CFM International partners GE and Snecma have officially launched core testing for the Leap X engine designed to power next-generation narrowbodies, but company executives remain ambivalent over making a firm commitment to a two-stage high-pressure turbine (HPT).
Adapting a two-stage HPT would be a significant departure for CFM, whose venerable CFM56 family of engines rests on a single-stage HPT architecture.
Leap X is the first application of the Leap 56 technology development programme CFM launched in 2005.
CFM is using a single-stage HPT on a Leap X core undergoing testing at its Evandale, Ohio facility. Roughly 2,200 parameters of data will be collected over the one- to two-month tests.
Using data from the current round of tests CFM next plans to develop a "core 2" featuring a two-stage HPT for testing, scheduled to begin in mid-2011. CFM is designing core 2 to reach a compression ratio 22:1 to 23:1 versus 16:1 for single-stage Leap X core testing. CFM models have a ratio of roughly 11:1. For the current round of single-stage testing, CFM plans to run a 20:1 compression ration to "find out where the stall line is", says Leap X programme manager Ron Klapproth.
CFM management is coy about the company's overall preference for a single- or two-stage HPT. Executive vice-president Chaker Chahrour says its decision on the HPT final design will be a function of results logged during the first two rounds of tests. CFM plans to run three core tests before settling on a final design.
Leap X manager of engineering Ted Ingling says that CFM is "keeping options available" on the turbine design as the company keeps a watch on the fuel burn environment. He stresses the importance of achieving the right balance of fuel burn and maintenance costs. "You can't have a 1% increase in efficiency and have the customer pay 10% more in maintenance costs."
While remaining uncommitted to specific architecture for the HPT, CFM has chosen materials for the turbine including an advanced turbine aerofoil material for the blades, and ceramic matrix composites for the nozzles and shrouds. Chahrour says CFM is not disclosing specifics details of the advanced HPT blade materials.
For the low-pressure turbine, CFM is examining use of ceramic matrix composite materials in the blades. Chahrour says that with the higher 1.8m (71in) fan diameter compared with current CFM56 models, use of those materials could lead to a 158kg (350lb) weight benefit per aircraft.
The Leap X core is responsible for roughly 7% of the overall promised 16% fuel consumption improvement for the new engine. CFM's targeted 10:1 bypass ratio accounts for roughly 7% of the improvement, with advanced engine systems accounting for the remainder.
CFM is all too aware of the targeted 12:1 bypass ration promised by rival Pratt & Whitney on its geared turbofan engines, but believes the advances in core design it plans to achieve for the Leap X engine will deliver the targeted 16% fuel burn improvement versus a 12-15% improvement targeted by the GTF.
© Pratt & Whitney
During gearbox and fan testing carried out late in 2008 on an Airbus A340 using an existing PW6000 core, P&W achieved an 11:1 bypass ratio. Company vice-president of next-generation product family Bob Saia recently told Flight Daily News the slight drop was not a "big difference" and once "you understand the technology tested on the A340 you can scale up or down pretty easily". The fan size used on the A340 was 2m.
But highlighting the divergent philosophies between the two manufacturers, Klapproth reiterates the gamble with the GTF, pointing out that if a problem with a gearbox occurs, an engine removal is required.
But P&W commercial engines marketing vice-president Mary Ellen Jones dismisses those concerns, explaining that after P&W shared fan tests results, "many customers told us that takes the gear issue off the table".
CFM is planning its own fan advances in Leap X, including blades manufactured through a 3D resin transfer molding (RTM) process patented by Snecma. The larger 1.8m-diameter Leap X fan features six fewer blades than the current 1.5m fan of the CFM56-7B.
Roughly 300 RTM blades have been produced, and CFM expects to develop three different iterations for the blades before it concludes design of the demonstrator engine. Similar to the blades featured in the fan of the GE90-115B, the smaller Leap X composite blades have a titanium leading edge. Those smaller blades need the capability to absorb the same frontal impact - a 1.1kg bird, for example - as the GE90. Snecma director of product strategy and market Fabienne Lacorre says that is "one challenge we have crossed over", and "the test rig actually in process in Paris will confirm our developments".
The Leap X "Mascot" fan was shipped to GE's Peebles, Ohio test facility in May for roughly a month of acoustic and crosswind testing. The test cell in Peebles is capable of running test with winds in the 60-70kt (110-130km/h) range, but current certification standards are 35kt. Once those tests are complete the Mascot returns to Villaroche, France for endurance testing after completing initial performance testing at the facility earlier this year.
All of CFM's tests are building towards a full engine demonstration in 2012 for a targeted 2016 certification. But the company admits much of Leap X's future depends on receiving clarity from airframers about their future narrowbody designs. "There is no need to certify an engine if there's not an application for it," says Chahrour.
Six to eight months ago he would have predicted launch of new narrowbodies by Airbus and Boeing in 2018. "Today I can't say that."
In preparation for a narrowbody debut to slide into the next decade, CFM is using its Leap X development as the cornerstone for its open rotor research. But at this early stage Chahrour says that the few carriers he has spoken to "are not that enamoured" with the open rotor. Instead those airlines are eager to achieve the 16% improvement in fuel consumption over the best performing CFM56 promised by Leap X "as soon as possible".
Chahrour admits to unknowns in open rotor design, including overcoming the challenges of noise reduction. "Based on what we know today, we don't know how to get rotor noise down to the turbofan level," he says.
CFM chief executive Eric Bachelet expresses a similar sentiment, explaining that open questions remain over an open rotor, including the ability to design an engine with the same reliability as current models. "We don't know that yet," he says.
But the company takes its study of open rotor technology seriously, and believes an ultra-high bypass ratio generated by the 4.3m open rotor fan diameter comprises the bulk of the projected 26% improvement in fuel burn. But Chahrour cautions that "efficient installation is critical to achieving this benefit".