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Aviation History
1960
1960 - 1957.PDF
Round the Stands ENGINESL AST week, in the section "Highlights and Sidelights of theStatic Show" we reported briefly upon present thinking in J the propulsion of supersonic aircraft. Here it is appropriateto say something of the manner in which engines are being developed for the currently more important market of high-subsonic aeroplanes. It is generally conceded that the opt'mum engine is a ducted fan, or by-pass engine, or turbofan—and thereare even other names for the same thing—with a cold/hot ratio of about unity (say Rolls-Royce) or at least 1.5 (say BristolSiddeley and the Americans). It is probably too early to assess which group is nearer the truth, and telling arguments could beadvanced on the Rolls-Royce and Bristol Siddeley stands in support of both. We were able to publish an inboard profile of the new BristolSiddeley BS.7S last week, and there is no doubt that this is on paper a superb engine. Moreover, the direct application of experi-ence with earlier gas turbines should ensure that it will also be good as hardware. As we noted on August 12, it has been sized to afan flow of about 2001b/sec; cold/hot flow ratio is approximately 1.7, overall pressure ratio of the order of 14:1 and turbine entryno hotter than l,200°K, so that design thrust comes out at 7,3501b. This was the optimum figure for the BAC-107 airliner, and theengine has now also been selected for the rival Avro 771. Both are twin-engined, so on this score Bristol Siddeley cannot hope to sellmore than half as many engines as Rolls-Royce did with the Dart (assuming that one new jet will be bought to replace eachViscount). Moreover, it is unlikely that anybody will cut metal on a four-BS.75 airliner, since this would duplicate the Trident. Onthe other hand, a three-BS.75 project seems attractive, and this might well be a future Caravelle variant. Returning to the school of unity by-pass ratios, the first itemto fasten upon is the remarkable growth of the Rolls-Royce Conway. It is unfortunate that the roots of this great engine extendso far back into the past, since they have tended to hold back the Derby engineers in their quest for higher by-pass ratios. Thereis no point in denying that the ratio of the engines currently in service is below the optimum, at 0.3 (but this is better than thestark "zero" of its competitors). Freed from the confines of the Victor's wing, Rolls have since pushed ahead, and in the RCo.42Mk 540 introduced a new low-pressure assembly giving a ratio of 0.6. Now it seems clear that they have taken the bold step of pro-ducing an engine known as the Conway 7 by scaling their latest existing transport engine, the RB.163. The latter has a total massnow only Zlb/sec greater than that of the BS.75, but owing to its lower flow ratio (1.0) gives ratings in the 10,0001b class. ScalingK up to a mass flow of 4801b/sec would produce an engine of around 24,0001b thrust, and it seems clear that this is whatRolls-Royce intend to do under the designation Conway 7— although they are not saying so openly. To a considerable degree the huge cost of developing a newfamily of advanced engines in the 24,0001b class (and doubtless having growth potential up to about 30,0001b) will be minimizedby the solid experience already gained with the earlier Conways, Kb.141s and RB.163s; in fact almost everything learned aboutwe 163 should read across directly to the big engine. Beyond the wet _ that the Stage-7 engine will start off with a guaranteedminimum rating of 24,0001b nothing has so far come out of the ucrby citadel—but one very interesting conclusion can be gleanedtroni a study of Vickers' brochures on the Super VC10. Whereas the original VC10 is being fitted with the RCo.42/2 ofM501b thrust, and the first series of "Supers" with the 22,5001b Flight" photographs and copyright drawings RCo.42/3, the heaviest versions so far envisaged will have thenew Stage 7. In parenthesis after this statement Vickers insert the words "Allison AR.972-2." It has long been known that thisdivision of General Motors are engaged in technical discussions with Rolls-Royce, and we have already published the fact thatthey would like to build a variant of the RB.163 known as the AR.963. It is not too much to deduce that engineers fromIndianapolis are co-operating in the development of the most powerful commercial engine in the world, and that AR.972-2smay well one day flow from the plant at Indianapolis. Switching to another topic entirely, it is worth commentingupon the outstandingly long overhaul life of several British engines. On August 22 the Dart 506s of TCA were cleared torun 3,000hr between overhauls—by far the greatest overhaul life of any aero engine. More than 12,000,000hr have been flown inairline service, and RR's stand contrasted the Dart 505 of 1953 (1,480 e.h.p.) with the Dart 542 of 1960 (3,030 e.h.p.). Close by,on the stand of the Smiths Group, were a pair of KLG KR.104 igniters, each of which had completed 2,500hr in Darts of BEA;this represents some 1,800 starts and at least lhr 32min in full icing conditions. Back at RR one could see a commercial Avon524, No 35088, pulled at l,995hr and due to go back into a BOAC Comet for another 3O5hr. Finally, Bristol Siddeley displayed aProteus 705, pulled from a BOAC Britannia on the completion of its authorized 2,400hr since last overhaul. Indicative of the manner in which gas-turbine aerodynamicistscan improve the performance of a given size of axial compressor were three examples of Gazelle compressors displayed byD. Napier & Son Ltd. The standard production engine has an eleven-stage spool known as the RA compressor; at 19,800 r.p.m.it achieves a pressure ratio of 6.2:1 with a mass flow of 16.61b/sec while absorbing 2,280 h.p., and its efficiency of 87.5 per cent risesto an optimum value of 91 per cent at 87 per cent of the design speed—a typical cruise condition. At the research station atLiverpool work has led to the ten-stage RB compressor, which at the same rotational speed handles 20.91b/sec with a pressureratio of 7:1 while absorbing 3,170 h.p., the corresponding efficiency being 86 per cent. More remarkable is the Rig 2Ahigh-pressure-ratio compressor, the fourteen stages of which achieve a pressure ratio of 11:1 at 18,250 r.p.m. while handling171b/sec with a power absorption of 3,500 h.p. and an efficiency of 84 per cent; at 10 per cent overspeed this compressor achieveda surge pressure ratio of 15:1, while absorbing 5,000 h.p. This suggests future Gazelles of over 2,000 h.p. Perhaps the most fundamental part of any air-breathing engineis its combustion system. For many years the largest British company specializing in this field has been Joseph Lucas (GasTurbine Equipment) Ltd, and this year three exhibits indicated current lines of research. The first, of a purely theoretical nature,was a small test chamber for the investigation of means whereby the maximum heat-release may be obtained per unit volume. Of more immediate importance to existing engines is the com-pany's work on equi-velocity sampling (EVS) and the achievement of optimum air distribution to the combustion chamber. Thiswork has been directed at overcoming inperfections from varia- tions in the velocity profile at the entry annulus to the combustionsystem. Since these variations are mainly across a radius, rather than around the periphery, air required for delivery to each sectoris sampled from across the full depth of the annulus. This system is directly applicable to an annular chamber, but when separateflame tubes are employed Lucas have evolved an air-distribution unit (ADU) in which curved scoops pass full-depth samplesalternately to the outside and inside radius of each flame tube to produce uniform air-delivery conditions upstream of the chamber.
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