Blade advances break the mould

Advances in both blade aerodynamics and in composites production technologies have enabled the rapid and cost-effective manufacture of thin, efficient blades with the required durability. Dr Neil Calder reports. There is significant current activity in developing technology and capacity for the manufacture of composite aero engine parts. The early relationship between polymer composites and aero engine applications was not an easy one, characterised by what, with hindsight, was probably a misguided and optimistic technology leap on RB211 fan blades - almost sinking Rolls-Royce in the early 1970s. The impact toughness of the Hyfil composite material which was developed for these was insufficient to provide a workable solution and so the company retreated to a safer titanium option, eventually developing this into highly-efficient SPF/DB hollow wide chord fan blade technology which underpins both the Trent series and the F-35 lift fan. Whilst the core of aero engines are pushed for ever higher temperature capabilities for combustion efficiency, the much cooler front end and surrounding structure, mainly the large fan and its containment case responsible for moving over 90% of the air in these systems, is a prime candidate for the application of advanced high strength carbon fibre composites. <Onwards and upwards> Advances in both blade aerodynamics and in composites production technologies have enabled the rapid and cost-effective manufacture of thin, efficient blades with the required durability. These efficiencies have allowed increases in bypass ratios of 12:1 and beyond. Safran cite an eight times mass reduction in switching from titanium to composite blades, which has enabled a corresponding reduction in the impact loading requirements for the containment casing, allowing greater use of polymer composite materials in these structures also. There are two very different camps emerging for the production of composite fan blades. Both are driven by the same considerations of ensuring that the fibre position in the finished blade is within a very tight tolerance, and that there is a capability for producing these at considerable volume. Takt times for fan blades for future engines for single aisle civil aircraft will be in the order of 15-25 minutes.  The groups working on composite fans are the main ones present in the large turbofan arena. GE and Safran are operating through the jointly-owned CFM International and CFAN, and Rolls-Royce through its subsidiary CTAL, which it initially formed with composites experts GKN before acquiring the organisation outright. GE has had a history with composite fan blades on the GE90 series of engines since the early 1990s for the B777 programme, although these have always had a very high labour content within their manufacturing processes. 8,000 blades per year are produced by hand layup from the jointly GE/Safran owned CFAN facility in San Marco, Texas. Every blade is essentially hand crafted from over 1,700 pieces of carbon fibre laminate, taking about 340 hours each. Although the CFAN workers take immense pride in their finished product, the ability to scale up production rates to meet the levels required for LEAP-1 have required a dramatic change in design and manufacturing philosophy. It has always been clear that the process of laying up many laminates by hand was not going to be cost effective at higher production rates so the man hour content of these things has had to be reduced significantly. This has been the challenge for all the organisations involved in this area of engineering and manufacturing development.  <The ABC of engines> The CFM LEAP-1 engine variants announced so far are ‘A' for Airbus A320neo, ‘B' for Boeing 737MAX, and ‘C' for COMAC C919, so all the new single aisle aircraft currently under development are demanding a massive production ramp-up for these blades. This has given rise to an expectation for some 1,800 engines per year. The philosophy and practice of shifting detailed part effort upstream from the cleanroom into the raw fibre processes has delivered complex 3D and contour woven preforming on automated Jacquard looms by Safran's technology partner Albany Engineered Composites. This allows most of the structural fibre positioning to be achieved in a manufacturing single stage, including the blade flanges, long before the material ever sees a mould tool or resin. Hexcel has been awarded the long term contract for the supply of HexTow IM7 fibres for these preforms, which are then Resin Transfer Moulded within closed die tooling without the need for an autoclave. Safran and Hexcel signed a long-term strategic agreement to supply advanced composite materials for both fan blades and nacelle structures on all the LEAP-1 engine variants. 3D weaving offers through-thickness fibres in the strength and toughness critical Z-direction, reducing susceptibility to delamination which is the principal failure mode for composites and providing overall component damage tolerance. Safran Aerospace Composites factories in Rochester, New Hampshire, and Commercy, France have been created to achieve production of 1,400 blades this year, ramping up to 31,000 by 2019. Albany Engineered Composites has signed an exclusive agreement with Safran for the supply of woven carbon fibre preforms for the lifetime of the LEAP-1 programme. Although lacking a current production fan in carbon fibre, coming up fast on the inside track is the Rolls-Royce CTAL facility on the Isle of Wight, which is likely to be developed into a full production plant elsewhere within the South West of England by 2019. Intensive work is currently under way to take the processes used in development to full production maturity. <Automate to innovate> The Rolls-Royce technical approach to large-scale composite fan blade manufacture through CTAL has been somewhat different, preferring instead to invest in extensive automation in the layup of very precise tapes of pre-impregnated fibres to build up the required blade geometries of constantly changing thickness and shape. Hexcel's HexPly M91 prepreg has been used by CTAL to manufacture DOVE demonstrator fan blades for new generation lightweight turbofan engines. Hexcel supplies HexPly M91 as slit tape which CTAL uses to lay-up the blade in an automated process that provides considerable design freedom in the placement of fibres just where they are required to carry loads. The result is as thin as a titanium fan blade - and lighter. HexPly M91 provides outstanding performance in this application, including very high toughness and high residual compression after impact (CAI). It gives improved tensile performance in combination with both intermediate modulus and high strength carbon fibres. Good tack life and out-life provide flexibility during processing on the shopfloor and low exothermic behaviour allows for simple cure cycles of thick monolithic structures without excessive heat build-up during manufacturing. Although GE's GE90 fan established the place of carbon fibre composite fan blades, there was still mostly a manual effort in laying up the blade profile. Safran and Rolls-Royce, with their respective technology partners Albany Engineered Composites and GKN, have brought automation to the production process, giving an end product which is technically more efficient and with greater quality through repeatability. neil@engineeredcapabilities.co.uk