguishes. The engagement is monitored by a power monitor circuit in the AFC accessory unit (or IFSAU). If ac power is lost for over 2 seconds, the YAW DAMPER light illuminates and the Yaw Damper switch returns to OFF.
(2) The yaw rate gyro senses oscillations in the yaw axis. The signal is filtered, programmed by the ADC for changes in speed, and amplified. It is then applied to the transfer valve. The transfer valve ports hydraulic pressure to move the Yaw Damper actuator. The Yaw Damper actuator output is summed with rudder pedal input to move the main actuator which controls the rudder, but there is no mechanical feedback to move the rudder pedals.
(3) As the Yaw Damper actuator moves, the LVDT supplies the feedback signal to cancel the signal from the yaw rate gyro. When the oscillation stops, the feedback signal returns the rudder to the original position. The feedback signal is also applied to the position indicator; however, during normal operation, the signals are of insufficient value to be monitored by the position indicator.
B. Operating Modes
(1) General
(a) The Yaw Damper system provides damping of movement about the airplane's yaw axis by shaping, amplifying, and coupling rate gyro and Yaw Damper actuator position signals to control the rudder through the hydraulic actuator. Yaw Damper operation is confined to synchronization mode, engaged mode interlocks and logic, and engaged mode operation.
(2) Synchronization Mode
(a) Prior to yaw axis engagement, the Yaw Damper coupler is in the synchronization mode. The purpose of synchronization is to provide a null output to the electrohydraulic transfer valve to prevent Yaw Damper coupler engage transients.
(b) Synchronization is accomplished by routing the demodulated, parameter-controlled signal from summing point 1 through switch S6, summing point 2 and integrator, and back to summing point
(1) Output from summing point 1 results in temporary outputfrom the valve amplifier. The valve amplifier output is reduced to a null as the integrator output increases and cancels the signal at summing point Yaw Damper actuator remains centered which nulls the 1. Prior to Yaw Damper engagement, the position feedback to the Yaw Damper coupler.
(3) Engage Interlocks and Logic
(a) Prior to engaging the Yaw Damper system, the Yaw Damper disengage warning light is illuminated. Before the Yaw Damper engage switch will remain engaged, the monitor must be good from the 115-volt ac power supply and the flight control B switch must be on.
(b) The Yaw Damper engage switch remains in the ON position after holding the switch to ON for at least 0.1 second. This allows voltage through the holding coil to ground as provided by the time delay gate S1.
(c) The Yaw Damper engage relay is energized by a ground through time delay gate switch S2 after a 2-second delay. Two sets of contacts apply power to the interlock circuit and to the rudder power control unit actuator solenoid. A third set of contacts open to extinguish the warning light.
(d) Transferring between 115-volt ac generators causes the monitor to momentarily be invalid. With a 2-second delay timer in the engage switch circuit, the Yaw Damper engage switch remains engaged. The engage relay is de-energized until 2 seconds after the monitor is valid. This delay allows the Yaw Damper coupler to synchronize with the new reference voltage. The Yaw Damper light will be on during this time.
(4) Engaged Mode Operation
(a) Rate gyro signals are processed in the rate gyro filter. The rate gyro filter consists of a demodulator, amplifier, and washout network. Filtering accomplished in these circuits eliminates steady-state signals and unwanted high frequency signals. Consequently, the Yaw Damper coupler does not oppose normal turn maneuvers and does not respond to airplane vibration and bending. Frequency response is tailored to minimize the airplane dutch roll. The rate gyro filtered output is routed to summing point 1 through a step gain in the ADC. Rate gyro signal gain is varied by this step gain to compensate for changes in airplane dynamics as a function of airspeed.
(b) When the Yaw Damper coupler is engaged, processed rate gyro signals at summing point 1 produce a valve amplifier output. This output is applied to the series Yaw Damper actuator transfer valve to command movement. Movement of the Yaw Damper series actuator results in rudder movement and an output from the position transducer.
(c) Position transducer output is demodulated and applied to summing point 1 to cancel the processed rate gyro signal when the corresponding change in rudder position is appropriate for the rate of yaw attitude change. Position transducer output is also applied to control the integrator thru switch S7 and summing point 2. The integrator lag is large enough so that the integrator output, which is applied to summing point 1, does not attenuate Yaw Damper action in the dutch roll frequency spectrum.