New materials play key role in next-generation turbofans

 - May 6, 2009, 8:20 AM

Boeing Business Jets and Airbus Corporate Jetliners could benefit from new engine technology planned to enter service on the next-generation 737 and A320 airliners from which they are respectively derived. CFM International is working on an advanced turbofan, targeting a 16-percent cut in fuel burn that could bring greater efficiency to these and other new bizliners. New materials such as three-dimensional (3-D) woven and ceramic-matrix composites (CMC) are key to the new powerplant being developed by CFM partners Snecma (Booth No. 1143) and General Electric.

The LEAP-X program is focusing on a very high-bypass ratio and advanced materials, according to Snecma senior vice president for system engineering, research and technology Jacques Renvier. Both its bigger fan and hotter core engine need such materials.

Weaving a fan blade in 3-D makes it possible to manufacture larger, more aerodynamically advanced blades without increasing weight. For a 71-inch fan diameter, each 3-D-woven blade weighs about eight pounds. The weight saved comes from the reduced number of blades. The LEAP-X’s fan has 18 blades, whereas the current CFM56-7 (which powers the Boeing 737 family) has 24.

A fan case using the same composite technology is now undergoing rig tests. Engineers expect to save 1,000 pounds on a twin-engine aircraft by moving from metal to 3D-woven composite fan blades and case.

The manufacturing process for this fan blade involves 7,000 needles working on the composite loom and a water jet cutting the fibers. Resin transfer molding (RTM) is the next step in this process by which CFM has produced approximately 300 fan blades.

The company also plans to demonstrate that detachment of the inner part of the blade is so unlikely it needs to conduct blade-off tests only with the outer half of the engine. CFM calls this concept “SafeLife.”

Full-scale testing of the LEAP-X’s fan is well under way. After being slightly modified, the fan was mounted on a CFM56-5C engine (which powers the Airbus A340-200/300 widebody family). Snecma assessed characteristics such as stall margins and performance at its Villaroche headquarters near Paris. Crosswind and acoustic tests are due to take place this month at GE’s facility in Peebles, Ohio.

Incorporating New Materials
GE already employs composite blades (albeit using multi-layer technology, rather than 3-D) on its GE90, which powers the Boeing 777. Rolls-Royce is sticking to titanium for fan blades on its latest programs; however, the UK-based manufacturer is partnering with GKN on a composite fan blades research program.

On the low-pressure turbine, the shrouded blades are composed of a ceramic-matrix composite material making them lighter, thus simplifying the rotor, which no longer requires a heavy disk, saving 350 pounds per aircraft.

One challenge, however, has been to weave the blades with their integrated shroud. The material’s fibers are made of silicon carbide, while the matrix is ceramic. This material plays the role of resin in conventional composites. Typically, the weight gain with CMC is said to be 30 to 40 percent more than metal. The company plans to run tests next year.

On the LEAP-X, reliability and maintenance cost are to be kept at the same level, according to CFM officials. Noise is expected to be 10 to 15 dB below Stage 4. Nitrous oxides (NOx) emissions are to be decreased 60 percent below the CAEP6 standard. The 16-percent cut in fuel burn translates into an equal reduction in CO2 emissions. These figures take “today’s best CFM engines” as the base reference.

About two thirds of the 16-percent improvement will come from increases in propulsive and thermal efficiency. “Propulsive efficiency is enhanced with the bypass ratio,” Renvier explained. The bypass ratio is increasing from five to six on current CFM56s to about 10 on the LEAP-X, which is said to be optimum in terms of fuel burn.

CFM engineers claim to have increased the bypass ratio without increasing the fan diameter (which would have increased the drag). The higher pressure ratio also enabled them to shrink the core. In addition, they have enhanced the fan flow through improved aerodynamic performance derived from using new materials to design larger blades without increasing weight.

Thermal efficiency is derived from the compressor’s pressure ratio, which may call for a two-stage high-pressure turbine to drive the compressor. GE is working on two design single-stage turbine and a two-stage. The outcome for the compressor is planned to be a pressure ratio of 22 with 10 stages. Current CFM compressors have a ratio of 11 with nine stages.

The second generation of the lean-burn, twin annular pre-swirl technology (TAPS 2) is to appear on the LEAP-X’s combustor. TAPS 1 is to enter into service on the new Boeing 787’s GEnx engine. “Rich burning technology could go slightly lower in NOx emissions than current CFMs, but lean burning is a quantum leap,” CFM’s LEAP-X program manager Ron Klapproth said.

The TAPS 2 combustor is shorter, making the hot area (where NOx forms) much smaller. “TAPS 2 also helps cruise NOx reduction. We anticipate there will be discussions about NOx at altitude, on top of current airport air quality issues,” Klapproth added.

CFM is targeting 2012 for the first run of a full demonstrator, with certification pegged for 2016. Exactly when Airbus and Boeing will replaced their existing narrowbody airliners is less clear, although both airframers have indicated that sufficient advances in engine technology will be the main springboard for any program launches.