Rolls-Royce is set to return to composite fan blades — with a partnership with GKN Aerospace that will set up the engine-maker for next-generation lightweight turbofan requirements. TIM ROBINSON reports. This is a full article published in Aerospace International: February 2012[caption id="attachment_6209" align="alignnone" width="140" caption="Composite developmental fan blade from Rolls-Royce and GKN. (GKN)"][/caption] On the eve of the Singapore Air Show this month, Rolls-Royce is set to open its first large jet engine factory outside the UK in Seletar, Singapore. A sign of the company’s global footprint (and the expanding Asia-Pacific market), the facility will also include production of titanium wide-chord fan blades for Trent engines. Yet, just as Rolls allows manufacturing of this key hollow fan blade technology to go beyond the UK, it is already working on the new generation of composite carbon fibre-reinforced polymer (CFRP) fan blades and cases, which promise much lighter, more efficient engines.
The blade that nearly broke the companyOf course, the company has been here before. The previous attempt to revolutionise jet engines with composite blades in the original RB211 for the Lockheed L-1011 TriStar bankrupted the company and led to its nationalisation — a bleak period in the firm’s history. The composite material Hyfil, developed by RAE Farnborough was not resiliant enough, with the ‘matrix’ failing after birdstrike tests. This experience switched R-R on to the path of perfecting hollow titanium alloy blades, first by using a honeycomb type structure, then by using an inert gas to separate the two halves of the blade using a super-plastic forming, diffusion bonding (SPF/DB) process. This, says Rolls, still remains the ‘datum standard’ for the most efficient fan blade in the world. While R-R trod this metallic path, in the meantime, competitor GE with its GE90 (launched in 1995) and latest GEnx engines carved its own niche with composite blade technology. Its GE90 engine, equipped with composite blades won it a monopoly on the 777-200LR and -300ER versions of Boeing’s airliner, shutting out competition.
The problem[caption id="attachment_6208" align="alignnone" width="265" caption="Wide-chord titanium alloy fan blades on a Trent 1000."][/caption] So why does Rolls think that this time is different and why change back to composites? Though composites can be lighter, the two key challenges have been in aerodynamics and in manufacturing — both of which have advanced almost beyond recognition in the past 40 years since the ill-fated RB211 composite blade. Modern computer fluid dynamics (CFD) means that engineers are more accurately and precisely able to design blades. In fact, they start by calculating the ‘gap’ for the airflow first, with the blade taking whatever is left. Manufacturing of composites too, in the form of 3D lay-up and automation, means that suppliers like GKN Aerospace can now deliver the complex 3D shapes of a constantly changing thickness across the whole length, while being thin enough to match titanium blades. Crucially composite blades, while promising lighter weight have so far always had to be thicker than equivalent metal blades to provide the necessary strength. Thicker blades mean reduced airflow and thus less efficient aerodynamics. Now Rolls believe that, with GKN, it has been able to merge the two qualities, created a CFRP blade that is as thin as a titanium one, yet lighter — a fan blade Holy Grail. Says Robert Nuttall, vp strategic marketing, Rolls-Royce, on the lessons of the Hyfil experience: “Laying up a composite blade is a complex issue — it’s not about a monolithic piece of composite. It is a layered-up, multilayered composite-composite, if you like. It has different structures to give it both the strength and the thinness.” Nuttall explains the company’s aims: “We had two targets. One is lighter but the other one is it has got to be aerodynamically equivalent.” He adds: “The hardest bit of that in some respects is getting the aerodynamic efficiency — because that is the difficult piece.” However, the company is confident that it is making the right choice, saying that ‘before we went ahead with composites we had to be sure it was better than the best — and we are the datum’.
Damage and durability[caption id="attachment_6210" align="alignnone" width="120" caption="The blade features a changing thickness and profile the whole length. (Rolls-Royce)"][/caption] New composite lay-up and manufacturing processes also means that the previous issue of durability has now been addressed. The previous Hyfil blade disintegrated because ithe structure wasv not designed to cope with the non-standard forces of a birdstrike. However, today’s composite manufacturing processes features layers in different directions to absorb the required stresses and loads. Rolls-Royce has already conducted blade-off and birdstrike tests for this blade which have proved the concept. In fact, composite fan blades have certain advantages for blade-off failures. Their unique deformation characteristics mean that, for a blade-out incident a blade shatters into smaller pieces, rather than as a single high velocity heavy piece for metallics. Though it retains the same energy, smaller fragments mean that the kinetic energy is distributed around the casing. This in turn leads to more weight savings, since the reinforced fan casing (metal or metal and kevlar) can now be made of composites too. The lighter blades and fan case also shift the CoG of the engine rearward. A reduction in weight at the front of the engine also means that a lighter pylon can be used. This in turn leads to a wing that needs less strengthening and is lighter too.
Lightweight enginesRolls is coy about how much lighter a single blade is compared to its titanium counterpart — preferring instead to consider the composite fan blades and composite fan casing together as a single system. Says Nuttall: “It’s not just lighter at the blade level, it’s lighter at the system level.” Indeed, he reveals that a new Trent-sized engine using a composite ‘system’ (blades and fan casing) could be some 500lb to 1,000lb lighter than equivalent engines — a massive weight saving per engine. He explains: “This sort of technology clearly lends itself to Trents and future widebody aircraft as the weight saving grows with size.” He adds: “That’s really substantial — that’s 4% of the engine weight by the time we’re done. Our designers will go after a pound of weight. That’s counted as a good achievement so getting 500 to 1,000lb is dramatic”. Furthermore, this ‘virtuous spiral’ of a lighter engine, leading to a lighter plyon and thus a lighter wing, brings a cascading effect of efficiency to the aircraft — reducing the fuel needed. A weight saving of up to a ton for a large twin-engined aircraft, could deliver extra payload, range and cut fuel burn.
Cutting edges[caption id="attachment_6211" align="alignnone" width="157" caption="Rolls estimate the total weight savings of fan plus casing could be up to 1,000lb per engine. (Rolls-Royce)"][/caption] Durability of the blade will also be enhanced thanks to titanium edges on the front, top and back of the blades. Though the titanium only makes up 3% of the weight on the new blade, the front edge of the blade, say R-R, is where most damage and pitting from FOD, bird ingestion or dust/ash particles occur. This will also help to mitigate accidents during ground handling with ladders or other of ‘ramp-rash’ that might damage the edges of the blades. This will also aid repair since removing a damaged blade and then reprofiling or ‘dress’ the titanium if need be will be straightforward. The curved root of the fan blade, a R-R signature design feature, also means the blade slides out easily — easing maintenance. The shaped titanium leading-edge strips also solve another problem, in that composites, unlike metals, are difficult to keep and maintain sharp edges. Indeed, Nuttall observes that today’s wide-chord fan blades are so efficient that much debris is ‘swirled’ straight through the fan without touching anything. Modern high bypass turbofans, such as the Trent 1000, barely give pause when ingesting a single bird according to R-R, perhaps over 95% of power. Finally the composite blade is coated with special coatings to protect it against normal environmental wear and tear.
Testing[caption id="attachment_6212" align="alignnone" width="350" caption="Flight tests with the new blades will commence in 2013 using a 747. (Rolls-Royce)"][/caption] But this focus on composite technology is not the result of a sudden ‘Road-to-Damascus’ conversion. Rolls-Royce says that it has been working on this technology over the past decade. This has formed part of the UK-government funded Environmental Lightweight Fan (ELF) collaborative research programme, which started in 2007. So far various sizes of development blades have undergone ground testing, including bird-strike and blade-off tests, and Rolls say that the concept has been actively tested over the past four or five years. These ground tests will continue this year. However, for the next technology readiness level (TRL6) they will need to be flight-tested. These flight tests, using a full set of blades, will take place using a modified Trent 1000 on R-R’s Boeing 747 test-bed in 2013, in the US. Rolls will use the 787’s powerplant to test the blades, as it is the most modern engine it has that has built up enough test data already — unlike the Trent XWB which is still in an earlier stage of development. However, “the XWB is the standard beyond which we need to progress”, says Nuttall. The flight tests with a Trent 1000 as a donor engine and equipped with composite blades and fan casing, will also see the engine equipped with low-weight LPT (low pressure turbine) components as part of the EU’s Clean Sky ALPS (Advanced Low Pressure System) project.
Opening of CTAL[caption id="attachment_6213" align="alignnone" width="375" caption="The UK’s Minister for Small Businesses and Enterprise, Mark Prisk MP, opens the CTAL facility on 12 January. (GKN)"][/caption] With the project now in its final stages, the focus now has switched to productionising the blade. To bring this technology to market, Rolls-Royce partnered with GKN Aerospace to create CTAL, a joint venture in 2008. This has seen a new dedicated £14·8m facility on the Isle of Wight which was opened on 12 January. This will pioneer the new manufacturing processes for composite fan blades and fan-cases. Key to this for the volume of blades and cases needed to meet global demand will be increasing production rates through automation and advanced lay-up techniques. The CTAL facility, which will employ some 70 engineers, will also provide synergy with the UK’s SILOET (Strategic Investment in LOw-carbon Engine Technology) programme which is aiming to deliver a 2% improvement in engine fuel efficiency and which will finish in 2013. Said Marcus Bryson, ceo and president of GKN Aerospace at the opening of CTAL: “The processes we develop here will be at the heart of the drive to improve the performance of tomorrow’s aircraft engines. This facility will help us ensure we and our supply chain sustain the level of technological progress necessary to meet major global aero-engine opportunities in the future.”
ApplicationsOther R-R technology programmes are likely to merge with this new lightweight technology, to develop a next gen engine likely to appear by the end of the decade. Says R-R’s Robert Nuttall: “It has the potential to appear in the next large engine” and adds: “we have to honour the threat of any new big engine [from its competitors] towards the end of this decade.” Though the engine will still be three-shaft and recognisably of Trent heritage, (although Nuttall says whether it would still be called a Trent would be marketing decision) the lightweight fan & case will not be able to be retrofitted to earlier engines. So what might be the first aircraft to fly with this? Intriguingly, this January saw Boeing issue a RFP to GE, R-R and P&W for a new 100,000lb thrust class engine for its revamped 777-8X and 777-9X (updates for the GE90-powered -200LR and -300ER). This new variant of the 777, expected at the end of this decade, fits in with R-R’s development timeline. Should R-R succeed, this will give it the opportunity to supply new engines for this next gen 777— potentially displacing rival GE as the preferred engine supplier and allowing customers an engine choice in the next evolution of the ‘Triple 7’. R-R also say the composite technology is scalable and may have a place in the new joint venture with Pratt & Whitney that will replace its IAE partnership on the V2500. Though Nuttall says it is up to the proposed JV to decide, he contends that the upcoming Rolls/Pratt partnership is intending to use the ‘best of the best’ technology from the two companies — and the composite fan/case system certainly fits that bill. However, Rolls says that for smaller engines, such as for business jets, the weight savings of composites become more marginal over traditional titanium, even for solid blades. Finally, the composite blade technology pioneered here will also have an application in any future open-rotor design which would also need a composite scimitar hybrid propeller/blade. There, too, the fragmenting properties of composites may benefit certification for this new powerplant.
ConclusionsRolls-Royce (with the aid of GKN) is set to open a new chapter in its long history. With composites technology now maturing it was only a matter of time before it made the leap from structures to engine parts and the advantages (of saving up to half a tonne of weight per engine) cannot be ignored. The lightweight composite system, when combined with other ultra efficient engine research programmes, also delivers strategic advantage in maintaining its position. It also potentially provides a winning product for a crucial battle with GE (and potentially P&W) for the next variants of the Boeing 777. In narrowbody powerplants too, it gives Rolls a strong hand to play in contributing to its new post-V2500 geared turbofan joint venture with Pratt & Whitney — better able to take on the might of CFM. Finally, while this can be seen as Rolls-Royce returning to a previous concept that modern manufacturing can now deliver, the DNA of these composite blades also stretches back to the VC10’s Conway engine, some of which used composite blades (with metal edges) from 1968. Revisiting this technology with today’s design and manufacturing will mean tomorrow’s blades could, indeed be destined for glory.
This is a full article published in Aerospace International: February 2012. As a member, you recieve two new Royal Aeronautical Society publications each month - find out more about membership.
Aerospace International Contents - February 2012News Roundup - p4 India's airline hangover p 12 Challenges for India's civil airlines Ever vigilant- p 16 A new modular AEW radar system Blades of Glory- p 18 Rolls-Royce embraces composites for next gen engines Learning new rules - p 22 MRO providers face up to composite repairs Tigers on the prowl- p 26 Focus on Asia-Pacfic air power Preparing for the UAS revolution - p 30 Unmanned systems in UK defence planning Letters - p 33 UK defence capability The last word - p 35 Keith Hayward on the importance of the Euro to the aerospace industry