Peebles, Ohio -- GE Aviation’s boot camp for new jet engines—has seen the GE9X, which is destined to power Boeing’s next-generation 777X passenger aircraft, undergoing tests during the past few months.
The engine’s components have been under analysis for much longer than just this summer, however. Testing of GE9X components began six years ago, including evaluation of the fourth-generation carbon-fiber fan blades and fan case, 3D-printed fuel nozzles and its special new lightweight materials, known as ceramic matrix composites.
Over the decades, hollow titanium fan-blade technology has served engine-maker Rolls well, from the RB211 to the Trent family of engines. Now Rolls is examining the possibilities of producing a carbon-titanium family of fan blades, a technology still in the development phase.
On modern high-bypass-ratio engines, most of the air passes around the outside of the core, which means most of the thrust is generated by the fan, not the core. Typically, the bypass ratio on today’s modern engines is at least 10:1. “That means 10 times as much air goes through the fan and straight around the outside of the core through the bypass duct, as opposed to the core.
Rival GE, meanwhile, continues to rely on carbon-fiber composite fan blades, which it introduced on the GE90 engine in 1995. Today, large fan blades are manufactured at GE using a carbon-fiber tape-layer process, in which engineers layer strips of tape to make up the shape of the blade. Meanwhile, for the CFM56 and Leap engines, GE partner Safran has been developing a 3D woven technology in conjunction with Albany Composites. Engineers pump resin into, and consolidate, a 3D carbon-fiber shape.
Carbon fiber is used for the blades because of its light weight. But whereas it is easier to shape thin titanium blades for aerodynamic performance, composites tend to have thicker cross sections. “With titanium, you can make thinner blades, which means the aerodynamics are slightly better. You have to lay down many layers of composites to form a fan blade, and it is quite challenging to form aerodynamically efficient complex 3D shapes with sharp curves using composites,”
to help with the bird-impact problem, manufacturers are migrating toward what might be described as a carbon-fiber composite-metal hybrid. Carbon-fiber composite fan blades tend to feature thin titanium edges, which give them the impact-resistance required for a bird-strike. Birds are not the only threat from the environment. “On composite blades, you need a metallic edge to protect the carbon-fiber composite, to deal not just with bird strikes but also erosion from the dust that is in the air, rain, snow and hail,” says Weeks. These factors “predominantly affect the leading edges of the blades, particularly toward the tips, which are the fastest moving parts of the blades,”
