ANALYSIS: A giant Leap for CFM

Washington DC
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A programme that began to take shape as a concept nearly eight years ago is finally taking real form inside CFM International's supply chain.

Launched at the 2005 Paris Air Show as a possible CFM56 replacement, the Leading Edge Aviation Propulsion (Leap) programme was at that time intended to supply the next generation of turbofans for all-new single-aisle aircraft by Airbus and Boeing. At that time, few expected a replacement for the A320 or 737 to appear before 2020.

Over the next six years, the single-aisle market evolved rapidly. A competitor, Pratt & Whitney, introduced a new innovation in propulsion called a fan-drive gear system, the Chinese entered the market with a new single-aisle airframe and Airbus and Boeing deferred plans for an all-new single-aisle.

Instead, the US and European airframers settled for re-engining and updating their products within this decade, with Airbus promising airlines a 15% fuel burn improvement compared to a standard A320, and Boeing - not to be outdone - vowing a 16% upgrade.

Such promises are based almost entirely on the performance of a new generation of single-aisle turbofans developed by P&W and CFM. P&W's PurePower-branded geared turbofan hit the market first. Bombardier selected the PW1500G to power the CSeries, a small narrowbody launched in the 110-149-seat market.

But engine selections for a much larger segment of the narrowbody market, ranging up to 220 seats, awaited.

In 2008, CFM partners General Electric and Snecma committed to launch the Leap engine series and to define the architecture of an all-new propulsion product that would be charged with replacing the most successful turbofan in history and confront the challenge from P&W.

So far, the Leap has kept CFM atop the narrowbody engine orders race, but the final outcome remains unclear. The Leap is the only engine on all three narrowbodies in development with at least 160 seats, which includes monopoly positions on the 737 Max and C919. But the PurePower PW1000G has established a monopoly on new jets in development by Bombardier, Irkut, Mitsubishi and Embraer, while also gaining a competitive position on the A320neo.

On the latter application, the results so far are equally murky. With 1,864 A320neos ordered, the Leap-1A has established a 4.6 percentage point lead over orders for the PW1100G.  But that is still five points less than the CFM56 lead over the V2500 on the current A320, and for 34% of the ordered A320neos, engine decisions have still to be made.

Clearly, the market is yet to deliver its final verdict on the new engine technology produced by CFM and P&W. Eight years after the Paris Air Show launch of the Leap programme, much will depend on what comes out of the tests that will soon begin on the first production-representative engine.

That engine's first components are in the production flow of CFM's supply chain, says Gareth Richards, Leap programme manager. These first parts will enter final assembly in April.

"We have a process where first we freeze the design, and that design freeze was already completed in the summer of last year," Richards says. "Then it goes to the phase of actually completing the drawings and releasing them to manufacturing. That phase I shall complete now; the designs have been released to manufacturing and the first part will be coming in for assembly in April of this year. We're past that point now and we're actually accumulating hardware through the manufacturing process."

This first engine - technically, a Leap-1A for the A320neo, but nearly identical to the Leap-1C for the Comac C919 - is scheduled to be assembled in August and be ready for testing by the end of September, Richards says.

The schedule for the first Leap-1B is not set until Boeing reaches the firm configuration milestone on the 737 Max at mid-year, but is generally running about nine to 12 months behind the Leap-1A. The schedule difference reflects the 10-month lag between the launches of the A320neo in December 2010 and the 737 Max in August 2011.

Even as the first components come together, another conversation is taking place between CFM and two regulators: the European Aviation Safety Agency (EASA) and the Federal Aviation Administration (FAA).

The Leap engine will be the first commercial turbofan to incorporate ceramic matrix composites (CMCs), which are installed as the shroud encasing the first stage of the high-pressure turbine (HPT). CMCs are a lightweight material that can survive temperatures that would cause even actively cooled metal blades to melt. GE has already used CMCs widely in ground-based gas turbines used for industrial power. The US Navy has also funded GE to demonstrate CMCs in the rotating parts of the F414 fighter engine.

But CMCs have never been applied before to a commercial engine in production, and that will require special scrutiny from the FAA.

"They are reviewing our test plans: both our component test and and our engine test plans," Richards says. "That will culminate with what is called a preliminary type board meeting, at which they will give us approval for the test plans we have proposed. It is up to us as a manufacturer to propose to them how we will certify and it's up to them as the agency to approve our test plan or say no."

The preliminary type board meetings for the Leap-1A and Leap-1C engines are scheduled in the second quarter. The same meeting for the Leap-1B is expected to follow about nine to 12 months later.

While CMCs are a new technology in the commercial turbofan market, the Leap includes several other technologies derived - and, in some cases, expanded - from the GE90 and GEnx.

Examples begin at the inlet of the engine. The Leap series features a carbon-fibre composite fan case, which surrounds an inlet fan comprised of carbon-fibre composite blades. The composite materials are lighter and stronger than metal, and so far have past a crucial blade-out test on a test rig engine.

"If we had to design a metal system at the Leap scale our total engine system weight would have to be 500lb heavier than it is," Richards says. "At the airplane level it's 1,000lb. That's not even counting the weight of the pylon supporting the engine. It would have to get heavier [with metal blades and fan case]. The wing structure would have to get heavier."

Reducing the weight of the fan case and the blades was even more important because of another change in CFM's propulsion architecture for Leap. One quick way to improve fuel efficiency is to increase the ratio of air passing through the inlet fan that bypasses the combustion process in the engine core. So, the 5:1 bypass ratio of the CFM56 expanded to an 11:1 bypass ratio for the Leap engine.

To increase the mass flow of air bypassing the engine core, it is necessary to widen the diameter of the inlet fan. For example, the 173cm (68.1in) fan diameter of the A320's CFM56-5B widened by 14% to 198cm on the Leap-1A.

Of course, as the inlet diameter increases, the low-pressure turbine (LPT) that drives the inlet fan has to do more work. CFM adopted a seven-stage LPT for the Leap, compared to a four-stage LPT for the CFM56-5B.

Another critical area of innovation for the Leap is the compressor system. CFM elected to concede to P&W an advantage with bypass ratio. By inserting a reduction gear between the LPT and the inlet fan, P&W was able to increase the bypass ratio of the GTF to from 5:1 to 12:1. Instead, CFM also focused on radically improving the efficiency of the compression process in a narrowbody engine.

The Leap engine is designed with 3D aerodynamic blades from the inlet to the back of the turbine section. These feature scimitar-shaped tips with blades curved from the inner to the outer sections.

"That technology of that 3D aerodynamics is carried all the way through the engine at every stage, and its impact is probably greatest in the compressor," Richards says.

It allows the Leap engine to more than double the compression ratio compared to the CFM56 to 22:1, roughly matching the compression levels achieved by the GE90.

Increasing the efficiency of the compressor was also the goal that drove CFM to install CMCs in the shroud of the high-pressure turbine, which is the mechanism that extracts energy from exhaust gases to power the compressor system. Metal shrouds in the same location would have to be actively cooled to keep from melting. The cooling air is siphoned out of the compressor section, reducing the airflow used for combustion and decreasing efficiency.

"By using CMCs in that shroud we no longer need that cooling air that is extracted from the compressor, and that's a debit to the efficiency of the compressor," Richards says.

Another result of the higher compression ratio saddles CFM with a difficult decision early in the design process. A key feature of the sales pitch for the CFM56 over the now-P&W-owned V2500 engine was lower maintenance cost. That advantage was driven mainly by a key difference between the rival engines in the HPT.

While the V2500 employs a two-stage HPT, the CFM56 is designed with only one. With fewer moving parts in one of the hottest sections of the engine, that small difference made the CFM56 inherently cheaper to operate and in some cases easier to sell. The CFM56 is the undisputed champion in the narrowbody engine wars. In direct competition with the V2500 on the A320, the CFM56 has a 10 percentage point lead.

The new bypass ratio requirements of the Leap engine, however, rendered CFM's single-stage HPT obsolete. CFM instead designed the Leap with a two-stage HPT, matching the number of stages designed into the competing engine. It was a necessary price to pay, however, to achieve the higher efficiency levels possible with a 22:1 compression ratio.

The last piece of the Leap technology puzzle is an advanced combustor. Besides reduced fuel burn, the airlines also want the new single-aisle aircraft to generate significantly less harmful emissions, such as nitrous oxide (NOx) and unburned hydrocarbons. The latter is produced when the combustor burns the fuel-air mixture too hot, and the former is caused by "cold spots" inside the combustor.

For the GEnx engines powering the Boeing 747-8 and 787, GE introduced the twin-annular premixing swirler (TAPS) combustor, which is designed to reduced NOx emissions by 50% compared to the CAEP/6 standard.

For the Leap engine, CFM is introducing the TAPS II combustor with improved mixers in a smaller geometry, yielding a 60% improvement in NOx emissions.