(4)
Solid state (microelectronic) interlock, and operating circuits.
(5)
Plug-in modules on hinged cards for ease of maintenance.
C. The autopilot and yaw damper attains optimum smoothness and stability of control by the use of error information based not only on the magnitude of the airplane and its control surface displacements from reference positions and angles, but also on the rate of change of these displacements. Optimum sensitivity and accuracy in maintaining system integrity are attained by integrating the displacement errors with respect to time and by making control surface movement a function of indicated airspeed. The following is a brief explanation of these principles of operation:
(1) Physically, when a sudden change in airplane or control surface position or angle occurs, a large rate of change of position exists before there is time for a significant displacement actually to take place. By applying control forces based on this rate of change instead of waiting for a significant displacement to build up, the initial control forces are greatest at the inception of the error and consequently are most effective when needed. Thus, considerable
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
corrective action is accomplished before the displacement becomes large and the possibilities of large deviations from reference conditions are greatly reduced. The rate at which these displacement corrections take place is damped by input rate information to assure return to reference conditions with practically no overshoot. When the displacement error is no longer changing, the rate signals fall to zero and the error existing at that instance is acted upon by control forces that are proportional only to the error magnitude. The combination of displacement control and rate control provides the desired smoothness and stability of operation.
(2)
Integration in the autopilot provides an extremely accurate means of automatically retaining reference flight conditions when sustained or recurring displacement from the references caused by wind or loading changes exist. A persistent displacement error, from a flight reference, may exist at such a low level that it will not actuate the associated servo channel to cause corrective action. Errors of this nature will produce increasing errors in airplane displacement as they are permitted to remain. Small signal errors are integrated against time to build up small displacement errors to usable values so they will correct the error through the associated servo channel. The integrated signal remains at the value required to overcome the displacement error, compensating for required changes in the original flight references.
(3)
The response of the airplane to control surface movement is aerodynamically a function of dynamic air pressure. As the dynamic air pressure increases there is a decrease in the amount of control surface movement required to produce a given change or rate of change in airplane attitude. Therefore, to maintain accurate control of the airplane at all airspeeds, continuous adjustment of the autopilot channel control surface gains is provided.
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D. Individual components of the autopilot and yaw damper system are shown in Fig. 1. The following are the associated airplane electronic systems upon which successful autopilot operation depends (Ref Chapter 34).
(1)
Attitude reference (INS or vertical gyro)
(2)
Compass (directional gyros)
(3)
Air data computer (ADC)
(4)
Low range radio altimeter
(5)
VHF NAV and glide slope receivers
(6)
Flight instruments
(7)
OMEGA Navigation System
NOTE: Refer to Chapter 27 for airplane rudder, aileron and elevator power control units.
E. Yaw axis control is provided by the yaw damper. The yaw damper minimizes dutch roll during manually and automatically controlled flight by providing rudder displacement proportional to, and opposing, the yaw rate of the airplane. Since a series hydraulic actuator tie-in is used, there is no damper control feed back to the rudder pedals. This allows the pilot to maneuver the airplane in a normal manner without having an opposing force from the yaw damper.
F. Pitch axis control is provided by the pitch control channel and associated components. The operating modes of the pitch axis are the manual mode, including pitch attitude control and pitch commands through control wheel steering; altitude hold; glide slope control during ILS, with automatic capture; automatic pitch (stabilizer) trim; and turbulence control. The pitch control channel operates in conjunction with the airplane vertical gyro, air data computer, glide slope receiver systems and the radio altimeter. The vertical gyro system supplies pitch angle signals, the air data computer supplies altitude error and altitude rate signals and the glide slope receiver supplies glide slope deviation signals, and the radio altimeter provides signals for glide slope gain programming. The output of the pitch control channel is applied to the transfer valve of the elevator power control unit to deflect the elevators. The pitch channel modes are selected by the mode selectors on the autopilot control panel.
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