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B.  Inputs to the feel computer are hydraulic system A and hydraulic system B pressure at 3000 psi, pitot pressure, static pressure and a stabilizer position mechanical input. Pitot pressures are directed to each system from individual pitot tubes on the vertical stabilizer. The outputs from the feel computer are two controlled hydraulic pressures, 200 to 2100 psi, to the feel control unit. The internal feel differential pressure switch illuminates an amber light on the overhead panel when an undesirable pressure difference exists between the two feel computer output pressures.
C.  The two output pressures from the computer are controlled by pitot pressure and stabilizer position. With the feel system pressurized, the force balance valve is hydraulically displaced to limit system pressure. Pitot pressure acting upon the Q-diaphragm, meters the flow by displacing the force balance valve until it contacts the droop spring and stabilizer actuated cam. As pitot pressure increases, the Q-diaphragm will deflect the droop spring and move the force balance valve to increase pressure to the feel control unit. Changes in stabilizer position will rotate the stabilizer actuated cam and reposition the droop spring. A relief valve provides protection against excessive pressure in the feel system. The valve will relieve at 40% above computer demand pressure.
500 
Aug 15/72  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  27-31-0 Page 7 

G11730G11063
 Elevator Feel System  551 
27-31-0  Figure 4 (Sheet 1)  May 01/75 
Page 8 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

504  Elevator Feel System 
Aug 15/73  Figure 4 (Sheet 2)  27-31-0 
Page 8A/8B 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

9.  Elevator Feel and Centering Unit
A.  The elevator feel and centering unit (Fig. 4) provides artificial feel to the pilot and centering for the elevator control system. The feel and centering unit is mounted to structure aft of the stabilizer rear spar and pivots about a lateral axis. The unit consists of a centering cam and roller assembly, centering linkage and an externally mounted dual feel actuator. The feel and centering unit receives two hydraulic inputs from the feel computer. The inputs act upon pistons within the free-floating dual feel actuator. A rod from one piston attaches to the feel and centering unit body. A rod from the opposing piston attaches by means of centering linkage to the feel control centering cam and roller assembly. The feel and centering unit neutral position changes as stabilizer attitude varies. Variations in stabilizer attitude are transmitted through the neutral shift mechanism to rotate the feel and centering unit about its lateral axis. A mach trim actuator is mounted on top of the feel and centering unit. The actuator output rod is linked to the horizontal stabilizer through the neutral shift mechanism. The mach trim actuator and rod function as part of the neutral shift linkage except when the mach trim system is operating. Mach trim command signals extend or retract the actuator output rod. This action provides a mechanical input to rotate the feel and centering unit and thereby reposition the elevators. If the autopilot system is engaged, a neutral shift sensor driven by movement of the feel and centering unit provides electrical signals to the autopilot system which causes the elevators to be repositioned.
B.  Actuation of the control columns in either direction will rotate the centering cam and force the roller up the face of the cam. A change in stabilizer position with the control columns held fixed, will rotate the feel and centering unit body and also force the roller up the face of the cam. Either action places a load on the control system and artificially provides feel at the control columns. The resulting load is removed by rotating the control columns forward or aft to run the roller back into the cam detent. A change in stabilizer position with the control columns free to move, will allow the roller to remain in the cam detent and cause the control columns to move to the neutral position.
10.  Elevator Power Control Unit
A.  Two elevator power control units (Fig. 5) provide actuation of the elevators in response to manual inputs from the control columns. The control units are mounted vertically in the empennage above the aft elevator control quadrant torque tube (Fig. 1). The two control units operate independently from separate hydraulic systems, the left unit from hydraulic system A pressure and the right unit from hydraulic system B pressure. The lower mounting terminals of each unit are attached to structure. The upper mounting terminals attach to lugs on the output torque tube, which is directly linked by pushrods to each elevator. Failure of either hydraulic system will render one control unit inoperable. The remaining unit will then drive both elevators through the full range of travel.
　
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