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Aviation History
1983
1983 - 2173.PDF
PROPULSION Son of JT9D takes shape EAST HARTFORD The first run of Pratt & Whitney's follow-on to the JT9D, the PW4000, is set for next April. All design work is completed, and the new, 11-stage, high pressure (HP) compressor was successfully tested in August. Certification of the first of this family of new turbofans in the 48,000-60,0001b thrust range is set for July 1986. Pratt & Whitney says that the PW4000 will burn "at least" 7 per cent less fuel than the JT9D-7R4. Results so far have shown extremely good correlation with computer predictions, the HP compressor proving some 3 per cent more efficient than that of the PW2037, due to be certificated in December. The improvement results both from the use of "second generation" con trolled diffusion aerofoils with thicker leading and trailing edges, and the introduction of three-dimensional design and manufacturing techniques not available at the time of the PW2037's design. The PW4000 has benefited even more than the PW2037 from Pratt & Whitney's Nasa-sponsored work on the recently completed Energy Efficient Engine programme (Flight, October 1, page 907). Component efficiencies throughout the engine have been increased, both by pushing up the rotational speeds of the turbomachinery and by better computer modelling. The core, for example, now spins at 10,000 r.p.m. — 2,000 r.p.m. faster than that of the JT9D-7R4. This means that more work can be wrung out of fewer components, resulting in a 30 per cent reduction in the number of HP compressor aerofoils, and a 50 per cent reduction in the aerofoils of the HP turbine which drives it. The PW4000 is to be certifi cated at 56,0001b thrust— the level demanded by the airlines. At this thrust, the use of powder metal HP turbine discs a la PW2037 was deemed unnecessary, although tip speeds of both engines are about the same. "The PW4000 is bigger, and so we had more room in which to work with conventional materials," says Jim Bruner, PW4000 engineering man ager. He says, however, that higher thrust versions of the engine are likely to require the use of discs made by the powdered metal process. Another interesting reason that Pratt & Whitney was able to stick with con ventional disc technology is that the PW4000 HP turbine runs some 100°C cooler than the PW2037. This is partly because the long-haul work for which this engine is designed is less demanding, and also because the HP compressor is more efficient, and therefore needs less turbine power. This advantage feeds through to the rest of the engine, notes Bruner, since it means that 26 per cent less air needs to be drawn from the compressor to cool the H.P. turbine. Better aerofoil cooling technology, and the use of single-crystal turbine blades in the first HP turbine stage, is also behind the reduced cooling requirement. Early tests have proved the worth of Pratt & Whitney's new way of actively control ling HP compressor blade tip clearances. Its "Thermatic rotor" is of a completely new design, replacing the bolted- together discs of the JT9D. Discs are now electron-beam welded, not only saving a considerable number of parts, but creating a "drum", into which warm air from the 15th compressor stage can be pumped during cruise. This expands the drum, forcing the compressor blades outwards and minimising blade tip leakage. The air is introduced to the drum via a hollow ninth stage stator, increasing the temperature of the drum internals by about 100°C and mimicking the conditions during take-off, when the engine is hottest. "The laws of physics have prevailed," says Bruner, commenting of the success of the idea, "although we've still got some fine tuning to do. We might, for example, find that 14th stage air is better." While the PW4000 fan is basically the same as that used on the JT9D-7R4, plans exist for the introduction of a wide-chord, snubberless version to both the PW4000 and PW2037 before the end of the decade. Bruner says that this reflects the progression of fan technology at Pratt & Whitney, since the narrow- chord, two-snubber fan came into service on early JT9Ds. This was developed to the point where only a single, part-span snubber is required for the -7R4 fan. Pratt & Whitney calls the PW4000 a "radically new engine". Asked what this means as far as certification is concerned, it notes that many of the engine's features are consistent with those of the JT9D. "We've made a fresh start only where it makes sense," says Bruner. "The PW2037 is soon to be certifi cated," he notes, "proving that many of the new technologies incorporated on . its bigger brother have already met with FAA approval". It had looked as though the PW4000's first new airframe would be the McDonnell Douglas MD100, in which application it would have been competing with General Electric's CF6-80C2. But now the engine is aimed at a market of 4,000-5,000 aircraft through to the end of the century. This covers new versions of the Boeing 747 and 767, Airbus A300-600 and A310, and retrofit to aircraft currently powered by the JT9D-7R4. Comparison of the PW4000 with its predecessor, the JT9D-TR4 reveals A increased sweep of core/bypass splitter to reduce particle ingestion B stiffer fan/low-pressure compressor support drum C extra H.P. spool bearing for stiffness D strengthened intermediate casing E stage 9 Thermatic rotor air inlet F revised combustor system with fewer parts G and H improved HP and LP turbine with fewer blades. Note the clearance control ducting JT9D-7R4G PW4000 FLIGHT International, 3 December 1983 1473
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