Patent GB-985978-A (GB)
Title: Improvements relating to navigational apparatus
IPC: G01C21/00
Filed: 1961-02-08
Abstract: 985,978. Aircraft heading indicators. MINISTER OF NATIONAL DEFENCE OF H.M. CANADIAN GOVERNMENT. Feb. 8, 1961 [Feb. 15, 1960], No. 4706/61. Heading B7W. An aircraft heading indicator comprises a plurality of shafts, of which one is rotated in accordance with the output signal from a directional element, e.g. a magnetic or gyro compass, a second is positioned in accordance with the calculated value of the variation of the element from a given basis of observation, e.g. true north, and a third is rotated at a speed corresponding to the rate of change of the variation, and a fourth shaft is positioned in accordance with the algebraic sum of the positions of the other three, to represent the aircraft heading with respect to the given basis at any time. In Fig. 1, a true heading pointer 12 and demanded heading pointer 13 work over a fixed azimuth scale 11. The known local value of the magnetic variation is set manually by knob 15 and displayed by index 15a. Deviation is added using a cursor 10b on a transparent disc 10a rotatable by knob 10c, the cursor being set on index 15a, and knob 15 rotated until the deviation is shown on scales 15b, 15c. The rate of change of variation expected during the flight is set manually by knob 17, and displayed on scale 18. When the instrument operates as a free gyro azimuth indicator, e.g. during transpolar flights, the rate used is the change of heading due to convergency of the meridians, and is shown in terms of latitude on scale 16. A demanded course, shown by pointer 13, is set manually on knob 14. Figs. 2 and 2a show the instrument operating mechanism. A magnetic heading analogue signal e.g. from a flux valve, is fed to a control transformer 30, the output of which is fed over amplifier 45 and switches 46, 47, to a torque motor 48 arranged to precess a directional gyro 49. The signal from the gyro pick-off 50 is fed through a control transformer 51 to amplifier 55 and a motor-generator servo 56 driving a magnetic heading shaft 32. Lack of correspondence between the angular position of shaft 32 and the heading input to transformer 30 results in energization of motor 48 and precession of the gyro at a maximum rate of 2 degrees per minute, until shaft 32, kept in correspondence with the gyro azimuth by combination 55-56, attains its correct position. Shaft 32 operates synchros 60-62 to repeat the magnetic heading in pilot's instruments and the like. Switch 47 is manually operated to provide a fast (sixty degrees per minute) precession of the gyro to achieve rapid error cancelling when a large error occurs, e.g. after free gyro operation. Shaft 32 provides one input to a mechanical differential 85. Shaft 75 is rotated by knob 15 in accordance with the value of the variation at the time of setting. The expected rate of change of variation set on knob 17 is available as a signal from potentiometer 103, and energizes a motor-generator integrating servo 109 which also rotates shaft 75 through slip clutch 116, so that the angular position of shaft 75 represents the expected variation at any time. Shaft 75 provides the other input to differential 85, and the position of its output shaft 71 represents the true heading, shown on pointer 12 and repeated through synchros 65-70 in various apparatus. The variation is also repeated as required through synchros 95, 96. Relay contacts 105, 106, and a clutch 118 can be controlled manually from a remote station, to apply a large signal to servo 109 and to increase the gear ratio between the servo and shaft 75, so that the value of the variation can be reset from the remote station. The demanded true heading set by knob 14 provides one input to a differential 90, and is corrected through a slipping clutch 144 by a synchro 140 supplied with a signal representing the convergency of the meridians, derived as in Specification 985,977. Shaft 75 provides the other input to differential 90, the output shaft of which drives a synchro 135 providing a signal for a pilot's demanded magnetic heading indicator. If the pilot follows this demand, the aircraft pursues a great circle course. A mechanical differential 150 has one input from servo 109, and another representing convergency from synchro 140. The sum is added to the magnetic heading analogue on shaft 32 in a differential 151, the output shaft of which can be connected through clutch 156 to a synchro 120 providing a heading error signal for an autopilot. A centring device 160 maintains this error signal at zero until clutch 156 is engaged. The output of differential 151 represents a datum heading continuously corrected for variation change and for convergency of the meridians, so that the autopilot steers a great circle course. During free gyro operation, switch 46 is opened, and shaft 32 is slaved to gyro 49 by the servo 56. The shaft position is, however corrected for convergency of the meridians by rotation of control transformer 51 by synchro 140. Shaft 71 is initially brought to the correct true heading by rotation of shaft 75. If the gyro drift rate is known, it may be set on knob 17. Specifications 863,412. 920,678, and 985,976 also are referred to.