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Aviation History
1983
1983 - 0048.PDF
Viggens test Gripen avionics JAS 39 Gripen will bristle with digital electronics first flown on Viggen test air craft. Highlights include digital displays, fly-by-wire flight controls, a multi-mode radar, and standard computer modules. The pilot will use four displays supplied by SRA—one of them headup—for almost all information. Four electro-mechanical instruments are provided for standby use only. The 120mm x 150mm head-down displays will use raster scan, with a special line-smoothing technique to remove stair- casing. The left-hand display will be for flight data, including artificial horizon, airspeed/Mach number, altitude, and approach-aid data. The right-hand display is a tactical horizontal situation and radar indicator. In the centre, the moving-map display will be all-digital. SRA says that current semiconductor memories have sufficient capacity for storing digital maps to satisfy Sweden's requirements, which cover only Sweden and its surroundings. The headup display will use diffractive optics to achieve high brightness and a wide field of view. Conventional stroke symbology will be used, along with a raster facility for displaying Flir imagery for night-time operations. SRA became involved with diffractive optics develop ment in 1975. This culminated in 150 flights of an experimental Hud in a Viggen from June 1977 to January 1980. Hughes made the optics under subcontract. SRA Saab-Scania is flying a demonstration fly-by-wire flight-control system on this Viggen intends to fly an improved diffractive optics Hud based on the previous example, but with a smaller field of view and im provements in optics manufacturing tech niques which reduce the colouring of the outside world. Horizontal instantaneous field of view will be 30° for the new Hud, compared with 35° for the earlier devel opment. JAS 39 needs only about 28° be cause of the aircraft size, according to SRA. Vertical field of view is 22° on all the SRA diffractive Huds. SRA will subcontract Hud production because it does not consider it worthwhile to acquire the necessary optics technology; the company argues that it is better to subcontract the entire Hud than just the optics. Diffractive optics Huds use a combiner SRA is supplying three head-down CRTs which, along with the Hud, provide the primary instruments 64 glass which comprises a layer of gelatin sandwiched between two glass layers which form a collimating lens. A split laser beam is used to generate a diffraction gra ting on the gelatin during manufacture. The resulting hologram is transparent to all light except at the narrow wavelength at which the Hud's cathode ray tube oper ates. The Hud imagery is reflected by the diffraction grating into the pilot's eye. SRA and Hughes use an off-axis optical arrangement which has one curved holo graphic combiner glass. Marconi Avionics favours quasi-axis optics which use three flat holographic elements. Although the latter system has been chosen for the US Air Force's ground-attack Lantirn pro gramme, SRA feels that the obscuration of the outside world is too high for the fighter application. SRA is now working on the use of holographic optics in head-down displays. The high transmissivity of the optics could allow liquid crystal diodes to replace cathode ray tubes as the light source, reducing weight, volume, and power requirements. JAS 39 will have relaxed static stability, and all controls—elevons, canards, leading-edge devices, rudder, airbrakes, and nosewheel steering—will be operated by the triplex digital FBW system. An early development model is already flying on a Viggen. The canard will be used as a pitch control surface as well as the el evons, improving manoeuvrability and re ducing drag. LM Ericsson is developing the multi- mode pulse-Doppler radar which forms the primary sensor for JAS 39. The sub contract policy works here too, though Ericsson admits only to "working with a number of companies on elements of mu tual interest". The company will not com ment on suggestions that it is working with Ferranti, but the latter company's electronic miniaturisation advances would be appropriate to this radar, which is claimed to be three times as effective as the PS.46 of the JA 37 fighter Viggen, but occupies only 60 per cent of the volume. FLIGHT International, 8 January 1983
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