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Aviation History
1954
1954 - 0574.PDF
260 THE BIG FLASK . . FLIGHT (Top) The "great door" moved aside from the insulating shed, visible on the extreme left; the fan motors, and part of the refrigeration plant, can also be seen. (Centre) The testing of meteorological balloons, which included the simulation of solar radiation at height. (Bottom) The space below the main pressure shell, at the same temperature as the working section, is used for the testing of the Viscount tailplane at -60 deg C. vacuum pump, driven by an electric motor of 140 h.p. This provides an equivalent initial rate of climb of l,500ft/min with the chamber air at +15 deg C, and gives a maximum simulated height of 80,000ft (0.395 Ib/sq in), or an initial climb of 1,000 ft/min with the air at -60 deg C, and a ceiling of 60,000ft (1.037 lb/sq in), or intermediate values. Emergency refilling of the chamber by opening a 12in valve in the vacuum pipe gives a descent from 50,000ft to sea level in 160 seconds. To obviate icing in the chamber at low temperatures, due to moisture in the circulating air, an air-drying plant has been pro vided. This consists simply of a bed of silica gel, through which the air is passed before entering the chamber. The plant is of sufficient capacity to supply three charges (each of 30,000 cu ft) of dry air per 24 hours, this air having a frost point of about — 40 deg C at sea level pressure and temperature. With expansion to simulated altitude this value is improved, actual air frost points recorded at an I.C.A.N, pressure equivalent to 35,000ft being of the order of —60 deg C. The rate of refilling the evacuated chamber with dry air is normally 360 cu ft/min. Plant engineer in charge of the refrigeration, vacuum and air-drying equipment is Mr. G. M. Stapleton. Air circulation is by means of four-bladed fans, one in each duct, driven by variable-speed D.C. motors of 150 maximum h.p. Usual airflow speed during high-altitude tests is about 6 kt, with a normal maximum of 40 kt, or up to 60 kt with the use of special nozzles when such speeds are required. The concrete floor of the insulating shed around the chamber is itself supported on brick piers about 8ft above the main con crete foundations. The chamber is supported on steel cradles, the mounting at the enclosed end being on rollers to allow for overall expansion or contraction effects. Differential expansion between the working section and the air circulation ducts is accom modated by flexible steel bellows situated midway along each duct and supported on independent pylons. The opening of the "great door," which has its own insulation shell, is effected by means of a 40-ton hydraulic jack built into the door of the building. A detachable strut connects with the chamber door, which is drawn away 12in from the sealing face of the chamber structure. A hand-operated winch built into the carriage of the great door then enables a sideways movement of 45ft to be made. At the time of our visit, the forward part of a Viscount fuselage was attached to the mounting ring around the upper air-lock door. Moving out of the chamber through the lower air lock (both air lock compartments can be used also as "sub-chambers" for testing small specimens under conditions of rapidly changing pressure), we crossed the connecting bridge into the control room and spoke with Mr. T. V. Small, A.F.R.Ae.S., who is in charge of the actual test work being carried out in the chamber. The arrangement of the overall programme of work is the administrative responsibility of Mr. V. H. Mole-Dennis. An immediate indication and a precise control of the test conditions in the chamber are available to the controller in the well-designed and roomy control room. To the left of two long desks are continuously recording instruments for pressure, tem perature and humidity. On the left-hand desk are no fewer than 260 pairs of electrical terminals, with connections inside the chamber, enabling the measurements of control-surface Desynns, electrical thermometers and strain-gauges on the test specimen to be noted or recorded in the control room, photographs taken, or other electrically actuated equipment operated. Dials for a selected number of measurements are built-in. Between the two desks are a press-button temperature indicator for various points in the chamber, a mercury U-tube giving the chamber pressure, and an accurate laboratory barometer. At the controller's position on the right-hand desk are intercom faci lities, chamber altimeter and oxygen supply dials, and a panel which carries a simplified plan of the working section on which are shown all intercom and oxygen positions. As the test per sonnel move around between these positions in the chamber, and plug in the intercom and oxygen facilities, small bulbs on the controller's panel light up at the appropriate points, giving an indication of the test workers' movements. At the other end of the controller's desk are the controls for adjusting the speed of the four fans in the air-circulation ducts. Since the chamber became fully operational in 1951, a variety of test projects have been performed in it. These include engine cold-starting trials, tests of powered control units and turbo- starters, generator brush-wear investigations, and studies of the behaviour of a number of aircraft and guided-weapon structures, control surfaces and systems and items of electronic equipment, under various low-temperature and high-altitude conditions. The ability to mount an aircraft pressure-cabin in the chamber is, of course, an important advantage (the Viscount cabin at the time of our visit was being used for a number of investigations into minor operational problems). In addition to its use for Vickers' own aircraft work, the
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