(b)
The modal suppression accelerometers are composed of a detection circuit that consists of a cantilever surrounded on both sides by metal plates. The cantilever moves with the changing lateral velocity of the aircraft. An output signal is produced by comparing the difference of the spacial gaps between the metal plates and the cantilever. The maximum response of the YDM for the accelerometer input is | 1 degree of rudder movement.
H. Yaw Damper System and Block Diagram (Fig. 2 and 3)
(1) System Inputs
(a) The YDS uses the following inputs to determine existing conditions, rudder commands, and system operation. 1) The left, right, and center IRUs provide roll attitude,
roll rate, yaw rate, lateral acceleration, and ground speed. The left and right ADCs provide true airspeed (TAS), impact pressure (IAS), angle of attack (AOA), and overspeed discrete.
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MAINTENANCE MANUAL
Yaw Damper System
Figure 2
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MAINTENANCE MANUAL
Yaw Damper Block Diagram
Figure 3
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747-400 MAINTENANCE MANUAL
2) The modal suppression accelerometers provide lateral acceleration data to their corresponding yaw damper module.
3) The numbers 3 and 2 hydraulic pressure switches sense normal hydraulic pressure to the upper and lower RPCMs respectively. The air/ground relay provides an analog discrete signal indicating the airplane is airborne or on the ground.
4) Airplane type code is provided by program pins selectively grounded through airplane wiring. The yaw damper channel (upper or lower) code is also provided by program pins selectively grounded through airplane wiring.
(2)
Yaw Damper Modules
(a)
The yaw damper modules contain ARINC 429 receivers and transmitters (data bus input and output) plus software signal check and selection programmed routines. As multi-input parameters are received and transmitted, a median value selector provides single valid input parameters for microprocessor calculations.
(b)
The yaw damper modules provide mode control for various operational conditions encountered over the full flight regime. Rudder commands computed by the modules assist in turn coordination and damping of undesirable yaw. Internal circuits in the modules conduct tests and exercise monitoring circuits. These provide failure indications and YDS disengagement if a fault is detected.
(c)
The yaw damper modules perform signal conditioning for analog rudder command outputs to the RPCMs. Servo Linear Variable Differential Transducer (LVDT) feedback monitored by the modules determines rudder position. The two yaw damper modules are connected by an interlock to determine if both modules are installed. If one module is not installed, the interlock is open circuited. This causes the installed module to illuminate an INOP annunciator (for the absent module) on the passenger oxygen and yaw damper panel.
(3)
Rudder Power Control Modules
(a) Two rudder power control modules are connected in series to the rudder actuating linkage. The rudder power control modules provide the mechanical output to the rudder power control actuator input linkage. The modules receive electrical commands from the yaw damper modules and convert these signals to mechanical commands via an electrohydraulic servo valve (EHSV). The EHSV responds to electrical commands and delivers hydraulic flow proportional to the input signals. Hydraulic fluid then drives the yaw damper actuator piston. The actuator piston drives the LVDT, which provides position feedback to the yaw damper module.
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