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Aviation History
1934
1934 - 1446.PDF
SUPPLEMENT TOFLIGHT 898b -58 THE AIRCRAFT ENGINEER AUGUST 30, 1934 FIG. 6.—Rudder and Elevator Control from right-hand side, showing RudderControl Mechanism, Course-change Cylinder, etc. may be seen at E, Fig. 6, into operation. The piston of this valve is connected by a link to the azimuth ring (see photo.), while the valve body is carried on the framework. Movement of the piston in one direction or the other admits compressed air to one or other of the flexible pipes F, G, which lead to a double-acting servo-motor H. The piston of the servo-motor is connected by its rod to the dummy rudder bar J, which is connected by cables to the rudder of the aircraft. It will be noticed that the servo-motor piston rod is actually- connected to a lever which is pivoted about the same centre as the dummy rudder bar, and which can move independently between the top and bottom members of the rudder bar. This may be more clearly seen in Fig. 7. When the automatic con- trol is in use, these two levers are locked together by means of the spring-loaded plunger K, which is operated by a Bowden cable from a lever (k, Fig. 2) in the pilot's cock- pit. The purpose of this arrangement is to provide a complete mechanical cut-out between the automatic gear and the control surfaces for use in case of emergency. When the aircraft is dis- turbed from its course, the valve E operates on account of the relative movement be- tween the gyroscope and the aircraft, and admits com- pressed air to the servo- motor, which, in turn, con- trols the rudder so as to correct the course of the air- craft. It will be clear, however, that unless some means were adopted to con- trol the movements of the servo-motor, the piston would travel to the extremity of its stroke and thus apply full rudder angle irrespective of the amount by which the air- craft had deviated from its course. Such a control would cause the motion of the air- craft to be continually dis- turbed and would be quite impracticable. To achieve smooth opera- tion of the control, it is neces- sary to arrange the mechan- ism so that the application of rudder angle varies propor- tionately with the angle through which the aircraft has departed from its course, and thus, as the aircraft returns to its course, the angle is progressively reduced. This requirement is satisfied by the incorporation of a "follow-up" system, by the action of which the move- ment of the servo-motor piston causes the rudder valve E to " follow " the movement of its piston and so limit the travel of the servo-motor piston to an amount determined by the actual displace- ment of the rudder valve piston. This will be more clearly understood by the following description. The supporting framework L is not rigidly attached to the baseplate, but is carried on a bearing which permits a small amount of rotation about a vertical axis. The framework is connected by a pair of levers (which may be seen at M, Fig. 9) to the rudder bar. When the rudder bar moves, the gyro framework, together with the body ol valve E, rotates slightly on its bearing in such a sense as to follow the travel of the piston and close the valve. Suppose that the aircraft has received a disturbance. FIG. 7.—Rudder and Elevator Control-from Front End, showing Centraliser Unit,and Rudder and Elevator Valves.
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