CFM bets on composites for future turbofan blades
CFM International (Hall 2 Stand B149) is studying a next generation of turbofans to power single-aisle commercial aircraft, hoping to secure a role in future replacements for the Airbus A320 family and the Boeing 737. Under the LEAP-X advanced turbofan program, joint venture partners Snecma and General Electric are pursuing innovations such as increased use of composite materials in engines. Environmental and oil price concerns have brought fuel burn to the top of the agenda, with a goal of a 16-percent reduction compared with the current best-performing CFM56 powerplant.
Carbon fiber, woven in three dimensions, is being introduced for fan and even low-pressure turbine (LPT) blades. For a 71-inch fan diameter, each blade weighs about nine pounds. Moving from metal to 3D-woven composite fan blades and case saves 1,000 pounds in weight on a twin-engine aircraft. So the overall goal is to reduce the number of fan blades. The LEAP-X has 18, whereas the current CFM56-7 (for the Boeing 737) has 24 fan blades.
In addition to the weight saved, CFM envisions a lower risk of blade-off events. Designers plan to demonstrate that detachment of the inner part of the blade is so unlikely that blade-off tests should be conducted only with the outer half of the engine.
CFM senior vice president of engineering, research and technology Jacques Renvier called this concept “SafeLife.” One challenge engineers face with 3D-woven composites is to reduce production costs to current levels.
Full-scale testing of the LEAP-X’s fan is well under way. Slightly adapted, it has been mounted on a CFM56-5C–the engine that powers the Airbus A340-200/300. Snecma has assessed characteristics such as stall margin and performance at its Villaroche plant, near Paris. Crosswind and acoustic tests started last month at GE’s facility in Peebles, Ohio. Endurance tests are planned for the third quarter in Villaroche.
In addition to the fan blades, another major modification for the test -5C is integrated outlet guide vanes (OGVs). Their role is both structural and aerodynamic. “Such dual-role OGVs already can be found on GE’s CF34 and GE90 [engines] but the challenge here has been to develop lighter ones,” Jérôme Friedel, chief engineer for the LEAP-X demonstration program, told AIN.
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 pointed out. The bypass ratio is increasing from five to six on current CFM56s to about 10 on the LEAP-X.
Thermal efficiency involves the compressor’s pressure ratio. The expected outcome is a pressure ratio of 22 with 10 stages. Current CFM compressors have a ratio of 11 with nine stages.
To rig-test new aerodynamic designs on the smaller blades, GE engineers create
an “aerodynamically identical” but bigger blade, which allows them to connect them to instruments. The actual blade on stage number eight is less than half an inch high.
In the 16-percent fuel burn cut, one third of the improvement will come from better component efficiency. This proportion partly overlaps the first two thirds. However, as component efficiency is already more than 92 percent, Renvier expects further improvement to be difficult.
The last part of the 16-percent cut will come from advances in engine and systems. For example, CFM is working on a core-mounted starter/generator. The challenge is that the gearbox needs to generate a lot of electric power. The benefit is the resulting smaller nacelle, which will have less drag.
One way to achieve the bypass ratio increase without increasing the diameter of the fan has been to further decrease the hub/blade ratio from 0.3 to 0.27. The bulk of the bypass ratio increase, though, comes from improved aerodynamics. The bottom line is that the difference in fan diameter between the 71-inch ground-test fan and a CFM56-7 is about 10 inches.
The minimal increase in diameter is significant because it means that the engines are still compatible with current narrowbody airframes, providing they are modified. More radical design changes, such as a geared turbofan or an open rotor, are compatible only with new airframes, CFM executive vice president Olivier Savin said.
On the low-pressure turbine, a ceramic-matrix composite (CMC) material saves 350 pounds per aircraft. The material’s fibers are made of silicon carbide, while
the matrix is ceramic. Tests are planned on a CFM56-7 next year
Typically, the weight gain over metal is said to be 30 to 40 percent. Snecma has demonstrated manufacturability of complex shapes with CMCs. A lobed mixer endured more than 700 cycles of testing “without any problem,” Renvier reported.
The anticipated outcome of the LEAP-X program is that it will succeed the ubiquitous CFM56 engine on narrowbody airliners. Snecma and GE do not feel under pressure since, apparently, Airbus and Boeing have no plans to replace their A320 and 737 designs until late in the next decade or even 2020. Moreover, CFM engineers have consistently shrugged off the threat posed by Pratt & Whitney’s geared PW1000G.
Therefore, the LEAP-X’s development schedule almost looks relaxed, with a full engine demonstration in 2012 and certification by 2016.
Between 2012 and 2016, a lot of work will be conducted on integration, which will involve the new GE-Safran joint venture in nacelles. The company was formed late last year and it is still to be named.
But what if the LEAP-X is ready in 2016 and both airframers target 2020 for their new narrowbodies? In that instance, the program would incorporate further advances, CFM officials promised. “Every year we improve aerodynamics,” LEAP-X program manager Ron Klapproth noted.
AIN has learned that development work has started for a CMC combustor that could also be incorporated into the LEAP-X program in the 2016-2020 time frame. Such materials are not deformed by creeping and have a small expansion ratio, resulting in a reduced need for cooling.
In the high-pressure turbine, CMCs could be also chosen in lieu of single crystal.
CMC blades also could eliminate the need for cooling systems: Inside the high-pressure turbine, temperatures are on the order of 1,700 to 1,800 degrees C, which is well above the melting point of current metal cast blades. That is why cooling systems draw air from the compressor. The cooling film blows at 600 to 700 degrees C and is less than 0.1-millimeter thick. Cooling blades use 6 to 12 percent of the core’s airflow, which impacts the engine’s efficiency. Eliminating the need for cooling is therefore good for fuel burn.
Moreover, CMCs have an auto-repair capability when exposed to heat, which is useful when a blade has been dented by a foreign object. Even composite blisks (blade integrated disks) seem realistic to CFM engineers.
CFM Keeps Open Rotor Option on the Table
In parallel with the LEAP-X, CFM also is studying open rotors that could power narrowbody aircraft. An open rotor holds the promise of a 26-percent reduction in fuel burn. However, progress on the project appears to be much slower than anticipated. The schedule loosely refers to “late next decade” for a possible certification. CFM executives would not put a figure on the number of engineers working on the open rotor and they made it clear that the concept is still at the feasibility study stage.