884,430. Attitude indicating instruments for aircraft. SPERRY RAND CORPORATION. March 21, 1958 [March 29, 1957], No. 9138/58. Class 4. An aircraft navigation aid indicating instrument for facilitating the steering of an aircraft towards and along a ground track that it is desired that the aircraft should follow, in which the pitch and roll atti- tudes of the aricraft are indicated by the position of an artificial horizontal line on a horizon-line defining member which serves to define, for observation by an observer through a window, adjacent areas representing the ground and sky respectively, and in which the position of the aircraft relative to the desired ground track is indicated by the position of a track-represnting index the indicating part of which is visible through the window against the ground representing area only. Instrument display, Fig. 1. An annular compass card 10 read against a fixed lubber line 14, is mounted in the instrument casing by rollers 11 and is formed with a peripheral gear engaged by pinion 12 of a train of gears 17 driven by a heading servo 13. Behind card 10 and mounted in a similar manner, is an annular member 15 which carries a selected heading indicator member 20 which is extended over the forward face of card 10 and is angularly adjusted by a manual heading-select knob 22, normally spring-locked, which operates through gearing 76, 26 and 28. which has a 1 :1 connection with gearing 17 through a differential 23. When knob 22 is being adjusted card 10 is held fixed by its connection to heading servo 13 and when knob 22 is locked card 10 and indicator 20 move in unison. A synchro 25 is connected to gearing 26 to generate a heading error signal for a purpose to be described. A fixed ring 21 is positioned behind member 15 to support in the centre of the instrument face an aircraftrepresentation index 30 and two cross-pointers 31, 32 are positioned behind the ring to be visible through its opening. Pointer 31 is moved transversely and pointer 32 vertically by meter movements 34, 35 respectively. Behind pointers 31 and 32 is positioned a pitch- and-roll indicator 40 comprising a roll-ring 50 mounted on bearings 51 and provided on its rear face with rollers 42, 43 which support a flexible band or screen 41 marked with a horizon line 44 dividing an upper light coloured area representing sky and a lower area which is either transparent or cut out so that when observed against the interior of the instrument it appears dark to represent ground. Ring 50 is driven through gearing 53 from a roll servomotor 52. A pitch ring 55 is positioned concentrically with respect to ring 50 by bearings 56 and it is provided on its rear face with a bevelgear ring 63 meshing with level gears 64, 65 operatively connected to rollers 42, 42 respectively to drive screen 41 in a pitch indicating direction. Ring 55 is driven through gearing 61 from a pitch servo 60. Viewable through the ground-similating portion 46 of screen 41 is a light-coloured ground track pointer 70 controlled by a meter movement 71 to simulate a predetermined ground track or course or a runaway along which the aircraft has to fly or land. Movement 71 is mounted on a platform 72 mounted in bearing 73 for movement about the longitudinal axis of the instrument by the output of a differential 74 through an angle that is the sum of the roll angle and the heading- error angle of the aircraft. This is achieved by supplying a measure of the roll angle obtained from roll servo motor 52 to differential 74 by gearing 75. A measure of heading error is supplied to the other input of differential 74 through gearing 76 from the output of differential 23. System operation, Fig. 2. A gyro magnetic compass 80, controlled from a magnetic reference e.g. flux valve 81, operates through a synchro transmitter 82 and a synchro receiver 83, the output of which is amplified at 84 to operate heading servo 13, the servo loop being completed by generator 85. The stator of synchro 25 is connected to compass transmitter 82 so that a signal voltage proportional to the difference between actual course or heading and set course or heading is generated in the rotor of synchro 25, which is applied as one impact to summary amplifier and limiter 87 which through amplifier 105 controls actuation of meter movement 34 relative to reference index 30 the movement being damped by the servo loop 107. Roll movement of screen 41 by means of roll servo 52 is obtained by the output of a synchro transmitter 91 operated by a gyro vertical 90, the output being compared in transformer 92 with the position of the roll ring 50, any error signal being applied to amplifier 93 the output of which drives motor 52 until correspondence is achieved through servo loop 94. The pitch output from gyro 90 at synchro transmitter 95 is compared at control transformer 96 with the position of screen 41, the error signal passing rhtough amplifier 97 to operate motor 60, the control loop being completed at 98. Meter movement 35 for horizontal pointer 32 is controlled in a manner similar to that described above for movement 34 by a signal from summing amplifier 99 amplified at 108 and stabilised by generator 100. A selector switch 111 is provided for selecting a desired flight plan from inputs such as a V.O.R. and L.O.C. receiver 112, glide-slope receiver 113 and altitude control 114. The inputs to amplifier 87 for controlling vertical pointer 31 are a signal proportional to the differonce between actual and desired aircraft heading, and a signal proportional to the roll attitude of the aircraft and, depending on the flight plan selected, a signal corresponding to lateral displacement of the aircraft from a selected radio beam. The signals supplied to amplifier 99 for controlling horizontal pointer 32 include a signal corresponding to vertical displacement of the aircraft such as from a glide slope beam or from a selected altitude and a signal proportional to pitch of the aircraft. The meter movement 71 for effecting lateral displacement of ground-track pointer 70 is responsive to the rectified output of a summing amplifier 120. The inputs to amplifier 120 comprise a signal from synchro 25 direct proportional to heading error which signal is applied also to a potentiometer 122 positioned by pitch motor 60, so that output of potentiometer 122 is a measure of the product of heading error and a measure of pitch, this product signal being applied also to the amplifier. A third input corresponds to a measure of the lateral displacement of the aircraft from a desired radio beam. Specifications 690,985 and 811,063 are referred to.