Patent GB-865523-A (GB)
Title: Navigation training apparatus
IPC: G09B9/24
Filed: 1957-06-11
Abstract: 865,523. Simulating compass heading on aircraft trainers. LINK AVIATION Inc. June 11, 1957 [June 11, 1956], No. 18464/57. Class 4. An aircraft trainer simulating the operation of a magnetic aircraft compass comprises a computor having input voltages representing the simulated true heading and the magnetic variation at the simulated location of the aircraft, which are summed together with an input representing compass errors to give a compass heading output. Fig. 1 shows three cams 1, 2, and 3 driven on a shaft 4 by a motor 5 energised by a simulated latitude voltage. Levers 6 carrying cam followers 11 are held in contact with the cams by springs 21, 22, 23 and are moved together axially of the cams by a threaded shaft 10 driven by a motor 9 energized by a simulated longitude voltage. The cams are so shaped that the position of wipers on the levers 6 working over potentiometers 15, 16, and 17 represent the magnetic variation, the horizontal component of the earth's magnetic field (Ixy), and the angle of dip (#), respectively, at the simulated location. The motors 5 and 9 may be energized by outputs derived as in Specification 738,452. The cams may alternatively be contoured so that motor 5 should be energized in accordance with longitude, and motor 9 with latitude. Sets of cams may be provided, each set being operative within a given range of longitude. In Fig. 2 (not shown) a true heading voltage from a conventional flight computer is added by a selsyn mechanism to the magnetic variation signal from potentiometer 15 to derive a magnetic heading #m output in the form of the angular position of a motor shaft. A total compass error signal derived from the apparatus of Fig. 3 is then added to the #m signal in a further selsyn mechanism which derives an output shaft controlling the indication of the simulated magnetic compass. Cam or relay mechanism is provided and operated by outputs from a conventional flight computer, to interrupt the operation of the latter selsyn mechanism when the flight computer outputs indicate such excessive simulated, pitching, banking, or yawing, that a real magnetic compass would become unusable. Fig. 3 shows apparatus for computing simulated compass errors, which are added by amplifiers 301 and 302 and energize a motor 303, the shaft position of which represents the total compass error, summed with the magnetic heading as described above. The first compass error is the " semi-circular error " Ese caused by permanent magnetism in the aircraft frame, and computed as Ese = K 1  sin (#m +)α 1 )/Ixy, where K 1  is a scale factor and α1 is a phase angle, these being constants for a particular aircraft. K 1  is set by the position of ganged wipers 317, 318 of constantly energized potentiometers 315, 316, the wiper voltages +K 1  and -K 1  energizing a resolver potentiometer 319, the wiper 320 of which is driven by a shaft the angular position of which represents #m +α1, deriving a voltage K 1  sin (#m + α1) fed to an amplifier 322. The feed back circuit of this amplifier comprises the potentiometer 16 of Fig. 1, so that its output is K 1  sin (#m + α1) /Ixy, fed to amplifier 301 over summing resistor 305. The second compass error is the " index error" due to incorrect setting of the compass lubber line, and is simulated by manually setting the wiper 312 of a constantly energized potentiometer 311, the derived signal being fed to amplifier 301 over summing resistor 306. The third compass error is the " quandrantal deviation error " Eq due to magnetism induced in the soft iron parts of the aircraft frame by the earth's magnetic field. This is computed as Eq = K 2  sin 2 (#m+α2) where K 2  and α2 are constants for a particular aricraft. K2 is set by the position of ganged wipers 332, 333 of constantly energized potentiometers 330, 331, the wiper voltages +K 2  and -K 2  energizing a resolver potentiometer 334, the wiper 335 of which is positioned by a shaft the angular position of which represents 2 (#m +α2) deriving a voltage K 2  sin 2 (#m + α2) fed to amplifier 301 over summing resistor 307. The fourth compass error is the " Northerly turning error," occurring during banking and computed as En+ = arc +an [+an 8 sin (Ï + h) cos #m) where Ï is the angle of bank and h is the ball bank angle. The potentiometer 17, Figs. 1 and 3 is constantly energized and has a tangent function winding, so that the wiper derives a voltage + an.#. This is fed to a buffer amplifier 343 and a polarity reversal amplifier 347, the outputs + +an # and - + an a energizing resolver potentiometer 348, the shaft of which is driven by a differential 341 having inputs Ï and h from the flight computer. The wiper output + an # sin (Ï + h) is fed to a buffer amplifier 354 and a polarity reversal amplifier 355, the outputs energizing a resolver potentiometer 356, the wiper 357 of which is driven by a shaft the angular position of which represents #m, deriving a voltage +an # sin (Ï + h) cos #m. This is fed over resistor 358, and amplifiers 363 and 364, to a motor 365, the shaft 366 of which drives the wiper 362 of a constantly energized potentiometer 361 providing a feed back signal to amplifier 363 over resistor 360. Resistor 361 is wound to have a tangent function so that the angular position of shaft 366 represents E  n  + and the wiper 368 of a linear constantly energized, potentiometer 367, which is driven by shaft 366, derives a voltage E n t fed to amplifier 301 over resistor 308. The fifth compass error is the " acceleration error " due to deviation of the apparent gravity vector from the vertical, and is computed as E A  = arc +an (+an # sin Z sin m), where Z is the angle between the apparent gravity vector and the vertical measured in the XOZ plane of the simulated aircraft. Sin Z is computed as Axa/Aza where Axa and Aza are the simulated aircraft accelerations along the OX and OZ axes. A -Axa voltage from the flight computer is fed over resistor 375 to amplifier 373 energizing a motor 372, the shaft 380 of which drives the wiper 379 of a potentiometer 378 energized by + Aza and -Aza voltages from the flight computer. The voltage from wiper 379 is fed to amplifier 373 over resistor 374, whereby the angular position of shaft 380 represent sin Z. This shaft also drives the wiper 371 of a potentiometer 370 energized by voltages + +an # and - +an # derived from amplifiers 343 and 347, so that the voltage on wiper 371 represents +an # sin Z. This is fed to buffer amplifier 385 and polarity reversal amplifier 386, the outputs of which energize a resolver potentiometer 387, the wiper 388 of which is driven by a shaft the angular position of which represents #m, deriving a voltage + an # sin Z sin #m. This is fed through amplifiers 390 and 391 to a motor 392, the shaft 393 of which drives the wiper 395 of a constantly energized tangent function potentiometer 394, providing a feed back signal to amplifier 390 over resistor 397, so that the angular position of shaft 393 represents E A . This shaft also drives the wiper 396 of a constantly energized potentiometer 398, the derived voltage E A  being fed to amplifier 301 over resistor 309. The shaft 217 of motor 303 derives a feed back signal from a constantly energized potentiometer 304, fed to amplifier 301 over resistor 310, so that the angular position of shaft 217 represents the sum of the inputs to amplifier 301, apart from the feed back signal. Specification 822,336 also is referred to.