Fan Case Manufacture
Huge sums of money are expended each year in wringing the maximum efficiency out of the
turbofans that power the world’s airline fleets. Usually, the structures that contain them are
regarded as an afterthought – if at all – by the public and even by many in the aerospace industry.
But creating a fan case for the 21st century is more complex than you might think.
Looked upon for decades as merely the container for a powerplant, and whose main function was to
be as aerodynamic as possible, fan cases are becoming recognised as important products in their
own right.
Increasingly, as with other parts of the airframe, a composite solution is taking the place of the
traditionally metal structures, but for the foreseeable future, metallic alloys will continue to have a
significant role in construction of fan cases.
It comes down to functionality: composites are lighter – always a touchstone of aerospace design,
but particularly today when their lower weights help offset the weight increases of the increasingly
large-diameter turbofans that power new-generation airliners.
Metal, however, retains particular qualities, notably in terms of resisting heat and in providing the
necessary strength for those parts of the casing to which engine components and ancillary systems
have to be attached.
Metallic aero-engine fan case
“The fan case part of our business consists of both metallic and composite solutions,” said Henrik
Runnemalm, head of engine technology at GKN Aerospace Engine Systems.
“In many cases there is a ‘best fit’ for the solution, depending on the functionality we are providing.
The fan case as such should do its job as a containment zone for the fan blades – which is critical, of
course. In many cases, the composite part does this very efficiently. However, at the rear of the
structure there are many parts that must be attached to the engine, such as the oil system and
wiring for different control systems. With all these fittings, in many cases a metallic solution has
proven to be a better choice.”
That section of the casing is also the connection point for the fan outlet guide vanes, static vanes
that steer the exhaust flow through the engine in the optimal direction. These aluminium or titanium
structures have load and stiffness requirements that are best handled through being attached to a
metal casing.
For those fan casing areas suitable for composites, either automatic tape-laying or resin transfer
moulding can be used to create the structure. Each has slightly differing qualities, such as
containment capabilities. Automatic tape-laying, for example, can present some challenges when
building specific geometries, while resin transfer moulding is a more flexible process, as more
complex shapes can be created.
Automated tape laying machine
In the metallic sector, meanwhile, both aluminium and titanium are employed in different sections.
One increasingly important factor for GKN Aerospace is to drive down the ‘buy to fly ratio’, in other
words reducing the amount of used material and time – and thus costs – involved in creating metal
components.
“Normally, you start off with a big ring forging that is then slimmed down to a slender ring and on
that ring you have flanges, which require a lot of machining. We provide that today, but we are also
initiating a product launch using other types of fabrication technology,” said Runnemalm. “This is
driven by the fact that we want to start with less material, to reduce the amount of machining
needed.”
Currently, the traditional form of machining means that sometimes 70 to 80 per cent of the initial
material is machined away. The waste can obviously be recycled, but the amount of time required in
the operation is substantial.
A more efficient method is to fabricate the necessary shape.
“Our vision is to come down, more or less, to zero waste. We’ve never really got down to producing
a ‘near net-shape-sized’ component. The ultimate solution would be a sheet of the metal you
require and build up the material you need, instead of removing it. Then, the only waste material is
from areas that need to have specific surface characteristics.”
There is now a drive underway to bring fabrication and additive manufacturing technology into
these products. Combining fabrication with traditional and more advanced welding methods,
together with additive manufacturing, or ‘3-D printing’, allows structures to be built up layer by
layer, with great precision.
Fabrication of engine components
GKN Aerospace’s plants in the US produce parts of the fan case – or sometimes the entire structure
– for many of the larger engines on the market today, such as Rolls-Royce’s Trent 1000 and the
GEnx-1B and -2B that power the Boeing 787 Dreamliner and 747-8. These fan cases are all produced
in a combination of metals and composites.
The latter material is, unsurprisingly, becoming increasingly popular. “There is definitely more drive
towards composite solutions,” said Runnemalm. This is partly because the use of composites helps
technical requirements, producibility and costs to be balanced.
“Engines are becoming larger and larger due to increased bypass ratios, which is good for CO₂
emission numbers. There is a drive towards lighter materials such as composites, not only to offset
the increased weight of larger powerplants but also in terms of functionality.”
Retaining highly-energetic components that fail and break off from an engine is one of the most
basic functions of a fan case.
The ever-increasing reliability of modern powerplants means that such incidents are rare. However,
the dangers of an uncontained failure are obvious. Very rapidly-moving engine parts can cause
severe damage to other systems or puncture a pressurised fuselage.
With fan blades becoming heavier as they grow in size, the materials needed to contain the energy
generated when one comes adrift are also changing. “The benefit we can have by producing
composite solutions is that we can tailor the section by laying the composite material in certain
directions and thus create strength,” said Runnemalm.
Composite fan case
This ability to precisely design the characteristics of different sections of a fan casing will mean that,
in coming years, composites will increasingly represent the way forward. “I think we will see a
situation where there is a full composite solution but I think, looking into the future, that there will
also be what we consider a new technology, a combination of the two. It’s driven by the balance on
what you need for the engine.”
Composites have been used in fan cases for at least 15 years but, as with all materials, they are
constantly being developed and upgraded as more advanced fibres and resins are brought to
market. This means, for example, that their strength and ability to withstand high temperatures is
steadily increasing.
However these capabilities, although better than those of just a few years ago, are not sufficient to
take on every section of the fan case.
“Somewhere in the case it needs metallics because as the engine goes from front to rear it becomes
hotter and hotter, so you have to use metals.” One of the challenges is to find the point in the casing
at which metal becomes necessary.
“Repairing composites is also quite challenging if you get some problem during manufacture or
operation. Metal we’ve dealt with for a very long time and it’s a very controlled process to repair it;
we have much more experience in doing so.
If patching of a composite structure is required, it can be “quite a challenge” to integrate it into the
rest of the material, especially as the patched section must have the same structural strength as
before the repair.
A further area where composites have to be carefully scrutinised is non-destructive testing (NDT). It
is not always obvious from external inspection if a composite structure contains an internal flaw –
for example, if it has been struck by a foreign object. “Therefore we’re investing in different NDT
techniques so we can make sure that our products are what we say they should be,” said
Runnemalm.
Non-destructive testing of an engine fan case
Several companies make fan cases, although GKN Aerospace is one of the two leaders worldwide in
engine systems generally; as such, it seeks to establish its differentiation from competitors by
ensuring its manufacturing technology is attractive to OEMs. “We’ve proven by the different
contracts that we are executing right now that we are quite good at these manufacturing techniques
– especially when it comes to containment on the composite side. On the metallic side, meanwhile,
we have this fabrication technique that OEMs find very attractive.”
The company hopes to increase that attraction further. “Both for composite and metallic solutions, I
think we’re looking for a dramatic change in manufacturing technologies, especially to deliver highrate volumes of the product. We’re investing in this area to meet this type of challenge.”
In terms of the metallic fabrication technology mentioned earlier, he believes that GKN Aerospace is
only at the start of a development path in this field. The ability to create components with minimal
waste will cut both waste and thus the cost of the component.
The other major new method for creating fan cases is additive manufacturing, otherwise known as
3-D printing, by which an object is built up by depositing thousands of fine layers of material and
creating the appropriate shape.
Additive manufacture - operation of an EBM machine
One of the great advantages of ‘3-D printing’ is that there is virtually no limit to the complexity of
shapes that can be manufactured. This can be achieved by two potential methods – using a laser to
create the layers of material either by metal deposition or by heating a powder base to a liquid form
and depositing it in the appropriate position. Advances in 3-D technology mean there is no limit to
the size of components that can be created.
Looking further into the future, Runnemalm believes there is a potential step-change in the sector.
Airframers are once again considering open rotors as potential powerplants for future airliners. The
concept appeared briefly in the late 1980s, when ‘unducted fans’ appeared as a potential moneysaving solution at a period of high oil prices.
The concept was tested in the air on a converted McDonnell Douglas MD-80. The main problem was
the high noise level generated, but if that problem could be resolved, the system could offer a fuel
burn roughly half that of conventional turbofans.
Should the open rotor – also known as an unducted fan or propfan – be taken up, the need for fan
cases as they are known today will considerably diminish. “That’s something we keep track of and
work on internally, to make sure we are well-positioned for the future,” said Runnemalm.