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Aviation History
1934
1934 - 1447.PDF
.AUGUST 30, 1934 S9 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT HqSc FIG. 8 FIG. 8.—Diagram showing Operation of Relay Valve. and in consequence has departed from its course. The gyroscope and the gimbal rings will remain "fixed," and relative movement will occur between the gyroscope and the supporting framework. This will cause the rudder valve to operate and, in consequence, compressed air will be admitted to one side of the rudder servo-motor. The servo-motor piston will therefore operate the rudder bar to apply a correcting movement to the rudder. As the rudder bar moves, however, the ratio levers M,M, cause the supporting framework of the gyroscope, together with the casing of the rudder valve, to rotate slightly in the same direction as the apparent displacement of the rudder valve piston. The rudder valve is thus closed, and, in consequence, no further movement of the servo-motor piston and no further application of rudder angle takes place. By adjusting the effective lengths of the levers M,M, it is possible to regulate the magnitude of the rudder correc- tion applied for a given departure of the aircraft from its course. As the aircraft returns to its course, the rudder valve causes the servo-motor to reduce progressively the applied correction until the aircraft is again on its true course, when the rudder will be in the neutral position. In prac- tice the aircraft usually '' overshoots'' the true course slightly, to which it returns by a diminishing oscillation. The accuracy with which the Automatic Pilot maintains the aircraft on its course is in no way effected by any inherent tendency of the aircraft to yaw off its course, since the mean position of the rudder is automatically kept at the requisite position for straight flight in much the same manner as a pilot frequently has to keep the rudder applied if he wishes to fly straight. Change of Course There are two possible methods of altering course on a gyroscopically-controlled aircraft. The first is to change the relative positions of the gyroscope and the aircraft. While this method has the advantage of accurately measuring the amount of the turn, it possesses several serious practical disadvantages. The second method has been adopted in the Automatic Pilot, and consists in pre- cessing the gyroscope itself, and since the aircraft is con- trolled by the position of the gyroscope, it automatically changes course as the gyroscope precesses. From the brief description of gyroscope theory earlier in this paper, it will be recalled that in order to precess a gyroscope about the vertical axis, a torque must be applied about a horizontal axis at right angles to the axis of the rotor. This torque is actually applied to the inner ring of the gyroscope by means of a small double-acting air cylinder, which may be seen at N, Fig. 6, just beneath the rudder valve. The piston of the air cylinder is con- nected to the inner gimbal ring by means of a link and the perforated arm O. When air is admitted to one side or the other of the course-change cylinder, the thrust of the piston applies a torque about the horizontal gimbal axis and so causes the gyroscope to precess in azimuth. The admission of air to the course-change cylinder is controlled by the "Course-Change Cock" (g. Fig. 2) which is under the control of the pilot. Balancing for Azimuth Accuracy The accuracy with which the Automatic Pilot keeps the aircraft on the predetermined course is adjustable by means of the rotor nuts shown at D, Fig. 5. By means of these nuts the position of the centre of gravity of the gyro rotor and inner gimbal ring may be adjusted so that a negligible torque is exerted about the horizontal pivots of the inner gimbal ring. It is possible to adjust these nuts so that the aircraft will not deviate from its course by more than four or five degrees per hour. Pitch Control As will be realised from the earlier description of gyro- scopic theory, a frictional torque about the horizontal axis of the inner gimbal ring will cause a precession of the gyroscope in azimuth. In other words, any friction about the horizontal pivots will be detrimental to the accuracy with which the Automatic Pilot can maintain the aircraft on its course. For this reason it is undesirable to connect the piston of the elevator valve direct to the pitch or inner gimbal ring, as, no matter how carefully the valve might be made, the friction between the piston and the casing would result in poor course-keeping qualities. In the case of the rudder valve this friction is not of serious consequence, since friction in the rudder valve is VALVE FIG. 9.—Rudder and Elevator Control from left-hand side, showing Connection between Pitch Ring of Gyroscope, Relay Valve and Main Elevator Valve.
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