725,551. Electric analogue computing systems. LINK AVIATION, Inc. March 12, 1951 [March 11, 1950], No. 5959/51. Class 37. [Also in Group XL (c)] Training apparatus for simulating the flight of an aircraft with respect to one or more very high-frequency radio-beacon stations or low-frequency beacon or broadcasting stations comprises means for generating alternating potentials indicating, by their respective amplitudes and phases, the simulated distance and bearing of the craft from each of the radio stations, these voltages being used by the apparatus to operate display apparatus in a manner simulating the operation of similar apparatus in a real aircraft. Considering V.H.F. operation, each radio station simulated may be either a very high-frequency omnidirectional phase beacon known as VOR or a overlapping-beam beacon providing landing facilities and known as ILS. Each of these may also have associated with it distancemeasuring equipment (DME) giving the range of the craft from the beacon. The apparatus comprises a plurality of switching circuits which simulate tuning of real radio apparatus, in order that a pupil may be able to select to which one of a number of " stations he " tunes," the switches operating relay circuits modifying the connections of the computing apparatus to achieve the desired indication on the display apparatus. Generation of trainer-station range-and-truebearing voltages. Operation of the flight controls in the trainer causes a pen 14, Fig. 1, to move over a chart 12 in accordance with the simulated position of the trainer, the N-S motion of the pen along the arm 18, and the E-W motion of the arm 18 on the shaft 22 controlling the wiper arms of potentiometers R1, R2, Fig. 11, respectively. Further potentiometers R3, R5 and R4, R6, Fig. 12, are provided; these are used to determine the coordinates of two radio stations respectively. Potentiometers R1, R3, R4 determining the N-S co-ordinates are excited by an alternating voltage from a transformer T4; potentiometers R2, R5, R6 determining the E-W co-ordinates are fed in quadrature from a transformer T5. The co-ordinate potentials for each station are added to those for the trainer and the two difference co-ordinate potentials for the trainer with respect to each station are added together, those with respect to the first station being combined in cathode-follower circuit V1, V3 and those with respect to the second station being combined in cathode-follower circuit V2, V4. The outputs from the cathode-follower circuits consequently consist of alternating potentials each having an amplitude and phase corresponding to the range and true bearing respectively from the trainer to the corresponding station. Insertion of station parameter and actuation of corresponding follow-up servo system. The rotors of magnetic resolvers 312, 314, Fig. 11, corresponding to station 1 and 2, are positioned according either to the simulated magnetic variation at the stations or to the runway direction depending on whether VOR or ILS type of operation is being simulated. The stators are energized in quadrature from transformers T4, T5. When, for example, the pupil " tunes " to station 1 the outputs from the rotor of resolver 312 are applied to the rotor of a similar resolver 326, Fig. 16. Also the trainerstation range-and-true-bearing voltage from cathode-follower circuit V1, V3 is applied to valve V5 which together with valve V6 performs a limiting operation to apply square waves, having a phase corresponding to the true bearing of the station from the trainer, to the grids of a discriminator valve V7 which drives a follow-up motor 340 through a servo amplifier, Fig. 15, to so position the rotor of the resolver 326 that the voltage applied to the anodes of V7 from one of the stators is in quadrature with the square-wave voltage from valve V6. The rotor is consequently positioned in accordance with the setting of resolver 312 and the left-hand stator has induced therein a constant-amplitude voltage which is in phase with the range-and-true-bearing voltage applied to the grid of V5. Brief indication of function of remainder of apparatus. The voltages developed in the apparatus described above are used in apparatus in a manner selected by relay circuits to simulate any one of a plurality of operations corresponding to those obtained by the use of standard radio-navigation equipment. In addition facilities corresponding to conventional radio-telephone and wireless-telegraphy facilities are provided, Figs. 25-31 (not shown). Apparatus for selecting the type of operation required in accordance with simulated station selection and tuning is shown in Figs. 20-23 (not shown). Orbiting, Figs. 32-35. This provides means for giving a predetermined indication when the simulated distance of the trainer from a " destination " point having predetermined co-ordinates with respect to a VOR station has a predetermined value as selected by the pilot. The trainer then simulates flight on a circular path at a given radius from the destination point. The rotor of resolver 500, Fig. 33, is energized through PL 169, Fig. 35, from the output of resolver 312 (or 314), Fig. 11, by means of voltages whose phase corresponds with the simulated magnetic variation at the station. The pupil rotates the rotor of resolver 500 in accordance with the magnetic bearing of the destination point from the station and adjusts potentiometer 504 in accordance with its range so that a voltage is taken from PL186 having magnitude and phase corresponding to range and true bearing of the destination point from the station. This voltage from PL186, Fig. 43, is applied to cathode-follower V26B, Fig. 40, while the trainer station range-andtrue-bearing voltage (the derivation of which has been already described) is applied to cathode-follower V26A. The sum of these inputs, having amplitude and phase corresponding to range and true bearing of the destination point from the trainer, is taken from the addition circuit 508 and is applied through amplifiers V27A, V27B, to the right-hand grid of valve V28, Fig. 32. The amplified output is rectified in V25 and then applied to the right-hand grid of V29 having applied to its left-hand grid the output from a D.C. energized potentiometer 512 which is set by the pupil in accordance with the desired orbit radius. The outputs from the cathodes of valve V29 are applied across the coil 514 of a vertical-pointer instrument forming part of the indication ID-249, Figs. 33 and 35. The pointer 490 is consequently not deflected when the inputs to the two grids of valve V29 are equal, i.e. when the simulated flight is at the desired radius from the destination point. Radio beacon (VOR) operation, Figs. 32-35. The pupil sets the rotor of the resolver 520 of his indicator ID249 in accordance with the magnetic heading of a desired course to the radio station. The stators of the resolver are energized with voltages whose phase corre. sponds to magnetic variation at the station derived from resolvers 312 or 314, Fig. 11. The output from the rotor consequently has a phase corresponding to the true heading of the desired course. This voltage is amplified in the left-hand side of V28, Fig. 32, and thence applied through transformers T16, T19 to the centre tap of transformer T18, Fig. 34, feeding a phase-discriminator V32. The primary of transformer T18 is excited via PL215 and Fig. 18 (not shown), by the output from the lefthand stator of resolver 326, Fig. 16, having a phase corresponding to the trainer-station true bearing. When the two inputs to the phase-discriminator circuit have the same phase, the valve V32 gives two equal outputs which are applied to valve V29 and thence to the coil 514 of the vertical pointer as previously described, so that the pointer 490 remains undeflected when the desired course is being followed. Instrument landing system (ILS), Figs. 32- 35 and 49-51. Means are provided for simulating the operation of the instrument-landing system, indications being given of departure from the desired course in azimuth, Figs. 32-35 and in elevation, Figs. 49-52. Azimuth indication.-The output from resolver 312 (or 314), Fig. 11, set according to true approach bearing of the simulated runway is applied through PL190 to transformer T16, Fig. 32, and thence to phase discriminator V32. The output from the the left-hand stator of resolver 326, Fig. 16, is also applied via PL215 to the phase discriminator so that as explained pre. viously the pointer 490 will remain undeflected so long as the desired course is being followed. Elevation indication.-A potentiometer 548, Fig. 50, is energized from an A.C. source through terminal G, PL215, Fig. 51, and is set in accordance with the simulated glide-path angle. The output from this potentiometer is applied to a further potentiometer 550, Fig. 42, the slider of which is positioned, in a manner to be described later, in accordance with the trainer-station range. The resultant productvoltage measuring desired altitude at the particular simulated location of the trainer, is applied via terminal PL188, to the right-hand grid of valve V42, Fig. 50. The signal applied to the left-hand grid is taken from a potentiometer 554 also energized from the terminal G but in anti-phase with respect to potentiometer 548. The rotor arm of the potentiometer 554 is controlled by the trainer flight controls so as to be set in accordance with the instantaneous simulated height above sea-level, while the stator is positioned by the instructor in accordance with the simulated altitude of the runway. The combined output from the cathode circuit of valve V42, which is zero when the desired glide-path is being followed is fed via amplifier V43, Fig. 49, to the grid of a variable mu valve V44, the gain of which is controlled in accordance with simulated range from the station. The gain-controlling voltage which is a negat