GOING TO THE BIRDS
Innovations such as these have enabled Rolls to use titanium fan blades for a long time, and their strength is a key attribute. A main reason is that fan blades must be able to cope with bird strikes.
“It is not uncommon for fan blades to go for years without repair,” says Sam Rice, sales and business development director for engines at aircraft spare parts specialist AJW Group. “However, when we do have to send them to our network of shops, the reason is often foreign-object damage—particularly bird strikes.” Following this type of damage, fan blades will be subject to nondestructive testing to ensure there is no blade-cracking, measurements to make sure they still have the correct shape and strength-testing to ensure they can still perform correctly.
Weeks says “one of the main challenges [for fan-blade technology] is tolerating ingestion of birds.” Aero engine-makers must ensure the powerplant can continue to operate safely after bird strikes. In the unlikely event that a fan blade comes off because of a severe impact—with a very large bird, for example—engine-makers must demonstrate that the loss of a fan blade can be tolerated. That is one of the reasons why manufacturers deliberately detach a blade during a test to ensure an engine can continue to operate safely.
In part 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,” he explains.
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,” Weeks stresses.
As in the case of the GE9X, a composite carbon-fiber fan-casing can be employed if the blades themselves are carbon fiber. Composite fan blades just require a composite engine casing, rather than a titanium engine-casing, which also helps to cut engine mass dramatically. A carbon-fiber casing can be used because the impact on—and potential for damage to—the casing in the event of inflight fan-blade loss is much less than for a titanium blade.
For metallic blades that experience foreign-impact damage, additive manufacturing can be used to build up a whole new section, machine it back, shape it and restore the blade to its original shape. “Ultimately, additive manufacturing may be used to create the blades themselves,” says Rice of AJW. Engineers would take advantage of the technique’s design flexibility to produce new forms with superior aerodynamic properties.
The UK’s Aerospace Technology Institute, meanwhile, is studying new techniques for repairing composite fan blades. Weeks concludes: “Composite blades typically need less maintenance than metallic ones. You find that composite structures are quite robust and can take a lot of abuse, but you do need to be able to inspect them, and impact damage is not always as apparent as it is on a metallic blade.”
Excerpt from and article in MRO Network.com by Ben Hargreaves