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Aviation History
1956
1956 - 1521.PDF
FLIGHT, 26 October 1956 677 ORION First Details of Bristol's New Turboprop DURING the early years of the present decade, the designengineers of what is now Bristol Aero-Engines, Ltd., fre-quently held informal discussions about the optimum future design of propeller turbine engines. It was then alun-dantly apparent that development of military turbojets such as the Olympus had resulted in the achievement of advanced thermo-dynamic cycles with pressure ratios and efficiencies appreciably higher than had been attained in any turboprop. By 1952 it wasclear that it would be possible to design a relatively straightforward turboprop with a "straight-through" layout, two-spool compres-sion, short combustion chambers and a highly efficient turbine in order to obtain very high power from a relatively small bulk, with aspecific fuel consumption no greater than that of tae most efficient piston engine. A prerequisite to any engine design is a clear knowledge of thetype of work which the eventual engine may have to perform. The obvious application for a new Bristol turboprop was thepropulsion of later types of Britannia and its projected successors. At the outset discussions centred upon a project study, designatedBristol B.E.25, with a maximum sea-level rating similar to that of the Proteus, namely rather more than 4,000 e.s.h.p. Comparedwith the Proteus, the first B.E.25 project differed in having a straight-through, as distinct from reverse-flow, layout and com-pounded axial compressors. With a mass flow fixed at 40 lb/sec (the Proteus is about 43 lb/sec) and with a higher maximumtemperature and pressure ratio and improved component effi- ciencies, it was immediately apparent that the engine could, forthe same output, be much lighter than the Proteus, with a smaller installed diameter and 20 per cent better fuel consumption. Thisinitial B.E.25 project was studied in three basic configurations: a single-shaft engine; a "single-spool" free-turbine engine; anda two-spool unit with the reduction gear driven off the low- pressure assembly. Of these three arrangements only the third showed real promise.The first was rejected on various counts, including the inflexibility of the high-pressure compressor and the requirement f< r blow-offvalves and variable whirl, the large starter power and the need for high idling r.p.m. to reduce acceleration time and increase pro-peller drag in the approach condition. The free power turbine was also rejected, since it was considered probable that easy startingand engine handling could readily be inherited from the split- compressor turbojet, and that it would be possible to avoid thefull severity of coupling the propeller to the compressor. Thus, instead of receiving the full force of either of a pair of problems,the chosen configuration suffered a small problem from each, the rotating mass attached to the propeller being only quite moderatebut the mechanical linking of the propeller and low-pressure assembly preventing the perfect matching of the two compressorspools in the manner which takes place automatically in an engine like the Olympus. . _„,. ,. Havin<* decided upon the geometrical layout for the B.fc.25, tneprincipal problem remaining was to find the optimum size for the new engine. Investigation revealed that the Britannia with fourProteus was nowhere near its Mach limit and could in fact utilize considerably greater cruise power. Furthermore, future long-range transports were likely to require substantially more power than could be provided by the Proteus. Fig. 1 shows the actualdesign point of the final B.E.25 project. Dr. S. G. Hooker, the chief engineer of Bristol Aero-Engines,set his sights at a cruising power of 3,500 e.h.p. at 350 kt at 30,000ft, or exactly double the cruise power of the Proteus 755.Also written into this basic specification was a specific consump- tion not greater than 0.4 and the minimum of weight increase overthat of the Proteus. Extrapolation from the design point back to sea level results inan engine with a maximum output of over 8,000 e.h.p., and it was abundantly evident that this would not only pose a difficult develop-ment task, but would prove an inordinately weighty engine with excessive potential take-off power, either for the Britannia or forits intended successor. Fortunately, however, a very happy solu- tion to this anomaly of excessive power presented itself. Partial-power curves of the projected high-pressure, two-spool engine showed that the specific consumption of such a unit would berelatively insensitive to throttling. In the Proteus, for example, restriction of power results in an increasingly rapid rise in specificconsumption, but the B.E.25 project promised to exhibit this characteristic in a much more restrained form, primarily as aresult of its more efficient thermodynamic cycle and better com- ponent efficiencies. The way was thus opened for the Bristoldesigners to plan an engine which, from the Britannia take-off viewpoint, would be markedly oversize, and then throttle it toa specified maximum output while still showing a substantial improvement over the consumption figures for the Proteus. Owing to the propulsive characteristics of propellers, the turbo-prop provides a very great take-off thrust in relation to its cruising output, and this characteristic further accentuated the excess oftake-off thrust available from the projected turboprop. This was another pointer to the solution of restricting the take-off powerin order to effect appreciable savings in weight and bulk in the reduction gear and propeller. In fact, it was soon apparent that,as the shaft-power would never exceed 4,400 s.h.p., the new engine could quite readily be designed to incorporate the existing Proteusreduction gear and drive the standard de Havilland 16ft propeller as now in production for the Britannia. Material advantage wasthus gained by designing a large engine, throttled for take-off, and, in the case of the Britannia, also under cruising conditions. Com-pared with the basic Proteus, the B.E.25 project was expected to weigh slightly more owing to the fact that the gas generator portionhad to be increased in size in order to handle double the mass flow. At the same time the basic B.E.25 promised to offer amplereserve power for future Britannia development and also—for the first time in any gas turbine—to maintain its full sea-level ratingsunder all airfield conditions of altitude and temperature. The latter, which forms an interesting analogy with the power curveof a supercharged piston engine, has given rise to the appellation "the supercharged turboprop," frequently applied to the B.E.25by Bristol spokesmen; other adjectives which have been employed are "constant-power" and "power-limited."In any split-compressor engine one of the first design tasks is the determination of the optimum work-split between the twocompressor spools. In the Olympus turbojet the low-pressure (front) spool has a lower pressure ratio than its counterpart, the
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