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Aviation History
1960
1960 - 0473.PDF
FLIGHT, 8 April 1960 First photograph of a genuine Rolls-Royce RB.141. This example is one of the three which ran last November Airline Engines AN AUTHORITATIVE OPINION UPON TURBOFAN AND BY-PASS ENGINES AT the 1960 National Aeronautic Meeting in New York of the Societyof Automotive Engineers, a paper entitled "Turbofans and By-Pass Type Engines for Jet Transports," was read on April 8 by A. A. Lombardand D. Gerdan, respectively directors of engineering of Rolls-Royce and Allison. The joint authorship confirms the partnership betweenthe two firms in the design and development of airline gas turbines. The text, however, appears to have been written entirely in Derby. ALL the patents of Sir Frank Whittle—the paper opens—t\ displayed a remarkable degree of foresight, but perhaps L M. the most significant was that of the ducted fan or by-passengine, applied for on March 4, 1936. Ten years later an aft-fan engine was built and run by Metropolitan Vickers,1 and in thesame year Rolls-Royce became interested in the by-pass formula as a result of the Air Ministry B.35/46 specification for theV-bombers. The terms of this [not previously published.—Ed.] included M0.87 cruise at 50,000ft over a range of 2,450 to4,000 n.m. From the BJ.80 of February 1947 has been evolved the Conway,8 which first ran in August 1952 and has now beendeveloped to thrusts greater than 20,0001b. Last November the RB.141 went on to th» bench, and the second member of thisnew optimized family, the RB.163,9 will run in October. Of the factors which determine the suitability of an engine fora particular airline task the most important are fuel consumption, weight or bulk, and cost—in each case assessed on the fullyinstalled powerplant. In Fig 1 are plotted consumptions for a 473 /SPECIFIC FUEL"\ CONSUMPTION V LB/HR/LB BASIC ENGINE (NO OFF TAKES.REVEHSEH OB SILENCER) K> H 1-2 1-3 14 15 16 (BY-PASS RATIO) Fig 1. Specific consumption curves, cruising at 35,000ft at 495kt (i.s.a. + 20'F) range of by-pass ratios, assuming a pressure-ratio and turbine-entry temperature higher than current American practice. The fourth curve in this figure shows that, were fuel consumptionthe only criterion, the optimum by-pass ratio would be about 1.4. However, Fig 2 shows how weight penalties modify the picture;in fact on the score of weight alone the best engine is the pure jet. In Fig 3 the curves are again plotted for an airliner perform-ing a given mission. At the 1,000 n.m. stage chosen the best by-pass ratio lies between 0.65 unsilenced and 0.8 when silencedto very low noise levels. The optimum by-pass ratio rises slightly as stage length is increased, but at 4,000 n.m. the variation withby-pass ratio is small and the advantages of going beyond unity are negligible. Price is brought in with Fig 4, which assessesdirect operating cost for a 1,000 n.m. stage. For the D.H.121 BEA have specified a ground perceived noise level 2i miles frombrakes-off of lOOdb (6db quieter than the Comet 4 or Viscount). The overall result is that the by-pass ratio should lie between0.7 and 1.1, the actual value depending on the allowable noise. Jet-transport operations have shown that, although perceivednoise level can be brought below that of the worst piston-engine transports, public annoyance cannot be avoided and performancelosses are disappointingly high. The value laid down for the D.H.121 represents a tremendous advance. Analysis shows thatthe by-pass engine is no noisier than the pure jet, whereas the fan engine appears to have a higher intake noise. The only clearsolution to the latter at present is to design the first compressor stages with moderate loading and tip speed. Mixing the hot andcold flows in a by-pass engine not only improves cruise consump- tion by over two per cent but also reduces noise level by about5db. The overall configuration9 specifies a single direct intake and a homogeneously mixed efflux passing through a reverserand common propelling nozzle. The reverser6 is engineered as an integral part of the engine and deflects the jet forwards atabout M0.5 at some 45° to the engine axis to give approximately 50 per cent reverse thrust. By selecting an intermediate reverserposition it is possible to obtain values of thrust at or near zero while maintaining high engine r.p.m. for boundary-layer control. One factor which can be seen in Fig 5 is that as by-pass ratiois increased so does the beneficial effect of increasing temperature become more pronounced. (Assuming that the engine is notshort of power to meet natural aircraft growth, a simple com- mercial turbojet shows virtually no gain from increased tempera-ture.) In order to permit high turbine temperatures, while preserving normal metal temperatures consistent with long bladelife, Rolls-Royce have developed a technique of cooling the first- stage turbine rotor blades with air tapped from the high-pressurecompressor. Since 1948, both the cooling technique and the Fig 3. Results applied to a particular task (iflOO n.m. stage) (BY-PASS RATIO] (BY PASS RATIO) Fig 2 (left). The effect of allowing for installation penalties (engine + fuel weight at landing for the ARB. 140-11): A, aircraft accessories and pod systems; B, engine net dry weight; C, reverser; D, pod structure and landing i» gear variation; E, silencer weight; F, fuel reserves; SF, silenced for flyover; SR, silenced for runway noise; U, unsilenced
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