This summer, the largest jet engine ever built was being put through its paces in Peebles, Ohio.
Peebles—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 new engine has had a lot thrown at it, including 160-hr. icing tests. Meanwhile, engineers have gathered reams of data from more than 50 test points that will be used to inform subsequent manufacturing and design.
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.
The combination of these components “decreases engine weight, boosts efficiency and will also decrease fuel burn,” says GE. Airlines apparently agree: Nearly 700 GE9X engines are on order.
As development of the GE9X illustrates, jet engines have been getting bigger. In tandem with this trend, fan blades also have been growing. In fact, thinking big is a good way to approach the capabilities of a modern jet engine: Each fan blade carries a load of air when sucked into the engine that is equivalent to nine double-decker buses and swallows a huge amount of air—in fact, a squash court’s worth of air—every second.
Over the years, fan blade designs have been transformed from flat plates to complex, three-dimensionally curved shapes. That has been made possible thanks to the advances in computational fluid dynamics (CFD) modeling. Today, extensive computer modeling simulates the flows through the fan itself and determines how efficiently the blades pump air through the fan. The aim is to attain optimum efficiency. The fewer the blades, the cheaper the engine. The desire is for high aerodynamic efficiency while minimizing the number of blades required. The latest engines have fewer fan blades than older ones because the aerodynamics have improved.
Over time, the engine diameter also has increased. This is because the diameter of the fan—the length of the fan blades—has grown in relation to the core diameter of the engine. What is known as the engine bypass ratio always is being boosted to achieve higher propulsive efficiency. The core is the high-temperature, high-pressure turbo-machine that sits behind the fan blades and generates the power to drive the fan. It contains the higher-pressure compressors, the combustor and the turbines.
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. Consequently, around 90% of the thrust is generated by the fan itself,” says Simon Weeks, chief technology officer at the UK Aerospace Technology Institute.
Bigger fan engines generate thrust much more efficiently than older engines that have lower bypass ratios. Fan blades necessarily grow to pump a much bigger volume of air. The actual surface area of the blades has increased with the chord. The latest turbofan blades have very wide-chord blades, for example. The hollow, titanium wide-chord fan blade, pioneered by Rolls-Royce and introduced in the 1980s, set new standards in aerodynamic efficiency and resistance to foreign-object damage. “Designed specifically for high-bypass turbofans, the breadth of these blades sets them apart from the narrow and less efficient earlier equivalents,” says Rolls. Some fan blades have very large surface areas and resemble paddles rather than blades.
One of the compromises with high-bypass-ratio engines is that aerodynamic drag tends to increase. The casing of the engine and nacelle grows in diameter—and higher in surface area, optimizing the overall architecture of the aircraft so the bypass ratio does not adversely affect aerodynamics. The bigger the engine, the more complex the flow of air and the bigger the potential to interfere with the aerodynamics of the wing. CFD, along with some physical wind-tunnel testing, is used extensively to try to understand what is taking place.
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.
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.
Rolls has other strengths. Historically, it has pushed development of and manufacturing with hollow titanium fan-blade technology further than many of its competitors. Rolls boasts what is said to be a unique superplastic formation process for producing hollow titanium fan blades. They are made from a sandwich of three sheets of titanium that is selectively diffusion-bonded and then super-plastically formed into a complex 3D shape in a mold at high temperature.
“You inflate the blade in the mold, and that expands the blade out to fill the mold form, the full 3D shape. At that temperature, you also make selective bonds within the titanium, so you end up with a bridged structure inside the blade itself. This gives you additional mechanical strength,” explains Weeks.
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.”