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Aviation History
1971
1971 - 0191.PDF
176 FLIGHT International, 4 Februaf^I971 mini DEFENCE Through a glass, steadily By JOHN BENTLEY IN THESE DAYS of ever-increasing com plexity in missile and weapon-plat form technology there is still room in modern warfare for the original basic standby instrument, the human eyeball Mk 1. Indeed most short-range air-to-ground or ground-to-ground missiles are still guided to their targets visually via signals sent through a wire which snakes along behind the missile after it is launched. Early missiles of this type were followed visually to the target with the aid of a simple monocular sight, and this is still sufficient in the case of troops launching missiles from a fixed ground position. But complica tions set in if the launching platform is mounted on a moving vehicle. Vibrations in it are magnified in pro portion to the degree of magnification of an optical sight; movement of the vehicle itself, especially in yaw and pitch, makes accurate tracking extremely difficult. To explain the philosophy and the usefulness of the new AF.120 sight, which is a joint private venture by Avimo and Ferranti, one must go right back to square one. Imagine standing beside a LandRover on top of a small knoll; on the back of the LandRover is mounted a guided missile which, after launch, is con trolled in azimuth and elevation by means of a simple miniature joystick. The target is a stationary tank out lined (conveniently) on another knoll about half a mile away. The "fire" button is pressed and the missile zooms away quite rapidly. Watching the tracking flare behind the missile the operator moves the joystick about (just as if "riding" the missile) and positions the flare so that it is super imposed on the target. Kept super imposed, and unless the wire runs out, the missile will score a hit. It's fairly simple, if one is standing still. All the operator has to do is to keep an eye, the tracking flare and the target in a straight line and every shot will be a hit. Moving targets present a bigger problem, but can still be managed at medium range with the naked eye. The real complications arise when the operator himself is being moved about, and particularly when he is being vibrated and is using monocular or binocular aids to see the target and track the flare. Wire-guided Nord SS.ll missiles are being fitted as standard armament to the British Army's Scout heli copters. Costing about £1,000 each, and with a range of around 6,000m, 19,685ft the SS.ll can stop a £660,000 main battle tank. But to employ this 6,000m range capacity, some form of optical magnification must be used so that the operator can see and clearly identify not only the target but the missile flare itself, which is difficult to distinguish with the naked eye at ranges greater than 2,000m, 6,562ft. If the Scout had to go as close as 2,000m to the target it would be in considerable jeopardy itself. To over come that problem the operator uses a magnifying telescope so that his visual range is considerably increased. A year ago Avimo and Ferranti were awarded a Government contract valued at more than £500,000 to sup ply their AF.120 sight for Army Scouts. Four sights have already been delivered and in tests with the sights at Castlemartin last autumn hit rates of better than 80 per cent were achieved, compared with rates of 25 per cent without the AF.120 installation. These "before and after" results are not the product of one magic ingredient but rather a series of logical and clever combinations of existing technology, providing the Scout-borne missile-aimer with a visual situation very nearly compar able with that of the man standing beside his LandRover on terra firma— and this despite rny jinking move ments the Scout pilot might feel to be necessary after launching to avoid enemy counterfire. In brief, the AF.120 sight is gyro- stabilised telescope fitted with a binocular eyepiece. The lower part of the sight can be retracted up into the casing when not in use, giving the aimer complete freedom of vision. The sighting head (a gyro-stabilised mirror) is enclosed in a double-skin casing outside the helicopter canopy and the whole system is mounted on a simple tubular sub-frame via anti- vibration mountings. Ferranti have used a standard air craft-type artificial horizon gyro unit with pick-offs to torque motors. The gyro unit stabilises the mirror which is mounted in the sighting head so that the helicopter heading can be up to 60° on either side of the sight heading and up to 30° off in eleva tion. In other words, at the lower power magnification available (X2:2), giving a 20° field of view, the aimer can scan through 140° in azimuth and 80° in elevation (without moving his head) while the Scout is flying on a fixed heading. Elevation stabilisation of the mirror is taken direct from the basic gyro, but stabilisation in azimuth is done indirectly. When an azimuth change is detected by the gyro unit the mirror is turned to compensate, but any small turning movement of the mirror is picked up and amplified before being fed into the motors which rotate the complete casing in azimuth. The aimer can elevate the mirror and rotate the casing via a throttle grip and milled wheel, and can thus track a moving target via a graticule at the centre of his picture. The mirror and the rest of the optics are Avimo's responsibility. Beneath the mirror is a Pechan prism which has the property of being able to provide automatic image tilt cor rection. Thus, whatever the angular relationship in azimuth between the sight mirror and the binocular eye piece, the horizon always remains at its natural angle, only tilting when the helicopter rolls. (Try rotating either mirror in a simple periscope about its vertical axis and see how much the horizon tilts.) Also in the optical path is a penta gonal prism, which rectifies the image, and two sets of magnification lenses. The normal low-power X212 lenses are used during observation
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