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Aviation History
1953
1953 - 0373.PDF
20 March 1953 37J On the line at Weybridge: with cowlings and nacelle fairings removed, the whole power-plant installation of the Viscount is here seen to advantage. cruise, corresponding to jet-pipe temperatures of from 515 deg C at sea level to 490 deg C at 25,000ft. To prevent over-heating at high ambient air temperatures, the variable datum fuel control device enabled the engine-speed fuel-flow relationship to be varied or "trimmed" by the pilot. Engine runs to 12,000 r.p.m. before take off permitted the fuel trimmers to be set to obtain the appropriate jet-pipe temperature, given on a cockpit card. The required cruising r.p.m. (below 13,300) to give 700 s.h.p. was obtained from r.p.m. selection charts. Thus a pre-take-off adjustment corrected the fuel flow for the particular airport altitude, and if the ambient air temperature were above I.C.A.N., an increase in jet-pipe temperature for cruising flight was permissible. The development of a simple presentation for the pilot of the calculations involved in these adjustments was a most important part of the Dart Development Unit's work. Premature engine removals were occasioned during the project by fatigue failure of the reduction gear lay shaft bearing cages— the only main problem experienced on the engine itself—and this was tackled by material and design changes by Rolls-Royce, and by 150-hr reduction gear inspections on unmodified engines by B.E.A. In general, the engines themselves were remarkably trouble-free, and the majority of defects experienced occurred in the various accessory systems. One such problem, by no means confined to the Dart, was that of generator brush wear at high altitudes: various types of brushes were tested in order to reduce this trouble. To increase ease of starting, especially in cold weather, the use of Murex 28-volt ground starter trucks was adopted. Two major Dart developments during the Dakota operations were the introduction of the engine and airscrew de-icing system, and of the water /methanol injection system, performed by Rolls- Royce and B.E.A. in close co-operation. Electrical surface heating is used for de-icing leading edges of the airscrew blades, the spinner, the oil cooler intake duct, and the engine air intake. The introduction of the new de-icing intake nose cowl caused the earlier concentric intake for the oil cooler to be replaced by the separate top-mounted unit. In the icing tests performed, the equipment's operation proved satisfactory, but some associated radio interference was reported. The water/methanol injection unit was introduced in order to restore power loss on take-off at high ambient temperatures, and so improve take-off distance and emergency single-engine climb at maximum r.p.m. Its development was studied not only on the Dart-Dakotas, but also to a large extent during the Viscount 630 and 700 trials. Much valuable information on warm weather operation with this unit was gained. Parallel with the steps to improve the pilot's presentation of engine information came the need to improve the system of measur ing and indicating the exhaust gas temperature, a problem effec tively tackled by Rolls-Royce. This resulted in the adoption of a new thermocouple system mounted on the exhaust cone instead of on the jet-pipe. This system overcame the variable effects experienced on the previous jet-pipe thermocouples, and was also a step nearer the ultimate aim of flame or nozzle guide vane temperature measurement. Temperature control to an accuracy of more than ±10 deg C could still not be guaranteed, however, since the gauge scale remained unaltered. Concerning presenta tion, the early charts were replaced by simplified charts and by direct scales; the idea of a special computor was not developed. ^ Over the period of B.E.A.'s Dart-Dakota operations, the engine overhaul life was extended from the initial 250 hours to 300 hours, and then to the existing 400 hours. At present, the remaining Dakota is flying at a trial cruising setting of 13,600 r.p.m.; it is hoped with this experience that the normal cruise setting of the Viscount will soon also be at this figure. Viscount 700 Series.—Some 550 hours' flying was completed by B.E.A. on the prototype Viscount 700 (G-AMAV), including 250 hours on an extensive series of proving flights, during which the engineering and operating experience obtained from the Dart- Dakotas was applied to the engines of the Viscount, with notable advantage. From this comprehensive series of flights on many different routes the following main conclusions were obtained. Engine starting proved simple, provided the necessary power (28 volts, peak load 900-1,100 amps) was available. Attempts to start the engine with insufficient power, however, could cause serious damage through over-heating. One disadvantage from the pilot's point of view was the problem of determining the power obtained from the engine. Instruments available for this were the torquemeters, j.p.t. gauges, and the flowmeters, none of which was accurate enough to determine the power exactly. This problem was particularly serious during take-off. Water/methanol was used for assisted take-offs at aerodromes whose temperatures ranged between I.C.A.N, conditions and I.C.A.N. 4 20. It proved very effective and restored the take-off power to that of I.C.A.N, conditions in practically every case. The electric engine and airscrew de-icers were given extensive trials and proved satisfactory (as did the thermal airframe de-icers), although serious icing was not encountered on many occasions. In general, the engines performed very well, and came up to expectations. Individual engines, however, often produced varying horsepowers over fairly wide ranges for the same r.p.m. setting, due to a number of reasons. Such variation in perform ance could prove to be a disadvantage in scheduled operations. In flying the aircraft, pilots again reported difficulty in maintaining synchronization of the engines, it being possible to move the throttle levers approximately iin without response from the engines. As part of the engineering development, a modified combustion chamber was tested in an attempt to reduce the specific fuel con sumption at a given power. Initial assessments of operating procedures were continuously checked. Hopes of taxying and stacking on two engines were discarded, due to engine starting and re-lighting uncertainties respectively (de-icing being a problem with engines stopped in the air). Fuel trimming while taxying out for take-off proved im practicable also. The fundamental ground and air procedures thus learned and refined were borne in mind during the checks and C. of A. tests of the first production 701 aircraft In regular service, B.E.A.'s Viscounts are to cruise at 13,300 r.p.m., and the engine overhaul life will be 400 hours, although an early increase to 500 hours is expected. With this impressive amount of Dart experience gained in continuous co-operation with Rolls-Royce, B.E.A. is able to bring both a tested operational method and thorough engineering knowledge to its first regular services with the Discovery-class Viscounts. The only radically different feature of the aircraft lay in its power plant, and the small number of major snags experienced on the Dart is a deserved tribute to its makers. Much further development lies ahead and the signs are that increased power and extended overhaul lives will accompany an enhanced reputation for reliability and economy. K. T. O. ENGINE SUPPLIERS The following is a list, supplied by the manufacturers, of the major suppliers of materials and equipment for the Rolls-Royce Dan. Light-alloy Forgings.—High Duty Alloys, Ltd.; Light Metal Forge, Ltd. Steel Forgings.—D. Doncaster and Sons, Ltd.; Firth-Derihon Stampings, Ltd.; Hughes-Johnson Stampings, Ltd. Steel Centrifugal Castings.—Sheepbridge Stokes, Ltd.; Firth-Vickers Stainless Steels, Ltd. General Components and Accessories.—Avica, Ltd., flexible piping; Automotive Products Co., Ltd., filters; Dunlop Rubber Co., Ltd., cowls; Fischer Bearing Co., Ltd., bearings; H. M. Hobson, Ltd., water/ methanol units; Hoffman Manufacturing Co., Ltd., bearings; J. W. Lawrence, Ltd., oil coolers; Joseph Lucas, Ltd., fuel systems; Plessey Co., Ltd., ignition plugs; Rotax, Ltd., starter motors; S. Smith and Sons, Ltd., electrical equipment; Wellworthy, Ltd., sealing rings. "CONSTRUCTOR AND OPERATOR" I T will be recalled that in Flight of January 16th, Mr. R. C. Morgan, B.E.A.'s chief project and development engineer, contributed an important article, under the above heading, on the liaison which has taken place between the Corporation and Vickers-Armstrongs, Ltd., during the Viscount's development period. A limited number of reprints of this article are now available, free of charge, from Flight's offices.
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