Aero engines lose weight thanks to composites (Part 2)

Aircraft engines have traditionally been the domain of metal but aero engine manufacturers are now saving substantial weight by using composites in the fan blades and containment case. We review the latest developments in this area.

The 18 blades for the LEAP engine's composite fan are produced by RTM. (Picture courtesy of Snecma.)
The 18 blades for the LEAP engine's composite fan are produced by RTM. (Picture courtesy of Snecma.)

In Part 1 of this feature we looked at why engine manufacturers are turning to composites and reviewed the use of these materials in the GE90 engine. Here we look at the new LEAP engine and Rolls-Royce's investment in this area.

LEAP forward

CFM International, the GE/Snecma joint venture that produces the CFM 56 engine powering many of today’s ubiquitous narrowbody workhorses, is adopting a new way of producing composite fan blades and containment cases for its advanced LEAP engine. This new powerplant is being developed for the Airbus A320neo (a re-engined version of the popular A320), the Boeing 737 MAX (a similarly upgraded B737), and China’s new COMAC C919 narrowbody.

The 15% more fuel-efficient LEAP engine will have a high by-pass ratio of 12:1.

Snecma, the Safran Group company responsible for the engine’s fan module, says it has spent 15 years and some $200 million developing the material and production system for its composite fan and containment case. It is working in close collaboration with Albany Engineered Composites, USA, a prime mover in developing and manufacturing the fan blades and containment case, utilising its strong experience in three-dimensional (3D) engineered composites.

The 18 blades for the LEAP engine’s composite fan are said to be the first to be based on 3D woven materials, resin transfer moulded (RTM), as opposed to being laminated conventionally in multiple plies as with current-generation blades. According to a Snecma official, the 3D woven material structure provides superior damage tolerance while the RTM process allows a thinner, more curved blade to be produced, with consequent aerodynamic benefits. Three years of testing have demonstrated the ability of the blades to withstand bird ingestion and fan blade off events as well as meeting stability and durability criteria.

In production, carbon fibres are woven into preforms, which are cut to shape by waterjet. The preforms are then placed in moulds where epoxy resin is injected. Cure takes place in the mould at 180°C and 8.6 bar pressure. Parts are then de-moulded and trimmed to shape with a 5-axis machining centre. According to the company, this out-of-autoclave technique facilitates the series production of complex curved shapes, with minimum material wastage.

To manufacture the LEAP composite fan case, 3D and contour weaving are combined to produce near-final net shape preforms, which are then moulded to final shape using RTM.

To manufacture the LEAP composite fan case, 3D and contour weaving are combined to produce near-final net shape preforms, which are then moulded to final shape using RTM. Utilisation of contour weaving allows the entire fan case preform, including flanges, to be formed as a single entity without the need for cutting and darting, while the continuously woven nature of the part eliminates fibre buckling after moulding.

Extensively automated production of the blades and other RTM parts – the fan containment case, the blade platforms and spacers beneath the under-roots of the blades – is due to take place in two centres, a new LEAP composites facility that AEC and Safran Aerospace Composites are constructing near AEC’s Rochester, New Hampshire, headquarters and a second new plant to be built in Commercy, France. Prospects for the LEAP engine and consequently for its composite components, are good, commitments having been received already for more than 3,500 engines.

Rolls’ U-turn

Until recently, Rolls-Royce has avoided the adoption of composites for the fan modules of its large turbofan engines, having resolutely stuck to the high-tech processes it developed for fabricating titanium blades and metal containment cases. However, this year it has announced that a new engine being developed for introduction towards the end of the decade will have a composite fan and case.

Although this looks like a U-turn, the company actually produced its first composite blades over 40 years ago, for the RB211 engine used on the Lockheed Tristar triple-engined passenger jet. Unfortunately, this bold venture proved ill-fated. The main problem was that those early Hyfil carbon fibre/epoxy composite blades, whilst satisfactory in normal service, too easily succumbed to bird strikes. Manufacturing repeatability was another issue. These problems could not be resolved at the time so there had to be a reversion to metal. Redesigning the engine for titanium blades and retro-fitting existing aircraft effectively bankrupted the original company, which had to be resurrected as a new corporate entity.

But the inexorable drive to reduce aircraft weight has caused today’s Rolls-Royce to re-evaluate composite fan solutions. Taking into account advances in composite materials and manufacturing that have taken place since the RB211 debacle, it has decided that it can no longer delay joining the composite fan set.

Any new blade must be at least aerodynamically equivalent to a metal counterpart as well as lighter, more resilient and more durable.

One of the factors inhibiting an earlier move has been that, although carbon fibre reinforced plastic (CFRP) blades can clearly be lighter than metal equivalents, it is difficult to make them as slender, so their shape has been aerodynamically non-optimum. Robert Nuttall, Vice President Strategic Marketing stresses that any new blade must be at least aerodynamically equivalent to a metal counterpart as well as lighter, more resilient and more durable. He accepts that new composite technology means that this combination can now be delivered and asserts that weight saving per engine will be substantial.

Rather than risk repeating earlier mistakes, Rolls has decided to ‘buy in’ certain composites expertise. Accordingly, it has partnered with GKN Aerospace, this UK Tier 1 supplier having extensive experience with composites, including 3D materials. The partners have developed a CFRP fan blade demonstrator that is as thin as a titanium aerofoil but can nevertheless survive bird strike and other critical events, as ground tests have shown. Next year, the blade is due to begin flight testing on a Trent 1000 engine, the Rolls-Royce power plant option on the Boeing B787. Rolls-Royce is confident that its composite solution will be suitable for all its large by-pass turbofans (including a new Trent derivative intended for Boeing‘s B777X), although titanium will still be competitive for small business jet power plants.

In tandem with the blade, a composite fan case has been developed. The inner skin carries an abradable inside coating so that if the tips of any blades should bridge the tight gap engineered between the rotating blade tips and the case during, say, vibration or turbulence, the resulting contact will not cause significant damage.

In January, Rolls-Royce and GKN Aerospace opened a new facility on the Isle of Wight, UK, to develop efficient, low-weight engine technology using composites. Composites Technology and Applications Ltd (CTAL) is a joint venture that will develop the processes needed to manufacture the new fan blade and containment case designs. GKNA’s experience with automated fibre placement and other automation will be valuable in securing the high-rate production likely to be required. About half the £14.8 million being invested in the CTAL facility and its programme comes from UK government sources through the Environmental Lightweight Fan (ELF) project, the rest being subscribed by the partners.

Here to stay

Overall, it looks as if the emerging partnership between advanced composites and aero engines is here to stay. Indeed it will proceed further as ceramic matrix composites (CMCs) start to be used in engine hot sections. It seems unlikely that aviation fuel prices will fall substantially any time soon, so the drive to reduce engine weight, in line with reductions that have already occurred in the latest airframes, is bound to continue. ♦  

This feature was published in the November/December 2012 issue of Reinforced Plastics magazine. © 2012 Elsevier Ltd. 

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