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Aviation History
1953
1953 - 0372.PDF
THE STORY OF THE DART . . . isolated from the fuel system, when an airscrew is feathered. The Airscrew.—The standard airscrew on present Viscounts is the Rotol Type (c) R.92/4-20-4/8, a four-bladed non-reversing left-hand tractor, equipped for electrical de-icing. Its variable- pitch mechanism consists of an hydraulically operated piston linked to the blade roots. The blade-angle range is from 4 deg for ground starting, through 21 deg for flight fine-pitch, to 85 deg 50 min fully feathered. The diameter is 10 ft. Each blade is of composite construction, and consists of a solid duralumin forging to which hollow beryllium-copper leading-edge sheaths are riveted. Electric anti-icing heater elements, consisting of resistance wires woven into glasscloth, are cemented within the leading edges, and current is supplied by the Rotax alternators mounted on the accessory gearboxes. The pitch-change mechanism is contained in the airscrew hub and consists of an annular double-acting piston enclosed within a cylinder attached to the front face of the hub. The piston is con nected through a linkage to the blade roots so that its linear move ment produces rotary action of the blades in the hub sockets. Forward piston-movement directs the blades into fine pitch, and vice versa. The controller unit governs selected engine speed by varying the power absorption characteristic of the airscrew, i.e., by changing the angle of the blades. It is essentially a conventional constant-speed unit, to which has been added certain electro- hydraulic valves dealing with the specialized pitch-control requirements of the turboprop It consists of a gear-type oil pump which delivers oil to the airscrew through a piston valve controlled by a spring-loaded centrifugal governor. Movement of the engine throttle lever, linked to the airscrew control rod, varies the spring loading on the governor weights, which in turn controls the oil supply through the piston valve into the airscrew dome. The pressure of the oil delivered by the governor to the airscrew is determined by two relief valves, set at 600 lb/sq in and 120 lb/sq in for coarse- and fine-pitch actuation respectively. The lower pressure is used for fine-pitch operation because the blade move ment in this direction is assisted by their centrifugal turning moment. When the fine-pitch stop is to be released, a switch is closed which energizes an increased-pressure solenoid in the governor casing. This solenoid opens a valve which admits oil at 600 lb/sq in. behind the fine-pitch relief valve, thus increasing its rating so that it becomes virtually locked. Oil at full governor pressure is thereby allowed to pass, via a spring-loaded valve opening at 250 lb/sq in, to the face of the lock piston. This pressure causes the piston to be withdrawn, allowing a spring collet to collapse, and the blade-control piston to move into the "ground fine" position. The feathering action of the airscrew is controlled by a separate pump and electric motor, mounted on the engine, which supplies high-pressure oil at 650 lb/sq in to the forward face of the piston. This pump is energized by the feathering push-switch in the cockpit, which is operated after the high-pressure shut-off cock has been pulled back to the "feathered" position. In addition to closing the fuel cock, this action lifts the governor piston valve into "positive coarse" pitch, allowing the feathering oil to enter the inner concentric oil-tube. Unfeathering is a reversal of the feather ing operation, the engine then being re-lit through a time circuit energized by the unfeathering button and throttle-lever micro- switch. Development in Operation.—On being adopted by B.E.A. for eventual use in their Viscounts, the Dart at that time presented two apparent problems. Not only was it to be the first turbojet for airline use, but it remained an entirely new engine with no extensive background of military service, which normally provided so firm a basis for the introduction of any engine into service with 370 FLIGHT Continued development: an impressive view of "Sir Henry Royce," first of B.E.A.'s Dart-Dakotas; 1,400 h.p. plus 300 lb thrust was now available for take-off (Dart RDa 3s). an airline. True, flight time had been gained on modified Lancaster and Wellington types, but much development and proving flying remained to be done in an attempt to work out operating procedures for the new engine. Little was operationally known at that time of either the turboprop problems to be expected by a civil operator, or of the Dart itself. The airline's Dart experience was gained on three types of aircraft j the prototype Viscount 630, a specially modified Dakota, and the Series 700 Viscount now going into service. We may glance with advantage at the brief details of each of these phases in turn. The Prototype 630.—During the early test and development flying of this type (described in some detail in "Development of An Aristocrat," pages 357-362), B.E.A. engineers were in close touch with the Vickers personnel; and a number of flights were made by B.E.A. pilots as part of the type's C. of A. trials. First main tropical trials were made in 1950, and in July of that year, the Viscount 630 prototype returned to this country after success fully completing these trials at Khartoum and Nairobi. During these trials and its earlier European demonstration tour, it had completed approximately 130 hours of simulated airline flying under the command of B.E.A. crews. With this background, a successful application for a commercial C. of A. was made, with the intention of operating the aircraft for a short period on passenger services on the London-Paris and London-Edinburgh routes. During this period of trial scheduled operations, from July 28th to August 22nd, engine maintenance duties were performed by Vickers and Rolls-Royce personnel, B.E.A.'s direct contribution lying mainly on the operating side. The 502 Darts of 1,250 e.s.h.p. were initially intended to be operated at 13,300 r.p.m. during climb, and at not more than 625 s.h.p. or 13,300 r.p.m. during cruise (the r.p.m. limit becoming dominant above 16,000ft). Following a test flight and the actual proving flight to Paris, however, revised limits were agreed, in order to obtain improved performance. These limits were 13,600 r.p.m. on the climb, and 13,400 during cruise (the latter corresponding to approximate chart-powers of from 790 s.h.p. at 2,000ft to 570 s.h.p. at 18,000ft). Although fuel consumption on the operational nights agreed in general with that estimated, the possible effect of small variations in climb and descent technique was seen to be great. Two of the conclusions drawn from the operations were that pilots should be fully aware of this fuel consumption variability and should be able quickly to select optimum procedures for any particular situation; and that methods of presenting performance data for pilot use should be further studied. Synchronization of the engines had proved difficult, particularly at night, when the airscrew blades could not be seen. On flights where many power changes were necessary to carry out traffic- control procedures, the lack of synchroscope or synchroniser tended to be a handicap. Fuel consumption summaries were provided and it was seen that, on operations with critical fuel reserves, a form of reference to compare fuel available with the planned reserve was essential. The general standard of engine serviceability for the period was remarkably high, admittedly due in some measure to the "personal attention" which the aircraft received. During the period, which included 138 flying hours, scheduled engine main tenance included pre-flight inspections, daily inspections (check 1) and 50-hour inspections (check 2a). Unscheduled maintenance was confined entirely to defects of a minor nature. The Dart-Dakotas.—To gain experience on the new engines under more or less normal operating conditions, a face-changing operation on two of B.E.A.'s Dakota aircraft was carried out, their normal engines being replaced by two RDa 3/2 (505) Darts. The first machine, G-ALXN, was collected from Hucknall in June, 1951, after completion of Rolls-Royce flight trials and issue of the C. of A. Performance tests and crew familiarization preceded a period of regular freight-carrying services by the two aircraft to Hanover, Brussels, Paris, Rome, Stockholm, Copenhagen, and Milan. It was during these operations with the Dart-Dakotas that B.E.A.'s engineers really came to know the Dart engine; although the normal scheduled passenger services with other aircraft had first claim on crews and on engineering labour, resulting in low utilizations for the two Dakotas, the experience gained on the turboprops was invaluable. The initial limitations set by Rolls-Royce on the Dart were based essentially on the criterion of jet-pipe temperature. Maximum specified r.p.m. and s.h.p. were such that the maximum permissible jet-pipe temperature was not exceeded in standard temperature conditions. These limitations were: 700 s.h.p., or 13,300 r.p.m. (whichever gave the lesser s.h.p.) for normal
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