Material ambitions

Aircraft makers are seeking to find lighter and greener materials for their planes.

Material ambitions

There is nothing like competition to drive innovation, and in the battle for supremacy of the skies major aircraft makers are pushing the barriers to the limits with the use of lighter and 'greener' composite materials such as carbon to construct planes.

No sooner had US group Boeing championed its new 787 Dreamliner as the aircraft of the future with its 50 per cent-composite construction, than European rival Airbus trumped this with claims of 60 per cent-composite usage in its planned A350 model.

But it is not just composites where the aerospace industry is driving forwards. Alloys of magnesium, aluminium-lithium and titanium are increasingly finding their way into aircraft construction.

With an airframe comprising nearly 60 per cent of new materials, chosen for their lightness and strength, Airbus claims that the A350 XWB plane has the most efficient structure in terms of design concept. It makes innovative use of panelled fuselage skins made of carbon fibre reinforced plastic (CFRP), which offers much easier maintenance and reparability.

Dr Henrik Roesner, a senior structural engineer at Airbus, says: "We will need both metals and composites in our aircraft and when we look at our next model, beyond the A350, we are keeping our options open and we cannot confirm that it will mainly be a composite aircraft - as far as I can see it will always be a mixture of materials."

There are advantages and strengths from metals that will be required because although composites offer excellent weight saving and manufacturing tolerances they lack other crucial factors, Roesner says. "Composites do not conduct electricity like metals, and as far as crash worthiness goes, composites do not provide the strength that metallic materials do, nor the same level of noise dampening.

"So the conclusion is that we need to combine materials and restructure composites to mirror the properties we find with metals. What is also as relevant as the choice of materials is how we are designing the architecture of the airframe. This is something that needs to be considered when designing and manufacturing a plane using composite materials, which was simply not the case years ago."

A case in point is the evolution of the A350 fuselage. In the early derivatives the aircraft featured a traditional aluminium alloy airframe but as composite technology developed, it was switched to a carbon fibre hull.

"[This change] simply reflected the current state of the research into these materials," Roesner explains.

"Very recently a composite material became available on the market that provided a much better toughness. That, together with the tolerances that could be achieved with this material, was what decided us to go down this route for fuselage parts.

"The wing structure, where the structural material is several centimetres thick, has long been of composite construction. The wing boxes are some distance from the ground and not prone to accidental damage when the aircraft is on the ground, and also the tolerances during the manufacturing process are not so sensitive. But the opposite is true for fuselage structures where the thickness is measured in single millimetres."

Even with the high composite content of the A350 there is still the requirement for traditional metal parts. But even there they bear very little resemblance to the metals traditionally used in aircraft construction. The internal structure of the wings is manufactured from an advanced aluminium-lithium alloy, while the pylons, the structure that connects the engine to the wings, is 100 per cent titanium alloys, a metal that also features heavily in the undercarriage assembly.

High innovation

"We should never make the mistake in thinking that when we talk about metals we are talking about being conservative, these are highly innovative materials," Roesner says.

"It is our philosophy in airframe development that we are building up evolutions. What we don't want is revolution. We started using composite structures in the 1970s in the A300 and then, step by step, in all the new programmes we introduced another feature, another step in composite applications. So when you think about the A350, it is the latest step."

A key challenge for the future, says Roesner, is to preserve the lightness of the composite material from the original sample to the whole airframe of an aircraft. One of composite's weaknesses, the lack of electrical conductivity, is overcome by adding a thin copper mesh as one of the construction layers. But that comes with a weight penalty that reduces the weight advantage.

"Combining this metallic material with the composite satisfies the lightning strike protection and allows a minimum electrical conductivity in the structure," Roesner says. "When you put a metallic mesh into a composite material you are reducing the weight efficiency. We will also need more acoustic dampening material between the fuselages to compensate for poor acoustic qualities of composites."

US experience

At Boeing's sprawling facilities in Seattle, US, they have had much the same experience as France-based Airbus.

Dr Al Miller, Boeing's leader of advanced technology on the 787 programme, says: "Our newest airplane [the 787] is made primarily of composite materials and is the first commercial jetliner to feature
a mostly composite fuselage and wings.

"That said, we still use advanced aluminiums, titanium and other materials in the construction of this airplane.

"The use of both composites and advanced alloys continues to grow. They offer superior performance in the aerospace environment - lower weight, reduced lifecycle costs, and resistance to fatigue and corrosion."

Miller says the extensive use of composites has dramatically changed Boeing's production system: "Together with our international partners we have had to invent all new ways to construct airplanes so that we take maximum advantage of the potential of the composite material. For example, in dimensional control in assemblies, we find advanced composites to be very attractive in reducing historical tooling costs and infrastructure. This allows easier design of very efficient production systems."

In the past, the manufacture of such huge composite structures has been painstakingly slow and the quality of the end product very much dependent upon the level of craftsmanship - a situation clearly not suitable to high-volume aircraft production.

To achieve the desired level of quality and production throughput, this once labour-intensive and time-consuming operation would need to be automated.

But despite these concerns, one of Boeing's composite experts, David Polland, says that, despite some weight issues, they plan to adopt more composites.

For manufacturing engineers at Boeing the benefits of composite fuselage sections are undeniable. One metal barrel requires about 1,500 sheets of aluminium held together by nearly 50,000 rivets. Whereas, when joining the same composite sections, only 20 per cent of these fasteners will be required.

Like Roesner, Miller also sees advancements in materials as an evolution, rather than a revolution. "In aviation we don't see new materials breaking on to the scene," he says.

"They are tested systematically and introduced slowly to ensure they meet our stringent safety and performance requirements. The materials we are using on the 787 have been in development, testing and initial deployment since the 1980s. We see continued improvement in both composites and metals for future aircraft use."

Getting smart

A key area of long-term interest is 'smart' materials. "To me the future is not about which material we finally use, but using the best one, building the appropriate design and finally bringing intelligence into the material and the structure," Roesner says.

"It is really like a bionic or natural system - these are not built simply as a material but are always an efficient structure, and that efficiency is provided through a combination of a nervous system - muscles and actuators. These are integrated together with the structure and it will only be this combination that provides structural efficiency.

"This will allow us to implement a health-monitoring network within the structure, to build in adaptivity ensuring optimum airflow. All this will only be possible with a smart structure."

A man well versed in the research into these smart materials is Dr Ian Bond, a reader in aerospace materials at the University of Bristol. "The big thing that is overarching everything is multifunctionality. I don't like the term 'smart materials'. [It's about] building in other functions, a lot of which is biologically inspired," Bond says.

"If you think of trees, they do multiple functions, they are structured. They carry nutrients and they can repair themselves. From a research perspective in universities there is a lot of work going on into things such as building in sensors for structural health monitoring, then taking it a step further and doing something with the information you gather, getting a reaction from the material."

According to Bond, composites, with their many-layered structures, are ideal for adding intelligent layers. "What composites give you is a structural hierarchy within them. It is not a lump of material, it has an internal structure, so you can see how you can build things into it [such as] fibre optics or sensors."

Future research might also lead to shape memory materials, or morphing metals, as they have been dubbed.

NASA is said to be tentatively working on materials that can make fighter planes morph from a fighter into a bomber and back again, but needless to say that remains very much science fiction for now.

"The ability for an airliner to move from take-off to cruise configuration by changing the shape of its wing-tips will have quite significant aerodynamic benefits. While civil aviation is justifiably conservative in UAVs [unmanned aerial vehicles] you may be able to use things like morphing wings," Bond adds.

With so many diverse strands of research into composites as well as exotic alloys, it is difficult to plot the path of aerospace material evolution. But one thing is clear: all the chief protagonists are keeping their options open with all the bases covered.

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