Patent GB-623721-A (GB)
Title: Improvements in or relating to ground or bomb sights
IPC: G01C21/18
Filed: 1941-10-20
Abstract: 623,721. Bomb-sights. SPERRY CORPORATION, and STEVENS, A. H. Oct. 20, 1941, No. 3873/48. Divided out of 13452/41. [A Specification was laid open to inspection under Sect. 91 of the Acts, July 3, 1946.] [Class 92 (ii)] A bomb-sight apparatus for aircraft has its line of sight stabilized in azimuth by means of a gyroscope so that it is independent of the yawing of the aircraft. Sight mounting.-As shown in Fig. 1, a skirt 2 rotates in a fixed framework 9 on rollers 10 and carries a gimbal frame 3 on fore and aft pivots 4, the frame 3 carrying a gyroscope casing on lateral pivots 5. The gyroscope is maintained vertical by a pendulum device 6 which has a copper ring 6<SP>1</SP> located in proximity to a stator winding 7 supplied with threephase current ; any eccentricity of the ring and winding caused by the tilt of the gyroscope produces eddy currents which brings the gyroscope axis back to its normal vertical position. The ring 2 is stabilized in azimuth by a shaft 13 and gears 14, 15, driven by gearing on the mounting of a gyroscope 12, Figs. 3A and 3B. The optical system includes a reticle 16 on the casing 1, and prisms 17 and 18 mounted on arms 19 of the frame 3, the prisms by this mounting being stabilized against rolling. Lenses 20 focus ground images in the plane of the reticle 16 via prisms or mirrors 21, 22, on the frame 2, and the target is viewed through an unstabilized eyepiece 23 fitted with a scale 152 at the bottom of a lens 24, Figs. 1 and 3B. The prism 17 is made to follow the target by a lever 25 whose lower end has a channel 28 fitting over a knob 29 on the prism-thus allowing relative movement during rolling-and whose upper end is actuated by a .member 31 on a rod 33 of the computing mechanism. Computing mechanism (Figs. 3A and 3B).- The rod 33 is moved by a cam 34 on a shaft 35 which also actuates a contact 36 of a bombrelease mechanism. The shaft 35 is driven from one side of a differential 45, but may also be moved by a knob 235 to set the prism initially on the target. The planetary arm of differential 45 is driven by a prism setting knob 46, and a third side of the differential is driven by gears 48, 49, of which the gear 49 is connected to the cylinder 50 of a variable speed device 51. The roller of this device is adjusted by a ground speed synchronizing knob 63 and a reading of the ground speed is given on a dial 58 when knob 63 is adjusted to keep the prism on the target. To take care of the altitude on the sight problem and to allow for changes in altitude, the disc of device 51 is not rotated at constant speed but is driven by the cylinder 66 of another variable speed device whose ball 67 is positioned by a shaft 70 rotated by a cam 73 on a shaft 74. The latter is geared to a shaft 77 driven by the cylinder 78 of a vertical velocity variable speed device which compensates for changes in altitude. Vertical velocity is set in by a knob 84 adjacent a dial 85, and initial altitude is set in by a knob 98 for turning shaft 77. The discs 86 and 94 of the variable speed devices are driven by a constant speed motor 53. Cam 34 is so arranged that its shaft 35 rotates proportionally to ground speed and inversely proportional to height ; hence, as above stated, the roller 54 is radially positioned in accordance with ground speed. This ground speed data is fed by a shaft 102 to rotate a cam 105 whose follower 106 lifts according to the formula Te x Gs/H where Te is the bomb's time of fall, Gs is the ground speed, and H the altitude. The Te and H data is set into cam 105 by an axial displacement produced by the followers 111 of a cam 112 which is rotated in accordance with H from shaft 77, a barometer 231 indicating the altitude, and which is displaced by gearing from knob 84, since Te is influenced by the vertical velocity of the aircraft. The cam 105 is also displaced by a knob 123 in accordance with the ballistic coefficient of the bomb. Another cam 127 is rotated by a trail knob 131 and displaced by the follower 128 of a cam 129 which is rigid with cam 112, the cam 127 being so designed that its follower 110 moves in accordance with 1/2 RTe<SP>2</SP>(1/H), R being the retardation factor of the bomb. The two cam followers 106 and 110 act on a lever 107 carrying the second contact 109 of the bombrelease mechanism. The pin 110 also acts on a shaft 135 which is geared to a trail dial 138 and which serves to rock a lever 141 about a pivot 143 fitted to a rack 142 which is geared to a drift angle dial 180. The other end of the lever 141 serves to rock levers as shown, to move a pointer 151 over the scale 152 on lens 24. Azimuth stabilizing.-A segment 159 on gyroscope 12 co-operates with air nozzles 160, 1601, which differentially control, through pipes 250, bellows 163, 163<SP>1</SP>, serving to move laterally a shaft 165 driven by constant speed motor 53. The shaft 165 is normally centralized by a device 170 but when moved by the bellows brings a roller 167 against one or the other of two drive discs 168, 168<SP>1</SP>, serving, through gearing, to rotate the gear 15 on frame 2 and also to drive the follow-up shaft 162 of the gyroscope 12. The driver to the shaft 13 also drives a shaft 181 serving to displace the rack 142 and also to rotate the drift-angle dial 180. The gyroscope 12 may be caged by means associated with a knob 225, such means serving to actuate a contact 304, and also switches, in circuit with the winding 7 ; details of the circuit are given in Fig. 13 (not shown). Action.-In use, the sight is first set on to the target by rotating a knob 185, and then maintained on the target by adjusting a knob 193 of a variable speed drive 189, such devices being geared, as shown, to the shaft 13. The pilot of the aircraft then images the gyroscope 12 to stabilize the line of sight, and if the reticle wire moves off the target he will adjust the knob 193 to cause the sight to turn towards the target and at the same time to give an indication, by way of a rheostat 202 and indicator 203, to turn the aircraft. When knob 193 is ultimately at zero, and the aircraft is therefore on the correct course, discs 206, 206<SP>1</SP> on the shaft of the knob will be rotated to a position in which fingers can enter notches in the discs. Finger 228 of disc 206<SP>1</SP> is in circuit with the winding 7 and until the knob 193 is at zero, i.e. until the aircraft is no longer turning, so acts on the circuit as to eliminate the erective force on the gyroscope 1. The other finger 2281 actuates a contact 204 which is in circuit with the contact 304 and also with a magnetic clutch 207 which locks, when excited, the centre arm of differential 209 to cause a shaft 208 to drive a further rheostat 213. The arrangement is such that when either of the two rheostats is off its actual position an indication at 203 will be given to the pilot to alter course. The switch mechanism, Fig. 4 (not shown), for the bomb-release mechanism includes means for moving a warning signal adjacent lens 24, and means for allowing the contacts to make circuit, but prevents them from ever breaking a circuit.