27-21-00
ALL ú ú 01 Page 6 ú Jun 15/84
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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757
MAINTENANCE MANUAL
(2)
The actuator uses an acme screw design for irreversibility and is powered by a self-contained dc motor that operates on 28 volts dc. Limit switches on the actuator control its stroke. An RVDT inside the actuator drives the flight deck trim position indicator on panel P8. The trim indicator shows units of trim since rudder trim in degrees is dependent on ratio changer position. The maximum trim authority is | 21.9 degrees (rudder travel rate of 1.1 degrees per second) and minimum trim authority is | 2.5 degrees (rudder travel rate of 0.13 degrees/sec). Actuator stroke is |1.37 inches (+/- 34.8 mm) and needs about 16.7 seconds for full travel in one direction.
G.
Rudder Thermal Compensating Linkage (Fig. 8 and 13)
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(1)
The rudder thermal compensating linkage consists of a upper and lower compensating rod, multiplier crank, input crank and associated hardware. The input crank attaches to the yaw damper summing lever extends through the fin rear spar to the upper compensating rod. The upper compensating rod (adjustable rod) is attached to the multiplier crank which connects both to structure and to the lower compensating rod which is attached to a support bracket attached to the fin rear spar at the lower end. The lower compensating rod is nonadjustable.
(2)
During climb, structure cools faster than rudder control linkage. This causes rudder to shift left due to different expansion rates of the structure and rudder control linkage, requiring right rudder trim to compensate. As temperatures equalize during cruise, rudder trim is gradually removed, leaving only the normal cruise trim requirements. Opposite conditions occur during descent, as structure warms faster than rudder control linkage. The rudder thermal compensating linkage eliminates the need for manual rudder trim in such situations. The lower compensating rod expands/contracts at the same rate as the rudder control linkage. Therfore, when the structure is expanding/contracting faster than the rudder linkage, the compensating linkage repositions the input crank, providing input to the yaw damper summing lever to counteract rudder deflection due to different expansion/contraction rates between structure and rudder control linkage.
H.
Autopilot Rollout Guidance Servos
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(1)
The three autopilot rollout guidance servos, located directly below the feel, centering and trim mechanism, are driven by the flight control computers. These servos are normally disengaged and are used for autoland rollout guidance only (Ref 22-13-00).
I.
Rudder Ratio Changer Mechanism (Fig. 5)
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(1) The rudder ratio changer mechanism is mounted in the vertical fin directly above the aft quadrant feel, centering and trim mechanism. The mechanism limits the rudder travel as a function of airspeed; as airspeed increases, available rudder travel decreases. The ratio changer output is driven by an electrohydraulic actuator and controlled by signals from one of two rudder ratio changer modules (RRCM).
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27-21-00
ALL ú ú 01 Page 7 ú Jan 28/02
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
SECONDARY CONTROL PATH ROD (RUDDER RATIO CHANGER POGO) PRIMARY CONTROL PATH ROD
324BL RIG PIN R-7
SEE A
MOUNTING SUPPORT BRACKET (2) SEE B
RIG PIN R-6 RIG PIN R-5 A FWD
ACTUATOR CHANGER RATIO RUDDER (2) CRANK BEARING SUPPORT
ADJUSTMENT LVDT NUT
AXIS ROTATATIONAL TORQUE TUBE (ADJUSTABLE) AFT QUADRANT INPUT FROM BELL CRANK (2) (3) SPRINGS RETURN INTERCONNECT ROD LVDT SHEAR PIN FWD
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