Engines at the core of new programs

  • 06-Aug-2008 10:27 EDT
FI 08 CFM56 engine.jpg
CFM is working on upgrades to its CFM-56 family, with a new Leap-X program looking beyond to next-generation requirements for future 737 and A320 replacement airplanes, including open-rotor and advanced turbofan designs.

Having reached a plateau in turbofan engine development in recent years, the scene is now being set for the next step up in conventional engine performance gains, with progress being made toward new-generation engines.

The biggest selling engine in commercial aviation history is currently the CFM International CFM-56 family, which powers all Boeing 737s and most Airbus A320s, plus upgraded KC-135s and first-generation A340s. With 18,500 engines delivered to 500 customers, the 50:50 joint venture between Snecma and GE Aviation is going to be a hard act to follow, especially as outstanding orders for thousands of engines guarantee deliveries through the next decade.

With such a sales record to uphold, CFM is aware that rival companies Pratt & Whitney and Rolls-Royce will want to offer serious challengers to power the next generation of single-aisle airliners, and they may also see an opening emerging to power new upgraded 737s and A320s. With this in mind, it has invested heavily in new technologies under the Leap56 program to keep the CFM-56 family competitive.

The first outline specification suggests a CFM demonstrator to run in 2012 that could deliver a 16% fuel burn improvement over current engines. Under the Leap-X program this performance gain would be delivered through a new 18-blade fan made from an advanced all-composite material that is based on technologies developed during Leap56 activity. The new technology would be applicable to either a geared fan or open-rotor design, and both routes are being explored.

A new demonstrator core is to be run next year and will have an eight-stage high-pressure compressor, raising the pressure ratio to 16:1. A new high-pressure turbine with ceramic blades will drive it, and the combustor design will benefit from the Leap56 program. Lightness will be a major feature. The open rotor seems to be rapidly gaining favor as the preferred future powerplant configuration, but at the end of the day, the practical operating and maintenance costs will be crucial, even if the aerodynamic advantages point in this direction. Low maintenance costs and low noise, as well as safety factors, will probably dictate which design emerges as the commercial winner.

According to Colin Smith, Engineering and Technology Director at Rolls, the company had invested over $22 billion over the last decade in technology and infrastructure; a growing proportion was spent in Germany and its U.S. plant at Indianapolis. Last year alone, it spent over $3 billion on R&D and its 2007 order book exceeded $72 billion. Future prospects look good as the company is well placed on all the major widebody programs from both Boeing and Airbus and is currently sole choice on the A350XWB.

So far, the GE-P&W Engine Alliance has not been able to conclude a deal with Airbus on an alternative A350XWB powerplant. The new Trent XWB engine is going to be 17% more fuel efficient than the Trent 700 used on the A330, which shows the degree to which an already efficient and reliable engine can be further developed. Rolls is not enthusiastic over the geared fan, as favored by P&W, and thinks its own three-shaft turbofan can deliver a similar performance while being both lighter and more reliable.

It is studying open-rotor designs and is participating as a leading contributor in the European Clean Sky initiative, with Snecma as a partner, but is looking closely at noise and safety issues. As it is difficult to effectively shield the open rotor, even with twin tails and high-mounted rear engines, noise is a challenge as the unducted fan proved two decades ago.

Noise will have to be engineered out of open-rotor engine design as much as possible, and applications for the powerplant will need to be of unconventional configuration. The next step-change in fuel burn performance that all airframers are waiting for will almost certainly emerge from this direction, providing the other operating issues can be satisfied.

With more interest in turboprops, Rolls is involved in the TP400-D6 program for the A400M military transport. It believes that it potentially has a new-generation prop engine that could find other platform applications once it has established a reliable reputation. There is talk of new high-capacity, short-haul turboprop airliners fitting into the market above super-stretched turboprop twins such as the ATR 72 and Q400 families. If fuel costs remain high, then a new generation of turboprop airliners could return, using composite structures and advanced ultra-efficient engines.

While others step cautiously down alternative engine configuration routes, P&W is making progress on its PurePower PW1000G geared turbofan (GTF) engine. Following more than 250 hours of ground tests since last November, the GTF has begun 30-40 hours of flight tests on the company-owned 747; it will then be transferred to an Airbus-owned development A340-600 for up to 100 hours of further tests. With this work under its belt, P&W will complete the detailed design of the GTF that will power the Mitsubishi MRJ 70-90-seat regional jet. Bombardier's GTF for its CSeries family of airplanes will require a higher thrust variant.

For a short-haul market, with high cycle operations, the GTF will have to establish a rapid buildup of customer confidence in reliability terms. P&W dismisses critics who suggest the GTF is going to be too complicated and expensive to maintain and points out that it will have fewer stages than a conventional engine; performance tests to date have exceeded expectations in terms of noise emissions.

It is confident that as the development continues, the engine will be available for further applications, obviously looking beyond the MRJ and CSeries to 150-seat airliner replacement opportunities. The shorter landing gear length and lower wing on the 737 would probably make application of the GTF somewhat difficult.

The GTF will certainly end up as a multi-version family of engines. Later, higher thrust models will take advantage of advances in materials technology and should be able to increase bypass ratios to increase fuel burn figures even more. For the present, P&W is not taking any unnecessary technological risks in what is already a radical new design and is moving ahead cautiously—always a wise move when an all-new engine is used in an all-new airplane.

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