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Aviation History
1960
1960 - 0008.PDF
FLIGHT, 1 January 1960 TOMORROW'S AERO ENGINES A Valuable Exposition of Bristol Siddeley''s Thinking ^:; •;• -^ THE James Clayton lecture for 1959 was delivered byDr. S. G. Hooker, OBE, BSC, D phil, technical director, Aero,of Bristol Siddeley Engines, to the Institution of Mechanical Engineers on December 16. He chose as his title "The Future ofAir-breathing Engines in Aviation." As is well known, he said, the 1957 White Paper on Defence saw little future use for mannedaircraft and placed the whole emphasis on ballistic missiles as a deterrent and guided missiles for defence. Yet [the lecturer con-tinued] there is still a great deal to do. This country has an opportunity to enter the medium jet airliner field and provide aso-called Viscount replacement. Nor does the future for manned aircraft seem entirely negative, approval for a tactical/strikeand reconnaissance aircraft for the RAF having been given this year. In the last 20 years the thrust of jet engines has increasedtwenty-fold. It is not expected that this trend will continue, because engines giving 20,0001b thrust are suitable for four-engined aircraft up to about 350,0001b gross weight, which can operate economically over the longest ranges and therefore thereis little incentive to go to a larger machine. On the other hand, supersonic military aircraft may requirereheat, which can give a thrust of over 30,0001b from an engine of the Olympus type. At an altitude of 36,000ft an engine of thissize without reheat will develop about 9,000 thrust horsepower at Ml and 29,000 thrust horsepower at M2, with specific fuel con-sumptions of 0.55 and 0.451b/t.h.p./hr respectively, representing an overall thermal efficiency of about 30 per cent. With reheatthe same engine develops the collosal power of 100,000 thrust horsepower at M2.2 with a specific fuel consumption of0.6251b/t.h.p./hr. Improvement in Specific Fuel Consumption. In parallel withthe increase in thrust, there has been a reduction in specific fuel consumption. Until 1946 most turbojets, being derived from theoriginal Whittle conception, had single-stage centrifugal com- pressors; these limited the compression ratio to around 4:1, andthe specific consumption was in excess of l.Olb/lb/hr (Fig. 1). Then came the Avon and Sapphire engines with single-spoolaxial compressors giving some 7 : 1 compression ratio and a higher efficiency, resulting in a consumption of 0.8 to 0.91b/lb/hr.In the early 1950s the two-spool axial compressor engine emerged, 16 1-2 I 0 08 06 04 02 DERWENT GOBLIN WHIT DERWEN1 GHOST TLE W.I. ENGINES WITH CENTRIFUGAL SA VON 'PHIRt f AVON • 1 .YMPL * r ENGINES WITH A>flAL COMPnc<:cnR<i - 5 _J DNWA / BYPA! OLYf Y 1 1PUS < BE. 58 S AN Ef1 1 3 DU GINE1- :TED- FAN 1940 1944 1948 1952 TEAR 1956 I960 1964 Fig. 7. Progressive improvement in turbojets: decrease in s.f.c. and increase in thrust/weight ratio, both plotted for sea-level static conditions; the s.f.c. of the new BS.7S fan is startling 16 14 £n Uu 8 Si 3a: I 4»- GHO *NEN (^" ^ERWENT WHITTLE W.I. 1940 1944 OR 1 ^VON1 I PHEUS |"DUCItD-hAN"| IFTING ENGINE rTDUCTEC 1 LIFTfT OLYMPUS* r X^\ -i—H^TlPERE ~AVON OLYMPUS I SAPPH 1 1948 IRE | i 1952 YEAR I9S6 HRUS WAY I960 -FAN r ENC;INE 1964 in the form of the Bristol Olympus, enabling compression ratiosin the range 10 : 1 to 15 : 1 to be achieved, and consumptions between 0.8 and 0.7 were obtained (in fact, an Olympus on test in1958 gave an s.f.c. of 0.681b/lb/hr). Concurrently the Rolls-Royce Conway appeared, embodyingnot only a two-spool compressor but also a by-pass of air from the low-pressure compressor round the engine and mixing withthe main flow in the jetpipe. This arrangement reduces mean jet velocity and temperature and results in an improvement in thepropulsive efficiency; and consumptions below 0.71b/lb/hr were obtained. The quantity of air by-passed in the case of the Conwayis by no means the optimum [it is being substantially increased —Ed.] and a later design, the Bristol Siddeley BE.58, with threetimes as great a proportion of air by-passed will give a specific consumption of less than 0.61b/lb/hr. By raising the compressionratio, a consumption of less than 0.51b/lb/hr can be foreseen. Such an engine is a real ducted-fan engine, and development ofthis type will form the main effort of most aero-engine companies in the 1960 era. The large boost in thrust which can be obtainedby afterburning in the fan stream will render these units attractive at both subsonic and supersonic speeds. Increase in Thrust /weight Ratio. In the early 1940s thethrust/weight ratio of the centrifugal engine was rather less than 2.01b thrust/lb weight. With the Avon, Viper, Olympus andConway, this figure has now risen to over 41b thrust/lb weight, and with the Orpheus a thrust/weight ratio of over 6 wasachieved. For military purposes, aircraft capable of very short or verticaltake-off are desirable, and this particular problem is at present receiving great attention. The type of powerplant which wouldbe suitable for such an application is the Bristol Siddeley BE.53 ducted-fan lift/thrust engine (similar to Fig. 2), which has a highthrust/weight ratio. If specialized engines used only for lifting purposes at take-off and landing are considered, then moderndesigns indicate that thrust/weight ratios of the order of 14 can be achieved. Such units would be ducted-fan engines of lowcompression ratio, a type in which Bristol Siddeley is actively interested (Fig. 3). Vertical Take-off Aircraft. Broadly, such developments canbe considered in two classes: single-seater aircraft and larger types. The single-seater would be of single-engine configuration,the powerplant being capable of the combined duty of both lifting the aircraft at take-off and landing, and propelling it in normalflight. Such an engine is shown in Fig. 2. The jet is deflected either downward to give vertical lift or in any inclined positionup to the horizontal in order to give a combination of lift and thrust. The airflow from the ducted fan at the front is bifurcatedinto two ducts, one on each side of the engine, as is the exhaust jet. These then terminate in directional control nozzles. An engine of this type has many advantages of a very practicalnature:— It can be mounted in the aircraft fuselage in the conventionalmanner. The jets can always be directed rearwards for conventional runwaytake-off and landing. The engine can be tested and the aircraft taxied with the jetsdirected rearwards avoiding problems of ground erosion and debris. For short take-offs, the aircraft can be accelerated with the jetsdirected rearwards and then the nozzles can be deflected downwards giving a combination of lift and thrust. In this way the aircraft willsweep its debris behind it away from the intake. Even for a true vertical take-off, the engine can first be openedup to full power and then the nozzles rapidly turned downwards to give the minimum time for erosion and scattering debris.Combining lift and thrust in one engine, the logistic and main- tenance problems are reduced to a minimum. For transport aircraft multiple powerplants are needed. Sincethe main problem is to obtain a thrust equal to the aircraft's weight, it follows that all engines should be capable of givingtheir full power in the form of lift. In other words, engines used solely for propulsion are undesirable. Once in normal flight mostof the engines can be shut down, since the cruising thrust required is perhaps one-tenth of the weight of the aircraft. The engineswhich will be shut down during cruise and only be used for take- off and landing must be specialized lift engines and have minimumweight. An engine of this type is shown in Fig. 3. Let us consider a transport aircraft (Fig. 4) with two BE.53lift/thrust engines, one under each wkig, and four specialized lift engines installed in the fuselage. If the thrust of the formeris 15,0001b and that of the latter 10,0001b, then the total installed thrust is 70,0001b. To provide for the possibility of an engine out,the weight of the aircraft for vertical take-off must not exceed
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