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Aviation History
1958
1958 - 0262.PDF
276 FLIGHT, 28 February 1958 DART up to DATE Power for the Viscount 810: the Rolls-Royce Dart RDa.7/1. I Development of the Famous Rolls-Royce Turboprop for the Viscount 810 IT is now some years since the first full description of theRolls-Royce Dart was published in Flight. The engine des-cribed on March 20, 1953, was the RDa.3, as was installed in B.EA's Viscount 701s under the mark number 505. None of these engines is still flying, as all have been converted to Dart 506s—which are de-rated to 1,400 sJi.p. for take-off; they have thinned and polished rotating guide vanes and improved guide vanes on the high-pressure turbine. This engine was described in an article on Dart development on January 14, 1955. Dart and Viscount development have, of course, gone hand in hand ever since the inception of the airframe; and when, earlier this month, the Viscount passed the landmark of one million revenue hours, so did total Dart hours exceed four million. Over 2,000 Dart engines of all marks have been built and the overhaul life in North America is now in the region of 1,700 hours. These are impressive figures by any standards, yet the Dart is still by no means at the end of its development life and patently only at the beginning of many new civil partnerships: A.W.650, Dart Herald, Friendship and Gulfstream, as well as installations that have been prepared for the Accountant and Breguet Alize. It seems likely, with many components now reaching lives of 3,000 or 4,000 hr, that it will not be very long before engine overhaul periods of over 2,000 hr might be achieved. This would give a gas turbine, for the very first time, a longer overhaul life than any piston engine has ever had: Chicago and Southern Air Lines (now Delta) eventually achieved 2,000 hr "pulls" with the Wright C.I8s of their Constellation 749s—although, as in all piston engines, plug changes and tappet checks had to be made at more frequent intervals. Trans-Canada Airlines, who have so far achieved the longest Dart overhaul period of any operator (1,800 hr with their RDa.3 Mk 506s) have already run two engines for a 2,000 hr period. The Dart engine in the Viscount 810 is an RDa.7/1 Mk 525. This is a basic Mk 520 engine with a recalibrated flow-control unit, increased cruise flame temperature and modified controls incorporating an additional pitch stop for the propeller. The shaft horsepower at sea level is 1,800 with 500 lb jet thrust. The designation system has become, over the range of engines, a little complicated to follow, but the table below should help to clarify Mk 505 506 510 511 520 525 S26 527 528 529 540 541 Rating RDa.3 RDa.3 RDa.6 RDa.6 RDa.7 RDa.7/1 RDa.7tt RDa.7/2 RDa.7/1 RDa.7/2 ROo.10 RDa.11 s.h,p. 1.400 1,400 1.600 1,600 1,700 1.800 1,910 1,910 1.800 1.910 2,400 2.100 Thmct (Ib) 365 365 370 310 495 500 505 505 415 505 670 655 t.«.h.p. 1,540 1.540 1,740 1,720 1,890 1.990 2,105 2,105 1,960 2,105 2,660 2.350 r.p.m. 14.500 14,500 14,500 14.500 15,000 15,000 15,000 15.000 15,000 15.000 15,000 15,000 Gearratio 0.106 0.106 0.093 0.086 0.093 0.093 0.093 0.093 0.093 0.093 0.093 0.093 Application Viscount 700 Viscount 700 Viscount 800 Friendship Viscount 806 Viscount 810 A.W. 650 H.P. Herald Friendship Gulfstream Viscount 840 the position. The mark number applies to the particular installa- tion; it is also almost completely accurate to say to the particular customer, since even the most minor change (which is almost invariably required) calls for a different designation. It may, for example, be necessary to run a fire-warning wire a little differently, or a label may be written in a different language. The Ministry of Supply rating nomenclature applies to basic variations in the development of the engine and the suffix to its tune. Thus the RDa.7/1 (1,800 s.h.p.) is a de-rated engine, although it is confusing that the standard engine is the RDa.7/2 (1,910 s.h.p.) and the 7/1 and 7 (1,700 sJi.p.) are variations. The final point to appreciate is that after a trial of suffix additions for rating variations from a particular standard of engine, the method of nomenclature was changed; the de-rated RDa.10, for example, is the RDa.ll. This is a much simpler method—always provided it can be remembered where the change took place. The RDa.7 was developed from the RDa.6 and the latter engine from the RDa.3. This earlier stage of development was described in Flight of March 20, 1953; but before bringing the development story up-to-date, it is worth recalling a little of the earlier Dart development history. The form of the basic engine, with its two stages of centrifugal compression, seven canted combustion chambers and two-stage turbine is too well known to need re-description here. It is the Dart 506 version upon which, perhaps more than any other, the wonderful Dart reputation is based; but now at least 24 customers (including B.E.A. and Capital, the two airlines with the largest Viscount fleets) have aircraft equipped with the Dart 510 engine, and by 1956, 93 per cent of all engines were achieving their full overhaul lives. Briefly, the RDa.6 differed from earlier engines in that it had a redesigned reduction gearing with a helical low-speed gear train, wider teeth and larger bearings. As the table in column 1 shows, this engine introduced a lower speed ratio between pro- peller shaft and turbine gearing, with a consequent reduction in noise because the propeller tip speed was lower. An increase in efficiency was obtained by one other modification: on both high- and low-pressure turbine blades triple tip-seals were fitted to reduce spillage. This engine went into production during the summer of 1955. Development work on every Dart component is a continuous process—particularly so with so many engines in service—and the next stage, the RDa.7, is best visualized as a logical coming- together of work and experience with the RDa.6. The manufac- turers steadfastly maintain that there is nothing in the least remarkable about the RDa.7, that every change from the RDa.6 was only to be expected if Rolls-Royce philosophy was properly understood, and that every mechanical change was made with the greatest reluctance. But then, the Derby firm have exceptional standards. The RDa.7, in spite of an increased mass flow of 10 per cent, an increase of 365 t.e.h.p. and improved specific fuel consumption, is certainly very similar to the RDa.6 and hence, to earlier engines. The RDa.7 carries a basic shaft-horsepower rating of l,910»'t achieved by an increase in mass flow from about 20 to 22 lb of air per second, and by an increase in cycle temperature. In com- parison with the RDa.6, the flame temperature has been increased by about 15 deg C. The additional air throughput has been obtained firstly by an increase in maximum r.p.m. for take-off from 14,500 to 15,000 (from which there is an associated temperature-rise) and also by physical changes. Of the latter, one of the most significant is an increase in the diameter of the second-stage impeller rotor by 0.4in to 17.6in in order to increase the pressure ratio and effi- ciency above its former value; it is now nominally 5.62 :1. The first-stage rotor remains unaltered; and, although the throat areas of both diffuser stages have been increased, and a double-sided steel diffuser ring has been adopted, the compressor casing is no larger than in early engines—indeed, the basic castings through- out are largely identical. [Continued on p. 277
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