Tests currently under way at the Airbus UK facility in Filton are exploring technologies aimed at extending the use of advanced composite materials on the main wing of future airliners such as the A350.
Airbus has steadily increased the proportion of advanced composite materials in its aircraft since it first used glass fiber reinforced plastic fairings on the A300 back in 1972. Today the A320’s vertical fin, horizontal tailplane, engine cowls and wing control surfaces are composed of carbon fiber reinforced plastic (CFRP). The horizontal tailplane on the A330/A340 serves as a fuel tank as well, while the A340-600 introduced a CFRP rear pressure bulkhead and keel beam. Meanwhile, the A380 adds a center wing box, unpressurized rear fuselage sections, some wing ribs and flap track beam sidewall panels of CFRP. The A400M military airlifter will have composite wing spars and skins, but the wing will not be required to accommodate the landing gear.
At the Royal Aeronautical Society’s recent annual conference in London, David Phipps, head of the composite development center at Airbus UK, talked about the possible next step. Researchers over the last 10 years have investigated materials and manufacturing techniques, and having built and tested a 20-foot wing box under the high load input element rubric, Airbus UK leads the effort to build and test a 60-foot wing box as part of the TANGO program. Only the outer 40 feet are composed of CFRP; the project began in the late 1990s, when making a carbon inner wing that could support the landing gear and a heavy engine was seen as a subsequent evolutionary step.
A total of 21 partners from across Europe and beyond contributed elements of the wing box using a range of materials, fabrication techniques and tooling. Airbus at Stade, Germany, made the lower cover using liquid resin infusion and non-crimp fabric (NCF), similar to the combination used for the A380’s rear pressure bulkhead, while Alenia made the upper cover using conventional pre-preg material. Where the lower skin has NCF stringers formed by resin transfer molding (RTM) and bonded to the cover in a secondary process, the upper cover uses stitched braided tube stringers film co-cured to the skin.
New Resin Techniques
Bombardier Shorts made the front spar of NCF using resin transfer injection, and GKN Aerospace used resin film infusion (RFI) technology for the rear spar.
There are four types of rib. Both Israel Aircraft Industries and Patria of Finland used RTM, the former with NCF and the latter with woven fabric. Australia’s Cooperative Research Centre for Advanced Composite Structures (CRC-ACS) used the thick film RFI process with woven fabric. And both Alenia and Hellenic Aerospace Industries of Greece used pre-preg.
Airbus assembled the parts at Filton for testing. It has finished upload and download testing and is now testing the wing box to ultimate load. The final stage will involve fatigue testing with deliberately applied visible damage; in the meantime, the exercise has significantly improved the partners’ knowledge of manufacturing processes, Phipps said.
The Airbus Stade lower cover, for example, used a family of modular single-size stringers to reduce tooling costs, which reduced manufacturing costs but at the expense of weight effectiveness. Technicians cured the rear spar at a lower pressure and with less complex tooling as a step toward dispensing with the need for expensive autoclaves: the process turned out to lack robustness but Airbus UK is working with GKN to improve it.
The cost and weight benefits of the CRC-ACS composite rib, a flat plate with dimpled stiffening made using a single film of resin, diminished with the need to use metal attachments and the associated manufacturing costs.
Making the outer wing of carbon and attaching to it to a metallic inner wing carrying the engine and landing gear is not weight-effective, Phipps said, because the heavy joint required for the attachment negates the weight saved by the use of composites.
The choice of material for the ribs appears less clear-cut, Phipps added, since factors such as crash stresses and over-fuel pressures limit weight savings and the manufacturing technology for metallic ribs is extremely cost-effective. Other lessons included the fact that the best choice of material depends on the properties required by a specific part and the importance of tolerances when it comes to assembly.
The TANGO team targeted savings of 20 percent in weight and cost over the last large aircraft wing Airbus UK designed from scratch. It met the weight target for the outer wing but even a 20-percent savings over pre-preg technology proved insufficient to make the composite structure cost-competitive with metal. Further work includes the construction of an RTM rib thick enough to form part of the landing gear support structure and an inner wing box as part of the advanced low cost airframe structures project.
The new A350 airliner will probably have composite wing spars and skins with metallic ribs and fittings, Phipps said, and the overriding goals remain achieving the full weight saving potential of advanced composites while driving down costs. In the medium term, all wings will likely be of hybrid construction.
Ultimately, whether wings are black or silver will likely depend on the application: carbon’s lower weight may prove more attractive on a long-range aircraft because of its effect on fuel burn, while lower manufacturing costs might make metal preferable on a short-haul aircraft whose acquisition cost is relatively more important. The point of the current research is to make both choices available to the designers of future aircraft.