(e)
Linear Variable Differential Transducer (LVDT)

 1)  The LVDT functions as a linear follow-up transmitter for closing the loop around the EHSV. It is operated by the yaw damper actuator piston. The LVDT is excited by 26 volts ac, 400 Hz and provides a feedback voltage to the yaw damper modules that is proportional to the linear position of the actuator piston. This feedback is used to null the servo loop. The LVDT is not a line replaceable unit.

 (f)
Electrical Connectors

 1)  The electrical connection to the rudder power control module is provided by one external and two internal connectors. One internal connector is used for the solenoid valve and the other for the EHSV. These electrical pins are mated when the solenoid or EHSV is bolted in position.
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 (g)
Check and Control Valves

 1)  Each check valve is designed for cartridge type installation to allow for replacement as a single unit. The valve incorporates a poppet and a hard seat. The main control valve controls three separate hydraulic outputs. The valve is considered a closed center, four-way metering valve and is installed in a separate housing for purposes of independent maintenance and testing.

 (h)
Compensator Assembly

 1)  Each assembly consists of three compensators which act as both an accumulator and relief valve. The relief valve will crack with a differential pressure of 60 |5 psi between its inlet and outlet port. There will be no flow from inlet to outlet until 2 to 3 cubic inches of fluid has accumulated in the compensator. Internal leakage shall not exceed 2 drops per minute within a 5-minute period.

 (i)
Linkage

 1)  The linkage is designed to withstand 200 pounds chip shear force at the valve without permanent deformation or 600 pounds without failure. The linkage includes an overtravel mechanism that limits the main control valve stroke which is produced by the captains input levers.
 2.  Operation_________
 A.  Functional Description
 (1) Yaw Correction Requirements (Fig. 5)
 (a) Sideslip
 1)  An airplane is defined to be in sideslip when the airplane heading is not aligned with the flight path. The sideslip angle is the angle between the airplane heading and the flight path. Airplane sideslip is caused by unstable air conditions, aerodynamic instability, or banking without coordinating rudder movement. When the airplane sideslips, a side force tends to retard the sideways velocity. This damping force comes largely from increased drag on the faster moving wing, the fuselage, and vertical stabilizer. The vertical fin and associated yaw damping tends to correct the sideslip by "weathercocking" the airplane.
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747-400
MAINTENANCE MANUAL

Yaw Correction Requirements
Figure 5

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 2)  When a sideslip condition is present, the airflow on the forward wing goes from an outboard point on the leading edge to an inboard point on the trailing edge. The airflow on the trailing wing goes from an inboard point on the leading edge to an outboard point on the trailing edge. This results in an increase in angle of attack of the forward wing relative to that of the trailing wing. A difference in angle means a difference in lift on the two sides of the airplane, which induces a rolling moment.
 (b) Dutch Roll 1) Dutch roll is a common oscillatory condition encountered due to low drag, high speed aerodynamic design and turbulance which can be created by air mass instability. If this condition is allowed to persist, it causes considerable discomfort. If the aircraft experiences a side force causing yaw relative to the flight path, sideslip is produced. The change in direction of relative wind causes one wing to produce more lift than the other, thus inducing roll. 2) The yaw induced roll and weathercocking moment combine to induce yaw in the opposite direction of the roll. This produces an uncoordinated turn. The change in direction of relative wind causes one wing to produce more lift than the other, causing the wings to level. The airplane then banks back in the other direction and repeats the cycle with oscillating motion. The frequency of this oscillation is in the range from 0.1 to 0.25 Hz.
　
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