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PRATT & WHITNEY DEVELOPS GEARED TURBOFAN

The development of turbofan engines has come to a turning point. The PW8000, which Pratt & Whitney officially launched in February of this year, will be the first large geared Turbofan available for commercial aircraft.

Conventional turbofans consist of a high-pressure spool and a low-pressure spool. Since the fan is normally part of the low spool, both are turning at the same speed. This speed however, often is a compromise. The fan really operates more efficiently at lower rpms while the rest of the low spool is more efficient at higher speeds. Putting a reduction gear in between these components makes it possible for the fan and the low spool to run at their optimum speeds. In the PW8000, the fan runs with only a third of the speed that the low spool does.

This principle is not new. The German engine manufacturer MTU, together with its partner Fiat, has built such a reduction gear in the frame of the ADP project (advanced ducted propulsor). While ADP itself was now shelved, the gearbox has become a vital part of the PW8000 development. However, a smaller unit will be used with the new geared turbofan.

While the ADP unit generates 29800 kW (40500 hp), the unit used with the PW8000 will have a power output of 23800 kW (32300 hp). The gearbox technology has become mature. The technology, which Pratt and its European partners developed for the gearbox, has solved typical problems of earlier designs in the areas of bearings, lubrication and cooling.

The latter was one major concern for the engineers. Studying the heat generation in gearboxes, the engineers found out that approximately 80 percent of the heat load in the gearbox actually comes from churning and not from the lubrication.

With the new lubrication concept, oil is only transported to the areas inside the gearbox where it is really needed for lubrication. The oil is then cycled out of the unit as fast as possible to prevent any build-up of heat loads. According to Pratt & Whitney, the lubrication oil is cycled twice as fast through the gearbox as with earlier designs.

The bearings are virtually self-aligning. In order to protect the gearbox from the flexing encountered during engine operations, a special kind of bellows structure was designed to isolate the gearbox from the rest of the engine.

Both, the ADP unit and the smaller gearbox for the PW8000, have already accomplished some comprehensive testing. The bigger unit has run for 600 hours, 500 of which under maximum load. The small gearbox has run for 400 hours. According to Pratt, there were no problems at all. The large unit has also accomplished 100 hours of full-scale testing in a PW2037 engine with a 2,9m fan.

The engine manufacturer has not yet made a final decision about the fan diameter. The original design was looking at a 1,93m diameter fan. However, a discussion about installation issues on narrow-body aircraft makes made Pratt look into a 1,83m-option also. While the fan diameter does not have an effect on the gearbox, it does affect the bypass ratio. According to Pratt, the 1,93m fan geared engine has an 11:1 ratio while the 1,83m-option gives a ratio of 10:1.

However, even 10:1 is significantly better than today's usual bypass ratio of six or seven. Also, Pratt sees other big advantages with the geared turbofan. For one, the manufacturer expects a fuel burn reduction of around nine percent. Also, Pratt forecasts a noise reduction of 30 Decibel (cumulated) below current Stage 3 limitations.

The geared fan will need significantly less stages than a conventional turbofan of conventional design. The PW8000 will have 13 stages (as compared to 20). Along with the fan, it will have a five-stage high-pressure compressor, a single-stage high-pressure turbine, along with a three stage low-pressure compressor and a three stage low-pressure turbine. Looking back at a long partnership with Pratt, the German MTU would like to participate in the program by supplying the entire low spool with the exception of the fan.

Another advantage, according to Pratt, is a 40 percent reduced emission level. This will be possible by using an advanced combustor. Also, since the fan is operating with a much higher efficiency, the core engine size can be reduced.

The PW8000 will share the core with the PW6000, which is Pratt's other new engine but is designed as a conventional turbofan. While the PW6000 is designed for the thrust class of 70 to 101 kN (15750 to 22950 lbs), the more powerful PW8000 geared fan is aimed at the 110 to 155 kN (24750 to 34875 lbs) category.

Pratt is convinced that, with this thrust scale, it will be able to supply a complete engine family to aircraft sizes reaching from 80-seat regional jets to 180-seat airliners. While Pratt has launched the PW8000 program without having a customer for the engine, the manufacturer says that the geared fan will save the operator of a typical 120 to 180-seat airliner up to 600000 Dollar per year in operating costs.

Flight tests with the new engine are reportedly scheduled to begin in 2001. With an estimated 5300 test hours (approximately 18000 cycles) the test program will probably even more extensive than the certification program for the PW4084 for the Boeing 777 was (4300 hours, 16000 cycles).

From page 54 of FLUG REVUE 10/98