Carbonfibre-based composite first migrated into production-rated turbofan engines 20 years ago out of necessity. The sheer size of the GE90-94B made it impossible to design hub-mounted dove tails to be large and strong enough to contain a solid metal fan blade. GE had already experimented with carbonfibre blades on a cancelled open rotor engine programme, so that became the material of choice for all GE fan blades going forward.
That was the first step in a steady, two-decade obsession with advanced materials by the Cincinnati-based engine manufacturer. The original planar blades of the GE90-94 soon gave way to the bowed and forward swept shapes made possible by the carbonfibre material and three-dimensional, aerodynamic design techniques.
The next major step in the progression of composite materials is now underway with development of the GE9X. The unprecedented 132in diameter of the front fan will test the strength of any design, but especially with a record low 16 blades doing all the work. GE has previously described the GE9X as featuring a “fourth-generation composite” blade, intentionally obscuring – until now – a new breakthrough.
The engine that powers the Boeing 777X will be the first to incorporate a hybrid composite fan blade, blending both carbon and glass fibres into the same part.
“Now we’re looking at carbon and glass fibres in the same resin system. It’s our first step in hybrid composites,” says Nick Kray, a consulting engineer on composites for GE Aviation.
Unlike a monolithic carbon-based composite material, hybrid composites blend two or more materials into the same component. That enables GE’s designers to tailor the material properties along the blade.
Airworthiness requirements imposed by the US Federal Aviation Administration make it clear that the biggest threat to a front fan blade is bird ingestion. No matter how long or wide a fan blade is, the material must survive a sudden encounter with a large bird at high-subsonic speeds.
The GE9X will feature the longest and widest-chord fan blades yet inserted into a turbofan engine, so GE needs more strength than provided by standard carbonfibre material to pass the bird ingestion test.
“Carbonfibre has a certain failure strain,” says Kray. “They can strain to a certain percentage and they’ll break. The glass actually has a higher strain capability. So it’s not as stiff. It’s allowed to flex more before it fractures.”
Between 5-10% of the GE9X fan blade will be composed of glass fibres rather than carbon fibres, Kray says, but that represents only a starting point.
Other combinations of materials are possible in future applications. Instead of only glass, GE could integrate different forms of carbonfibre – such as unitape, braids and weaves – into different areas of the same fan blade, Kray says.
Such hybrid materials are also not limited inside an engine to the fan section. The large frames inside the fan case bear heavy structural loads, and so must remain metallic. “Your flow path surfaces which are not heavily loaded could be composite,” he says.
In the future, hybrid composites could be developed to replace areas of a turbofan engine normally reserved for high-temperature metals.
A large portion of the compressor section inside the engine core is exposed to temperatures between 200˚C (392˚F) and about 425˚C.
“If I have a high-temperature composite, that certainly would up my design space in that part of the engine as well,” Kray says.
It took about 25 years of dedicated research and experimentation before GE was comfortable replacing metallic surfaces in the turbine shrouds and combustor liners with lighter ceramic matrix composites. A similar timeline may be necessary to reach a state of maturity for high-temperature composites, Kray says.
Source: Cirium Dashboard
