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.
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," Nuttall 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.
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.
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.
The technology step-change comes more than four decades after the engine maker initially attempted to introduce composite fan blades on the RB211 engine, which it developed for the Lockheed L-1011 TriStar in the late 1960s. Those blades, made from a carbon fibre-based composite named Hyfil, did not withstand foreign object impact.
The Trent 1000: testbed for CFRP blades
Rolls-Royce worked in parallel on a titanium alloy blade as a fall-back option, which was used for the RB211-22. Still, the development cost for the engine increased so much that, combined with adverse economic conditions, the company went bankrupt and was nationalised in 1971, with the government underwriting development cost. Since then, Rolls-Royce has refined the production of hollow titanium alloy fan blades to an art form.
The SPF/DB process starts with the front and rear surfaces of the fan blade being welded together along the edges. The aerofoil is then inserted into a mould and heated to a temperature where the metal becomes elastic. Inert gas is injected between the two surfaces, so that they assume the shape of the final aerofoil.
Additional titanium strips initially been welded to the insides of both surfaces thereby become an internal support structure.
But this closely guarded expertise might lose its significance in the future, at least on medium- and high-thrust engines - provided an airframer comes up with a new aircraft.