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Barbs and bluster as engine makers take aim

posted Jul 27, 2010, 6:52 AM by Rowan Hewitt

By John Croft


As expected, the marketing and rhetoric surrounding the favoured dual-spool turbofan engine for the re-engined and next generation single-aisle commercial aircraft heated up along with the temperatures at Farnborough.

CFM International took the first jab at its pre-show press conference, where CFM executive vice-president Chaker Chahrour discussed benefits of a slow-turning low-pressure turbine (LPT), a design necessity for the company's Leap-X 30,000lb thrust (133kN) class turbofan design, the engine of choice for the Comac C919. "Keep the LPT running slow," says Chahrour. "You speed up the LPT, you pay the costs."

Pratt & Whitney's geared turbofan has a low pressure spool [LPT and low pressure compressor (LPC)] that, by design, spins more than twice as fast as the CFM design - about 8,500RPM. Because it can spin faster, the low pressure spool for the Bombardier CSeries engine requires only three stages.

The CSeries is one of three airframer wins for the GTF so far. A 3:1 gearing system between the low pressure spool and the fan slows the fan to an optimum speed at just below Mach 1, says Pratt. Though the gearing system weighs 113-136kg (250-300lb), Saia says it saves up to 320kg per engine when considering fewer low pressure spool stages needed, a net gain of as much as 182kg.

P&W vice-president, next-generation product family, Bob Saia says the engine maker has a large knowledge base with such speeds - the Pratt & Whitney JT8D turbojet that powers aircraft like the Boeing 727 and Boeing 737-100/200. Note that Boeing selected CFM as the sole supplier of the 737 engines starting with the 737-300. CFM is designing a low pressure spool that turns at "just over" Mach 1, a number P&W disputes. "The physics won't allow it," says Saia.

Both manufacturers are scouring their engines for weight savings to counter the increase that naturally comes with the larger 18-blade fans and associated support structure. CFM estimates it is saving 450kg per engine through its new resin transfer mould (RTM) composite fan blades, each of which is protected by a titanium leading edge. Endurance testing of the blades is about to start, following a battery of tests including bird strike.

CFM plans to have the design freeze of its Leap X1C engine and fan for the C919 in 2011 and first engine to test (FETT) in late 2012 followed by certification in 2013.

P&W looked at composite blades but has decided on a bi-metallic blade, the exact details of which have not been revealed. "We found it had the same weight as composite but has the significant benefit of making the blade thinner," says Saia. "It has better [aerodynamics] and still meets bird ingestion requirements." Saia says bird ingestion and blade-out tests are complete and the first set of 18 fan blades are complete. P&W will begin building up its 24,000lb-thrust PW1524G engine for the CSeries in August, targeting a third quarter 2012 certification, about one year ahead of the Leap X.

Although there low speed spools differ, both companies have similar high pressure, or hot section designs, which CFM says historically accounts for 90% of the maintenance costs.

P&W says its design will provide a more than 20% reduction in maintenance costs compared with today's CFM56, a number it calculates to be $750,000 per aircraft per year.

Saia says the savings comes from having six fewer low spool stages and a cooler running core, a less thermally efficient option, but one that is counterbalanced by longer part life.

"There is scepticism, because it's a big number," says David Hess, president of P&W. "As we take them through that data, the scepticism kind of goes away. We show them everything that is coming out. We've convinced them, I think we've convinced Airbus and convinced Embraer and others that the number is real."

Both companies say they continue to work with Boeing and Airbus on designs for a potential re-engined 737 and A320. Trade studies include elliptical nacelles that could allow the larger fans to fit on both aircraft, particularly the lower-sitting 737. Saia says both CFM and P&W would have to offer a smaller fan size for the 737, noting CFM's smaller offering for the 737 versus the A320 today. Regardless, he says P&W would be able to deliver "double digit" fuel savings for the 737.