Composite class: developing the Airbus A350-XWB

The future is plastics, as the young Dustin Hoffman was memorably told in the 1960s classic film The Graduate. For the aerospace industry, it was certainly some distance into the future. It’s only now, more than 40 years later, that polymers are starting to make serious inroads in the construction of aircraft; but the era of the plastic plane is well and truly with us. Boeing’s 787, the first major airliner to be predominantly made from composites, is expected to be certified later this year. Meanwhile, the Airbus A350 XWB, which also makes heavy use of the lightweight material, is currently in the final stages of preparation before a planned first flight in 2013.

The XWB in the designation stands for ’extra-wide body’, and the A350’s cabin will seat nine abreast in standard class and eight in premium class the company claims that each seat will be 1.3cm wider than the equivalent configuration of the aircraft’s chief competitor, the Boeing 787 Dreamliner. In fact, the entire concept of the aircraft was originally conceived as a response to the Boeing 787. Initially proposing an updated version of its previous wide-body jet, the A330, the company responded to criticism of a lack of ambition by launching a programme to design a lighter-weight, more aerodynamic airliner that could match the Dreamliner’s claimed 25 per cent reduction in fuel consumption compared with its predecessor.

In fact, the company has ended up designing an aircraft that will compete more with the Boeing 777 than the Dreamliner, as it’s a slightly larger aircraft; the A350 family will seat 270-440 people, to the 787’s 210-330. There are three aircraft in the A350 family, with the basic model actually being the middle one, designated the A350 XWB-900, with a 65m-long fuselage and a maximum take-off weight of 268 tonnes; the smaller -800 is a shortened-fuselage variant, while the larger -1000 is stretched.

Most parts of the aircraft are a mixture of composite and alloy

The A350 also has a slightly higher proportion of composites in its construction than the Dreamliner 53 per cent, as opposed to 50 per cent. The remaining 47 per cent is composed of titanium and aluminium-lithium alloy. Most parts of the aircraft are a mixture of composite and alloy; in the wings, the main metal components are inter-spar ribs (the spars being the supporting structures running along the wing from tip to root, and the ribs running across the width of the structure). In the fuselage, the outer skin panels are carbon fibre, as is the frame, although the frame includes aluminium strips to ensure that lightning strikes can be dissipated. The supporting cross beams are metallic.

As this is the first composite fuselage Airbus has manufactured, testing is under way, with a composite panel replacing an aluminium one on an A340 testbed, providing information on the acoustic performance of the material. ’Composite is neither worse nor better [in terms of acoustics]; it’s just different,’ said Didier Evrard, A350 XWB programme manager. ’We thought we needed to get information on adjusting the acoustic dampeners.’

Airbus has built two fuselage demonstrators at its Hamburg facility, including a full-scale demonstrator that it is using to test installation and systems an approach it adopted after problems with the wiring systems of the A380, which were designed in digital mock-ups. Difficulties with translating this onto the real aircraft led to delays in the delivery of the super-jumbo. ’We are training our pre-final assembly line people and testing the system installation process on the physical mock-up at Hamburg,’ said Evrard. ’The teams train on installing the harnesses, pipes, tubes and wires before we move to the aircraft for real. It has been really useful, with the technicians and designers informing each other on changes and improvements.’

The wing of the A350 XWB is an integral part of the overall design for fuel economy, and may be the most distinctive part of the aircraft, with its thin cross section and upwards-curving tips. It is the second composite wing that Airbus has tackled, but the first for a commercial airliner; the first was for the A400M military transport aircraft.

The way the wings will be constructed does, however, represent a first for the company: they will be built in a horizontal configuration, instead of vertically, like previous wings. This is to allow access to the central wing box area and to make the holes for the components installed underneath the wings easier to get to.

The wings will be assembled at Airbus’s facility in Broughton, near Manchester, using parts from all over the world: front spar from Spirit Aerosystems in the US; rear spar from GKN in Filton, near Bristol; leading edge also from Spirit, but this time from Prestwick in Scotland; upper cover from Germany; and lower cover from Spain.

Once shipped to Broughton, the spars will be attached to the aluminium-lithium ribs, and the covers will be fixed temporarily into place for drilling. The covers are a single piece, rather than multiple parts as on the much larger A380 wing and, again in contrast to the A380, the skins for the wings are bonded rather than bolted into place, so far fewer holes are needed only 5,000 per cover, rather than the quarter of a million for an A380 cover. Once drilled, the covers are removed for deburring and sealing, and then replaced for fastening onto the structure. The internal structures fuel tanks and some of the hydraulics are installed, before the wing is sent for painting and then flown over to another Airbus plant, in Bremen, where the electrics, pneumatics, control surfaces and remaining hydraulics are put into place. When that is done, it’s off to the final assembly in Toulouse.

The shift to composite parts rather than metallic puts a different emphasis on the manufacturing process. Metallic parts are generally produced ’near net’ that is, close to the specifications needed but not spot-on, with machining providing the final touches to get within the engineering tolerances. But composite parts are made the right size from the start, so the most precise engineering has to go into making the tools to produce the parts, rather than into making the parts themselves.

The A350 has a slightly higher proportion of composites in its construction than the Dreamliner aircraft

Electronic systems for the aircraft come from Thales, which is providing both the cockpit and the in-flight entertainment systems. The cockpit will have a similar feel to other Airbus models which is part of the company’s policy to ensure pilots can make the transition to the new aircraft easily and Thales has customised its head-up display (HUD) system, used on the A380, for the A350 family.

The cockpit design, currently being tested in Toulouse, has six displays, with the HUD and the on-board airport navigation system integrated into the display system, which will host the software directly. This has allowed Thales to dispense with a separate processing unit for these systems, which is said to free up cockpit space while reducing weight by about 15kg and cutting electricity consumption by 150W.

HUD is a relative newcomer to commercial airline cockpits, although it has been used by military pilots for years. However, Airbus began using it in 2009, and had it on its wishlist for the A350 series from the start, said Michel Soler of Thales Avionics. ’Thales’s HUD was part of the initial design of the A350 and will get certification when the aircraft enters into service, meaning quicker delivery for customers,’ he said. ’HUD on commercial airliners is now a real trend. Our goal is for this type of product to become standard on all Airbus aircraft in the near term.’

In another similarity to the A380, the A350 will have its own engine, developed by Rolls-Royce. The latest version of the Trent range of turbofan engines, the Trent XWB, underwent its first test last June. ’The Trent XWB has the lowest carbon emissions of any wide-body engine, and will be the most fuel-efficient engine on the market,’ said Ian Crawford, director of Airbus programmes at Rolls-Royce.

“Our goal is for the head-up display to become standard on all Airbus aircraft”

Michel Soler, Thales Avionics

The Trent XWB, as its name implies, is a development, rather than a step-change, in engine design. It is a three-shaft turbofan, lighter than previous members of the Trent family owing to the use of bladed discs (blisks) in its compressor stages, rather than turbine discs where the blades are attached to the central section the first stage of the eight-fan intermediate pressure compressor is a blisk, as are the first three of the six fans of the high-pressure compressor. The main fan of the engine is huge 3m across, wider than the entire fuselage of a Concorde, and sized to keep the engine operating quietly when it has to handle heavier loads.

Other innovations include improved heat-resistant materials, which enhance efficiency, and larger bearings able to handle greater load.

One issue that remains to be resolved is whether the same engine will serve all models of the A350 XWB fleet. This was the initial plan, but airlines have expressed concern that the engine as currently designed will not deliver enough power for the largest version of the aircraft, the A350-1000. There is speculation that Rolls-Royce will develop a more powerful variant of the Trent XWB for the A350-1000, with news agency Reuters quoting a major A350 customer, the chief executive of Air Lease Corporation, saying the current configuration wasn’t sufficient for payload, range and runway performance on the larger aircraft and that Rolls-Royce had agreed to look again at a new design. An announcement is expected at the Paris Air Show, after this issue of The Engineer has gone to press.

Second nature

’Concept Cabin’ is supported by a bionic structure that is said to mimic the structure of bird bone

Airbus’s future designs department has looked into the future to present an idea of how flying might look in 2050, with a ’Concept Cabin’ design that it claims is ’inspired by nature’. Some of the details provide some tantalising glimpses of the engineering developments the company has in mind for future generations of aircraft.

For example, the most striking feature of the Concept Cabin is that it can become near-transparent, with an ’intelligent cabin wall membrane’ controlling air temperature that can transform the cabin into an all-around view. This membrane is supported by a ’bionic structure’, which, the company said, mimics the structure of bird bone and is optimised to provide strength where it is needed.

Engineers will recognise this description as being characteristic of additive layer manufacturing (ALM), where structural optimisation software is used to design a complex shape to bear the forces experienced by the structure, which is then built layer by layer by melting fine powder with a laser.

Airbus is developing ALM techniques at EADS Innovation Works in Bristol, and has developed parts such as ducting for air management inside the A380 wing and trials for components in landing gear. The company has an overall plan to be able to ’print’ an entire wing within 25 years.

The Concept aircraft also includes design features such as engines semi-integrated with the fuselage, positioned above a U-shaped tail to help reduce the noise of the aircraft, and wings shaped to provide the laminar flow of air over the aerodynamic and control surfaces, removing turbulence from the airflow, which increases the power needed to push the aircraft through the air. Thinner wings reduce the space available for fuel storage, but this is a trade-off with the improved efficiency of the design.

Other engineering features include fully recyclable materials within the cabin space; the use of self-cleaning materials to reduce maintenance; and systems to recover passenger body heat to power the cabin features.