RR Générique

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Rolls-Royce is preparing to move on from its long history of employing hollow titanium alloy fan blades for its turbofan engines, instead turning to manufacturing the frontline aerofoils out of carbon fibre-reinforced plastic (CFRP).

The British manufacturing giant has developed a composite fan blade design - together with its UK partner GKN Aerospace - which is to be flight-tested in 2013, and could become available on a new engine, beyond the Trent XWB, towards the end of the decade.

While US-headquartered General Electric introduced carbon fibre fan blades on the GE90 engine in 1995, and has since used a similar design for the new GEnx-family, its competitor across the Atlantic has held on to a titanium construction until now.


© Rolls-Royce

Blades from the black stuff: Rolls-Royce's composite fan blade demonstrator

It has thus far not been possible to produce a composite fan blade that is as thin as its metallic counterpart, says Robert Nuttall, vice-president strategic marketing at Rolls-Royce. The thickness of the aerofoil's cross section determines its aerodynamic efficiency. Titanium has thus far delivered the best balance between weight, drag and durability against vibrations, foreign object damage (FOD) - such as bird strikes - and erosion through sand, volcanic ash and rain. All of Rolls-Royce's large turbofans since the original RB211-22 have been equipped with hollow, wide-chord titanium fan blades, produced via a super-plastic forming, diffusion bonding (SPF/DB) process.

"Composite blades are light[weight] but, in order to have the strength to deal with the real-world requirements, they tend to have been thicker than a normal [metallic] blade, which means they are not as aerodynamically efficient," Nutall says. Rolls-Royce says the titanium fan blades are "lighter and more aerodynamically efficient than those on the [General Electric] GE90." Now, however, Rolls-Royce and composite specialist GKN have developed a carbon fibre fan blade demonstrator that is as thin as the titanium aerofoil, and fulfils the other criteria in robustness, manufacturing costs and production volume scalability as well. This carbon fibre fan blade has already undergone ground tests, including blade-off and bird strike tests, and is to begin flight tests on a Trent 1000 in the second quarter 2013. The Boeing 787 powerplant has been selected because it is the model that is best understood, he adds. With the ever-increasing availability of data, such as pressures, temperatures and flow patterns at multiple measuring points throughout the engine, the Trent 1000 has allowed more insight during ground runs in Derby and flight tests on the aircraft than any previous engine model.

WEIGHT SAVING

Nuttall declined to comment on how much weight the manufacturer expects to save by moving from titanium to composite fan blades. He says, however, that the total weight reduction will not result from the lighter aerofoil alone - a "significant" contribution will also come from a new carbon fibre fan case.

Like the blade, the case has to fulfil different purposes, and has thus been designed as a sandwich construction, where each layer takes on a certain function. The inner surface is to provide as little as possible clearance to the rotating blade tips, to avoid wasting energy in the fan stream, which gives approximately 90% of the total thrust. At the same time, however, damage to the blades must be avoided if their tips touch the sealing liner during vibration or turbulence. This is why the inner surface is typically covered with an abradable coating.


© Rolls-Royce

A fan blade about to undergo a bird-strike test

Moving outward, the next layer is designed to absorb energy in case of a blade failure. To be certified, the engine must contain any debris if a fan blade or segment of it detaches at maximum power. Finally, the outside part of the fan case has to provide structural support for accessory equipment such as pipes and wiring.

The fan blades and case are designed as an integrated system. Thus far, the case has undergone separate ground tests, including blade-off tests, Nuttall says. Both components will be tested together on the aircraft in 2013, as an ­advanced low-pressure system (ALPS).

The composite fan and casing will be available to become of a new engine "before the end of the decade", says Nuttall. However, a retrofit to current models, including the Trent XWB, will not be possible because the existing engine cores have been optimised for their dedicated fan blade and case systems, he says.

While the weight benefit of carbon fibre obviously increases with larger fan sizes - suggesting applications on high-thrust engines - the currently tested composite blade and case system could "quite easily" be used for a narrowbody powerplant, according to Nuttall.

Due to increased strength requirements for the smaller and lighter fan blades for medium-thrust engines, GE's CFM International partner, Snecma, will utilise a new CFRP construction process for the Leap-X - the successor ­generation for the CFM56-family.

Instead of laying-up multiple pre-impregnated carbon-fibre sheets, as for GE90 and GEnx fan blades, Snecma will produce the Leap aerofoils using a 3D resin transfer molding (RTM) process. In this, the carbon fibres are woven into a three-dimensional pattern, before resin is injected and the blade cured in an autoclave.

ONE SIZE FITS ALL

Rolls-Royce is certain that its composite fan blade will be suitable for all large by-pass turbofans, except for small powerplants on business jets, where titanium still offers a weight benefit. The manufacturing process is the main area of the partnership with GKN. Together with the South of England Economic Development Agency, both companies invested nearly £15 million ($23 million) last year in the setup of a pre-production facility at GKN's site in Cowes, on the Isle of Wight. ­Nuttall explains that, while the lay-up of current-generation CFRP fan blades involves a large degree of manual labour, the future process will be fully automated, which should guarantee high production consistency, and the possibility to scale up the output. It will also allow the carbon fibre material to lay-up in a three-dimensional fashion, he adds.

"We're the only enginemaker that has an optimized engine on the three latest engine programs, the Airbus (EAD.PA) A380, A350, and Boeing's 787," Robert Nuttall, Rolls-Royce's vice president for strategic marketing, told Reuters at the Farnborough Airshow.

"Pratt & Whitney (UTX.N) and General Electric (GE.N) don't."

Rolls' Trent 1000 engines power Boeing's new 787 Dreamliner, which entered service last year. Rolls plans to upgrade this engine to one dubbed the Trent 1000-TEN, which will deliver 3 percent better fuel burn than the existing engine and will be used on the 787-8 and newer variants from 2016.

"The smooth entry into service of the Trent 1000 engine on the 787 shows that we can be trusted and I think that showed to Boeing they can rely on us," said Nuttall.

"We have tremendous incumbency on the widebody market - half of the widebody order book that is out there is powered by Rolls-Royce."

GE believes it has the edge because it is the current engine incumbent on the 777.

Unconvinced by re-engining programs such as the Airbus A320neo and Boeing's 737 MAX, Rolls-Royce believes the future lies in developing projects that match engines and planes from the outset of their development.

As such, Rolls last year sold its share of the International Aero Engines (IAE) consortium to Pratt & Whitney for $1.5 billion and formed a new partnership with Pratt to develop engines for mid-size aircraft of 120 seats upwards.

Nuttall, who said the new venture with Pratt would focus on geared turbofan technology among other things, believes widespread use of geared engine technology on commercial jets is close.

"Airbus and Boeing are vying with one another to see which one is better - the A320neo or the 737 MAX - and it may end up that they are both the same, and that will tell us that we're at that tipping point so the future engines we think will be geared," he said.

"We haven't finalised our definite list of technologies, but we don't
need to do that until around the middle of this year. My shopping list
is more extensive than it was [before the Airbus decision to increase
thrust requirement for the -1000]. That's what allows us to keep the SFC
[specific fuel consumption] unchanged even though by making the core
bigger the cycle gets a little bit worse. It's also given us time to
drop the risk a little bit more on some of the technologies." Production
of the first of six test engines for the -1000 is due to begin in the
first quarter of 2014.

Composite fan blades are one example of a technology likely to reach
maturity in the required timeframe. "Until now we would argue that
Rolls-Royce has led the world in titanium fan blades - a blade
technology that has been lighter than its composite competitor," says
Nuttall. "If we can get a composite system - the blades and the fan case
- then we can save a lot of weight, but up until now we haven't been
able to get a composite blade that's as aerodynamically efficient as a
titanium blade.
"It has been easier to make a thin, slim aerofoil
with metal. We now believe we have managed to get a blade that is as
efficient as our titanium blade but allows us to have a composite blade
and containment system and altogether they will save us 500-1,000lb of
weight. In context, that's an amazing amount of weight to save. The
trick is how you lay-up the blade."

Rolls-Royce is also to become the exclusive engine supplier on the
-1000. The Trent XWB engine to power the -1000 will incorporate benefits
derived from the manufacturer's Advance 3 technology demonstrator
programmes.

The manufacturer will not increase the fan size, but will spin it 6%
faster and alter the internal aerodynamics - shaping the inner annulus
at the hub - to draw a larger airflow through the same intake.

It is scaling up the core with an annulus which is 3-4% larger. Among
the efficiency measures are a dual microstructure technique implemented
in turbine manufacture that enables the properties of the engine discs
to differ in line with the differing temperatures at the hub compared
with the rim.

The empty weight of the aircraft will increase by 2.4 tonnes, of
which each engine will contribute 250kg (550lb), although Rolls-Royce is
planning a weight-reduction programme in the fan.

R-R has progressed its composite fan development work to the point where it has completed tests on the first Trent 1000-based advanced lightweight low-pressure system (ALPS) demonstrator engine at its Derby, UK headquarters, and begun testing a second. These powerplants incorporate a carbon-titanium (CTi) fan system (titanium is used at the leading edges of the composite blades) as well as composite “rafts” that simplify the installation and maintenance of the engine’s pipes and cables.

The Trent 1000 uses a 112in-diameter fan with 20 blades, but this is reduced to 18 on the ALPS demonstrator.