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Aviation History
1960
1960 - 0006.PDF
SYSTEM SURVEY Self-setting Flight Log THE ultimate goal in the development of Decca Navigator air-borne equipment is obviously to eliminate all manual operations and present position information on a distortionless chart.Additionally, the position information can be put to further use by automatic data link transmission to ground stations and byprovision of steering information to the autopilot. While these are future developments of the basic system, the immediate pro-gramme is aimed at a drastic reduction in size and weight of the airborne units. Within a matter of months Decca will be able tosupply a lightweight Mk 8 receiver and Flight Log weighing a total of 471b compared with the present 951b. The receiver willbe contained in a long \ ATR case and the only power supply required will be 28V d.c. The necessary h.t. will be providedby a transistor oscillator which will be integral with the receiver. This remarkable reduction in weight, which effectively answersone of the major criticisms of the system, is achieved in part by transistorization, but also by a reduction in component weight.A new development is that Decca have designed their own miniature relays, four of which fit in 1 cu in and three of whichweigh loz. The relay contains no rubbing parts. It is of the soft iron type with a single contact and employs a permanent magnetas a return spring. The moving arm is set like a see-saw with the permanent magnet at one end and the contact at the other,the pivot being in the form of two rollers held in contact by 0.004in diameter Terylene thread wound round the rollers in a"figure-of-eight". Decca have hand-made about 200 of these relays and at least one example has undergone 25 million re-versals without a single fault during operation and without any sign of deterioration. The longer-term requirement of fully automatic operation isbeing approached by a number of different means and Flight was able to observe one of these, the Mk 3 self-setting Flight Log,during a trial flight from Gatwick in Decca's Valetta last week. As detailed in our issue of February 20 last year, the Mk 3 unitemploys a rather different computer which makes a digital com- putation of Decca co-ordinates_ once every second to produce Xand Y outputs in terms of a binary code. As can be seen in the accompanying illustration, a series of tracks on the edge of theFlight Log chart is marked black or white to represent binary codes, the values being detected by photo transistors beneath thechart material. The digital X and Y outputs from the receiver are compared with those derived from the photo transistors and theFlight Log spools are then driven by servo motors to cancel the difference between the two and position the pen. The traditional ink pen moving across the face of the chart isreplaced in the Mk 3 Flight Log by a stylus which vibrates against a typewriter ribbon beneath the rear face of the chart. Thechart itself is made of a transparent plastic material called Per ma- trace, and two small lighted arrows flash beside the pen toindicate its position behind the chart. The external appearance 6 FLIGHT, 1 January I960 Seen beside the flight deck of HMS "Hermes" is the UHF direction- finding CADF aerial made by Standard Telephones and Cables of the Flight Log is thus much neater than before and it is im-possible for the pen itself to be damaged by inadvertent move- ments by the pilots. Both the Deccometers and the Flight Logare controlled by a single set of push-button inching controls. The rows of black and white code markings on the edge of thechart remain hidden beneath the cover plate so that only a rectangle of chart is in view. Because the pen is positioned underthe chart in relation to the code marks on the edge of the Perma- trace the usual sprocket drive is unnecessary. In the usual Flight Log, the computer is adjusted for variouspattern selections, combination patterns and chart scales by a turret switch containing a number of labelled keys. This functionis replaced in the self-setting Flight Log by a band of black and white markings at each end of each chart section. A continuousblack/white boundary beside the strip of codes provides a position- ing signal to allow for chart movement or distortion, and a gapin this trace positively stops the key indications over the photo- cells until the computer has been set for the characteristics ofthe new chart. When changing charts, therefore, the appropriate inching button is pressed to move the Permatrace along. At thekey indications the chart automatically stops and then restarts, and the pen is moved to the appropriate position on the succeed-ing chart. On the usual Flight Log the pen must be put down either onthe exact position of the aircraft on the chart or on the nearest corner of the appropriate Decca pattern, but with the self-settingFlight Log the pen need only be positioned anywhere within a square of a given area centred on the position of the aircraft. Thecomputer now being tested is designed to handle a six-digit binary code with the result that this area is 1.28in square. Witha four-digit code it would be 0.3in square, but with a nine-digit code the permissible latitude would cover the whole exposedarea of the chart. Of particular interest is the fact that it has been found that a computer handling a nine-digit code would in factbe lighter than the present six-digit unit. For semi-automatic chart-changing with the present Mk 3 FlightLog, neighbouring charts must be arranged to be coincident to within 1.28in if the pen is to position itself without furtheradjustment. Should this limit be exceeded it cannot position itself less than 1.28in away from the actual location, the resultantposition error depending, of course, upon the scale of the chart. Although the necessity of making neighbouring charts sub-stantially coincident might appear burdensome in various ways, chart-change is so simple that many more such operations canbe accepted during a normal flight. Furthermore, projections giving much less distortion may be used at the expense of slightlymore frequent chart-changing. During the demonstration flight last week these features ofthe new Flight Log were explained and effectively demonstrated. Setting and chart-changing were far simpler than that requiredby the standard Mk 8 Flight Log and a markedly less distorted projection was in use. The equipment worked smoothly, as ithas done for some months. It is noteworthy that the digital out- put of the computer is eminently adaptable for use in data linktransmission and also for the rectilinear computation for a dis- tortionless presentation which is now being planned. Deccapoint out that the self-setting Flight Log is not necessarily the only approach to fully automatic operation, but it certainlyrepresents a very significant advance, particularly from the pilot's point of view. The Decca Mk 3 self-setting Flight Log with binary code markings alongside the chart exposed. The phototransistors are beneath the rectangular lamp assembly
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