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Aviation History
1953
1953 - 0382.PDF
38o FLIGHT ENGINEERING THE VISCOUNT . . . circuit-breakers. Through these, main feeders are led to supply the pilot's, radio operator's and steward's services, each circuit being individually controlled and protected by circuit-breakers. In the event of an abnormal impact load on the aircraft, inertia switches isolate all supplies other than the fire extinguishers, the emergency saloon lighting, and the actuators to close the low-pressure fuel cocks and water/methanol cocks. These emergency circuits are closed automatically. A three-pin socket is mounted under the belly of the aircraft to take the engine-starting connection. When this connection is made, one of the pins operates a relay circuit to isolate the aircraft accumulators and off-load the generators. The accumulators can also be isolated manually. At present the engine-starting cycle is automatically controlled by a timing switch. A Dart will usually light up in from 12 to 15 seconds, and the button-holding and warning-lights will both be cut after this period; the pilot will then be able to start the next engine after about 20 more seconds—just long enough to set the controls. Although, as stated above, the generators are off-loaded by the act of plugging-in the ground starter, they can be brought back into circuit if required. At a minor airfield, for example, with only a small 24-volt trolley available, it is possible to start one engine and then bring the generators back onto the line. Two invertors are fitted in the main freight hold to provide an alternating-current supply for the Pacitor fuel-contents system and for the several electric instruments—the compass, Zero Reader, horizon, and the autopilot and A.D.F., for example—which take 3-phase, ri5-V current at 400 cyc/sec. A completely separate three-phase supply is provided for the electric de-icing of the engines and airscrews. For this purpose a 7.3 kVA generator is mounted on each accessory gearbox, supplying 400 cyc/sec current at 208 V. Each nacelle houses a two-speed cyclic switch which feeds a steady supply to the heater pads over the engine intake and an intermittent supply to, alter nately, the airscrew and the engine intake. The cycle switches over every 60 or 120 seconds. The de-icing pads themselves are discussed on page 370. It is quite impracticable to discuss here the electrically-operated services in any detail; but enumeration of the distribution of power at least serves to give an indication of the complexity of the electric system of a modern airliner. The engine and airscrew de-icing is, as described above, a completely self-contained system —owing to the high power consumption and intermittent nature of the demand. The main generators feed the power panels, from which the following services are led: Crash services circuits (4); landing-flap motor; feathering circuits; engine starting circuits; D.C. radio" supplies (14); steward's supplies (18); supply to the A.C. invertor for instruments, autopilot, and similar units (9); pilot's No. 1 supply (34); No. 2 supply (75); and No. 3 supply (30). And each of these supplies may, in turn, be subdivided to feed a large number of individual units. Hydraulic System.—The services actuated by this system comprise undercarriage retraction, nosewheel steering and main- wheel brakes. The hydraulic reservoir is situated in a cupboard"in a corner of the forward vestibule, together with a selector valve, Plessey actuator and an emergency hand-pump. From the reser voir, oil is gravity-fed to two Lockheed Mk 7 pumps driven by the gearbox of each outer engine. Both pumps deliver to a common supply pipe and a cut-out valve which maintains pressure between 2,000 and 2,500 lb/sq in. The system includes three air-charged accumulators. Each of these accumulators can provide sufficient pressure-oil for eight brake applications after failure of the main system. All the external charging and test connections are fitted on the starboard side of the nosewheel bay. Pneumatic System.—Easily the simplest system on board, this is provided solely for inflating the seals around the doors in the pressure cabin. It is supplied by a 300 cu in reservoir in the nosewheel bay which provides air at a pressure of 300 lb/sq in. From this reservoir, air is led through niters to a reducing valve from which the supply emerges with a pressure 4 lb/sq in above that in the cabin. A sequence valve is incorporated to permit or prevent the pressure flow when the door concerned is, respectively, shut or open; it also provides an exhaust for the air in the seals when the doors are opened. Pressurizing and Air Conditioning.—The Viscount em bodies a system engineered by Vickers, and embodying many Normalair components. The 6$ lb/sq in pressure differential can maintain 8,000ft cabin altitude at 30,000ft and pro rata. Three engine-driven blowers are fitted (both starboard engines, and the port inner), the delivery from which is filtered and passed through non-return valves, silencer and spill valve to a common duct which leads under the cabin floor to the temperature-control system at the rear of the fuselage. The temperature is controllable, by the steward or stewardess, to heat or cool the cabin air to any required degree. Generally, the compressor delivery will be too hot for comfort, notwithstand ing the very cold air at the cruising height of the Viscount. The three cabin blowers deliver whenever the engines are running, the excess being discharged through pilot-controlled spill valves. Any one compressor can, if required, maintain full cabin-pressure. From the temperature-controlling units the conditioned air passes to two main ducts along the sides of the cabin between the windows and floor, behind the skirting. From this duct the air can take two courses: entry to the cabin through a concealed vent in the duct adjacent to each seat station, diffusing through the wall- duct covering; or passage through individual controllable louvres above the windows. The latter feature is very unusual in a pressurized aircraft. A slight modification to the eleventh, and subsequent, aircraft is that the main airflow will flow into the cabin through holes in the base of the wall duct. The used air is extracted around the ceiling lights and through roof ducts to the underfloor compartment, by way of the rear toilet and luggage hold. The air from the flight deck and pantry is vented through the pantry floor and aft to the same outlets, thus preventing any food odours from entering the cabin. Warm air for windscreen de-misting is tapped from the flight- deck supply and accelerated by a blower. The blower delivery is then passed to louvres and diffusers around the pilot's windows, the supply being under the control of the captain. A ventilating fan is inserted in the delivery duct to the individual silent louvres and flight deck, for use during ground conditioning. Pitot-Static System.—A particularly complete installation is provided, served by a triple static-vent unit on each side of the nose and two electrically-heated pitot heads, again mounted on each side of the nose. The system is used in conjunction with the differential-pressure switch, undercarriage-lock switch, Zero Reader and V-g recorder, in addition to the usual instruments. Fire Extinguishers.—The extinguishing system is generally confined to the engines and is, therefore, dealt with in "The Story of the Dart" (pages 368-371). De-icing System.—Unusually, the Viscount employs three basic systems: thermal, fluid, and electric. The first system, on the leading edges of the wing and tail unit, uses exhaust-heated air which is taken in by intakes on the inboard side of each inner engine nacelle. This air is passed through a Gallay light-alloy cross-flow, single-pass heat-exchanger mounted in the exhaust branch from the inner engines. The proportion of exhaust passed through the heat-exchanger is thermostatically controllable. The heated air is then taken through ducts in the leading edges of the wing and tail surfaces from which it escapes by way of a slot—at the tail, a series of holes—in the front of the duct. After negotiating the slot, the hot air flows between \ the duct wall and the A section through the rear part of the B.E.A. cabin, looking aft, shows the following furnishings :— (A) red mottled pile carpet and underfelt; (8) light grey window surround; (C) red Vynid* piping; (D) aluminium satin finish; (£) off-white P.V.C. string racks for light luggage; (F) painted cream; (G) extraction duct; (H) cream Vynide; (J) Fibreglass soundproofing in flameproof bags; (K) duct to individual air louvres; (L) mid-grey Vynide; (M) woven fabric, mid-grey with white stripe; (N) main air duct; (0) dark-grey Vynide; and (P) wardrobe bulkhead.
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