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Aviation History
1956
1956 - 1103.PDF
FLIGHT, 17 August 1956 FOUR DERBY WINNERS Rolls-Royce Commercial Gas Turbines urn The new Dart 520 (KDa.7) turboprop. By THE TECHNICAL EDITOR IT has long been appreciated that the supply of any equipmentto an airline is a basically different business from that associa-ted with most other branches of aviation. To a high degree the position is akin to that in the longer-established fields of industryand surface transport in that the operator generally shows sales- resistance to "new-fangled" equipment or to the products of amanufacturer who does not already enjoy a considerable back- ground of success among airline customers. This philosophyapplies particularly stringently to the supply of airline power- plants. The engine is really the heart of any aircraft and noairliner can be truly attractive unless it is powered by an efficient, long-lived and trouble-free engine. Furthermore, in most cases,an engine starts off at a disadvantage if it is not the product of a company with which the airlines have already established along and generally happy partnership. Within these limits there has always been room for severalmajor engine manufacturers to do business. In die past the bulk of the market has been satisfied by America. This still holds truetoday and more than half of the four-engined equipment at present on order is to be powered by engines made by Pratt andWhitney or Wright. Nevertheless the picture is changing in a remarkable manner. By any standard, the advent of the gas tur-bine is a factor of the utmost consequence, and it has offered opportunities which British manufacturers have been shrewdand capable enough to grasp. Rolls-Royce in particular have established themselves as the only company in the world who have"got their feet wet" in the commercial gas-turbine business; and it is generally agreed that lasting success in the airline businessattends only those who get their feet wet. (The converse, of course, is not necessarily true, neither does it follow that wettingone's feet must involve a protracted fight against a sea of troubles.) Having soaked themselves up to their knees in the river Dart,the great Derby company have been sufficiently encouraged to spread the whole basis of tiieir civil-engine development. In factthey are increasing the scope of their operations to such an extent that another five years may see them ousting all the greatAmerican companies as the source of most of the world's air- transport power. Never before has any company in any countrybeen on the brink of so much business in this highly competi- tive field. Neither has any company previously been able to offerfour quite different families of truly commercial engines matched to the requirements of virtually the whole spectrum of airlinerdesign. In this article it is proposed to recount some of the Rolls-Royce experience and to outline their philosophy. Dart Turboprop, As is well known, the bulk of the company'sexperience of commercial turbine operation has been gained with this engine. It has been argued by American engine companiesthat the value of this experience is reduced by reason of the fact that the Dart is a relatively small engine of early design. Sucha contention largely misses the point of the commercial-engine argument, for actual airline experience over a period of years isinfinitely more valuable than any amount of military flying. As an example it is possible to cite the famous Merlin piston engine.After World War 2, Rolls-Royce believed that the millions of hours flown by the Merlin in all kinds of military operation wouldmake it a simole matter to develop a commercial derivative. It was soon made painfully clear, however, that, from the airlineviewpoint, the Merlin was an almost totally unknown quantity and the company might almost have had to start from scratch.Much the same is true of the Dement turbojet, for, in spite of its record of more than four million hours as a reliable fighterengine, scarcely any Derwents have been flown in airline fashion and few Derwent components have exceeded 1,500 hr. The remarkable development of the Dart is shown in Table 1.The history of this engine is unique. There never was a military Dart. The design grew from nothing—unless one includes thesupercharger of the last Rolls-Royce piston engines—and grew up with the airlines, with particular emphasis on British Euro-pean Airways. It is typical of the former chief executive of this corporation that he once referred to the Dart as "agriculturalmachinery." He said this not with acidity but with affection, for in the airline business the dominant factor is not so muchbrochure performance as the way in which the engine works. In the Dart, operators have an engine that they can really livewith in comfort. Rolls-Royce have already collected quite extraordinary testimonials from over a dozen commercial opera-tors who have described the Dart's behaviour in glowing terms. Already the operating figures on the Dart are exceptional. Theoverhaul life of 1,050 hr was first achieved by B.E.A. almost two years ago, when the engine had been on the routes for only18 months with an operator working at the low utilization of five hours per day. Operators can now reach the 1,050-hr markin a much shorter time; last October Hunting-Clan reached this figure after four months.Outside North America the Dart is generally established at 1,050 hr. Throughout 1955, flame tubes were normally changedat 500 to 600 hr, but today a revised design of flame tube (referred to below) is employed, with the same 1,050-hr life asthe remainder of the engine. In North America competitive airline operations tend to demand higher powers at lower alti-tudes, imposing rigorous operating conditions with high flame- temperatures and mechanical loadings. Past operations byT.C.A. and Capital have been on a 1,400-hr basis with a change of the first-stage compressor impeller at 700 hr. Next year the Dart will certainly be placed on a basis of1,500 or 1,600 hr with no parts-change whatever, except for the various filter elements. This type of performance fullymatches the best which can be achieved with reciprocating engines. In the gas turbine, however, quite a different kindof cost-structure prevails. Each piece of the engine has a particular authorized life and cost, and something like 80 percent of the operating cost on the Dart is in pieces lifed at more than 2,000 hr. It is, therefore, impossible to determine a com-pletely representative cost-per-hour figure until the carcase-life of individual engines has reached at least 3,500 hr. Turning to the mechanical features of the engine itself, thebasic Dart 505-506 has already been fully described in past issues, that of January 14, 1955, being particularly informative. Virtu-ally all the 505s have now been converted to 506 standard during normal airline operation. Relatively recent modifications to the506 include a new design of first-stage impeller and a flame tube incorporating skin cooling along lines suggested by Dr. J. S.Clarke of Joseph Lucas (G.T.E.), Ltd. As an indication of the beneficial effect of skin cooling, seven such tubes (a complete setfor one engine) were recently returned for a further 500 hr after inspection at 1,230 hr. Thus flame tubes, which last yearwere removed after 500 or 600 hr, are already cleared for one full engine life and are considered to be good for double this lifewithout attention. Dart 505s and 506s were fitted to 124 Viscounts which have gone "out of the door" at Vickers and itis on these that most of the airline experience has been built up. Most Darts at present in production are of the series 510,in which an increased-capacity helical reduction gear provides for greater take-off horsepower, and other modifications improvethe power and specific fuel consumption under all conditions of operation. It is pertinent to emphasize that certain short-hauloperators are particularly glad to receive the 510 owing to the increased climb performance which it offers. The restrictionimposed by the Dart 506 reduction gear on low-altitude shaft horsepower has been of little consequence in Europe, where theairline accent is on economy; in America keen competition has led to sustained Viscount operations at high power and at relativelylow cruising altitude and here the 510 has proved invaluable. Into the 510 has been built all the experience gained with the
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