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Aviation History
1977
1977 - 0144.PDF
134 RIGHT International, 15 January 1977 TOMORROW'S POWER •4- page 116 An improvement of between 3y and 5j per cent in specific air range of the TriStar is reported as a resuit of fitting the RB.2I I with a 15 afterbody in place of.the original 11c design approach might also make it possible to dispense with, or greatly reduce, the nacelle acoustic lining. The length of the fan nacelle could therefore be much less, its design being governed by various performance requirer ments, and the need to engineer a thrust-reverser might be eliminated by using reversible-pitch fan blades or even by relying on the fan's substantial idling drag. With the considerable reduction of installation drag and weight offered by a "minimum cowl," and the de coupling of the fan and 1-p turbine design constraints, we could then go for a further significant reduction in a specific thrust to about 50N/kg/sec—about one-third of the current figure. The corresponding bypass ratio is about 20, with fan pressure ratio reduced to 1-25 from the present value of about 1 • 6. Cycle pressure ratio would be similar to the previous example, with perhaps a 50-100°C further increase in turbine-entry temperature. Although the engine itself would be about 70 per cent larger in diameter than a current turbofan for a given cruise thrust, the overall diameter would not increase as much as this because of the "minimum cowl" design approach. With similar provisos to those given for the first example, the installed s.f.c. of this class of power- plant should be perhaps 20 per cent better than the cur rent turbofan figure. Both of these turbofan concepts would make extensive use of composites in the nacelle. No return to the propeller The third example represents a more radical change. The cowl is discarded altogether and we return to the open propeller. But this is a "propeller with a difference." It embodies transonic aerodynamic technology and multiple blades to achieve both a high disc loading and a reasonable efficiency at flight Mach numbers up to about 0-8. Six or eight blades might be used, swept back towards the tips and featuring an advanced form of construction with a spar inside each blade. Estimates for the fuel consumption of such powerplants make them as much as 20 per cent more economical than present engines, depending on the efficiency of the "prop-fan" itself and the cycle used for the gas generator. Becent Hamilton Standard tests of a model prop-fan in a wind tunnel indicate an efficiency of 0-77 at Mach 0-8 and suggest that the target efficiency figure of 0-8 used in project studies should be attainable. Although the prop- fan has some important potential drawbacks—lower standard of ride comfort; possibly higher cabin noise at Figure 6: Roll-Hoyce's geared mid-fan scheme cruise; higher weight; cruising speed limit of around Mach 0-8—its attractive fuel economy constitutes an incentive for further research and evaluation. It is too early to forecast with confidence which of these powerplants is most likely to materialise, or when a project might be launched. My personal view is that we are rather unlikely to return to propulsion by open rotor for the larger transport aircaft, and that some type of advanced tubofan will form the powerplant of the early 21st century. The fan cowl and outlet stator blades perform very valuable functions. Together they control the flow of the propulsive airstream, largely removing exit swirl and its associated losses, and minimising the periodic flow interactions with the airframe which give rise to vibration. Some of the design problems of future turbofans may look formidable now, but it is worth remembering that serious attempts to predict future development have quite often turned out to be pessi mistic. A good example is a belief which prevailed quite widely at the beginning of the 1960s. It was then felt that bypass ratios for main propulsion engines were unlikely to rise above a value of about two, at least for a very long time, due to the weight and installation- drag penalties which seemed inevitable with lower- specific-thrust designs. In the event, advances in high- temperature technology, aerodynamics, materials and mechanical design led within that decade to the launch ing of the current large engines, which have bypass ratios of about five. Much time has been devoted to studying alternative layouts aimed at economising on the total number of turbo-machinery blade rows required. Figure 6 shows the geared mid-fan engine recently studied by Bolls-Boyce. Its low-pressure turbine is divided into two sections: a high-speed forward portion is geared to a lower-speed rear section mounted on a rotating casing which also carries the fan blades. This scheme bears some relation to the "Bostat" idea put forward by Howell at NGTE in the 1940s. The latter featured rotation of the turbine "stator" blading (hence "Bostat") but did not embody the gear system. Although such schemes offer a saving in the number of blade rows required, and also remove certain gas-generator design constraints because of the absence of an 1-p shaft passing through the system, they introduce problems associated with the sealing of the rotating casing. Conclusion In the 50 years which have elapsed since Griffith's "first practical proposal to use a gas turbine '.:.'," the aero-engine world has undergone a vast trans formation. The gas turbine quickly fulfilled its promise of high unit power and light weight. The BAE-inspired development of the axial compressor provided a basis for today's efficient, high-pressure-ratio engines. Gas- turbine powerplants now appear in a variety of forms to suit the wide range of modern propulsion require ments. For the future, we can certainly anticipate a con tinuation of intensive and highly competitive develop ment under the spur of military requirements and the huge business potential of civil air transport. Technically, there is much still to come. As ever, we face an array of opportunities and problems. There is a continuing need for design ingenuity and for perseverance in research and development. With much at stake, good judgement of both engineering and commercial issues will be vital.
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