“Weight is the enemy.” Structural weight has been an obsession since the beginning of aviation history. All aircraft manufacturers have searched for robust but light materials to build their machines. Today, composite materials that associate fibres and resins are the most promising. They have started to become standard for a growing number of parts of aircraft structures because they are far lighter and more resistant than the aluminium alloys they replace. Airbus has played a major part in that revolution and it is still ahead.
Forty years ago when Airbus was created, composite materials were in their infancy. Nobody was really considering Carbon Fibre Reinforced Plastic (CFRP) as an option for major structural elements. In fact, a mere three to four percent of the original Airbus A300 was made of composites. A large part was glass fibre reinforced plastic, and it was applied only to secondary parts such as fairings. None had to carry important loads. However the situation changed rapidly by the turn of the decade with the development the A300’s shorter derivative, the A310.
“We had a couple of weight problems,” remembers Hartmut Mehdorn, who at the time was in charge of production before his appointment as president of Deutsche Airbus. “Not only was there a question of overall weight, but the aircraft was a little bit too heavy in the rear.” It translated into increased fuel consumption at a time when Airbus needed to extend the range to cope with a longer-range competitor. The solution came from Hamburg – a CFRP vertical fin that reduced the weight by more than 250 kg while reducing the number of individual parts. It was more expensive than the traditional metallic structure but ultimately it paid off.
To develop, produce and obtain the certification of that very first primary structure element entirely made out of carbon fibre composite, Airbus had to work very closely with its suppliers. “Nobody anywhere in the world had done it before. We had to set up almost a new factory for this,” says Hartmut Mehdorn. “Nothing existed, so we had to invent it ourselves step by step.”
After that decisive breakthrough, the whole commercial aircraft industry has turned to the possibilities of carbon fibre composites – but Airbus led the way. Not only have composites accounted for a growing percentage of the structural weight of each new Airbus model, but larger and more complex components were developed. Composites represent about a quarter of the A380 structural weight and will account for more than 50 per cent on the future A350. Parts as vital as the long keel beam of the A340-600 or the very large centre wing box of the A380 have been made using CFRP.
However, despite all their positive qualities, carbon composite materials are not panaceas. They are relatively light, insensitive to corrosion and fatigue, and modern manufacturing techniques permit them to be designed and produced into large complex components, drastically reducing the number of joints and parts. On the other hand, they still have a number of shortcomings such as the absence of electrical conductivity or sometimes the difficulty of damage assessment. Lastly, they are significantly more expensive than aluminium alloys. In other words, composites are not always the optimum solution.
That has led Airbus to keep open the competition between the different advanced materials: metallic, fibre composites or laminates. “Our approach is not technology for the sake of technology. It is to use the right material at the right place, which is where it brings value,” explains Alain Tropis, the head of the Airbus Centre of Competence Structure who is in charge of R&D on new materials. “That is what we call the Intelligent Airframe concept.”
The Intelligent Airframe concept goes far beyond the simple choice of a given material for a specific part. “In fact, we are looking for the best solution for the entire aircraft, not the best for the parts. In that frame, the best overall solution may not be the best locally,” explains Alain Tropis. An example of that philosophy is the design of the so-called electrical structure network within the carbon fibre composite fuselage of the A350 in order to fulfil the electrical requirements that the carbon composite cannot. Another example is the A350 cross beams that would have been designed in CFRP if their load-carrying function alone had be considered, but because of other airframe requirements, including electrical conductivity, it is an Aluminium-Lithium solution that has been selected.
These examples illustrate what is really expected from advanced materials and, in particular, from composites. It is a fact that, at present, no material offers a universal solution to designers. For years to come, research for overall entire airframe optimum solutions will involve both metallic and composite components. However, the trend is clear – what is sought is the integration of multiple functions; in other words, advanced materials which are not only capable of carrying loads but also fulfil other requirements such as electrical conductivity or noise abatement. At the same time, Airbus is working on processes to enable the integration of several parts into one single component in order to bring down the total number of parts.
In parallel, much research is conducted to reduce costs. In that respect composites that permit the manufacture of large integrated components that are corrosion and fatigue free are very promising. “By using composites, on the A350 we have enlarged the service intervals for the aircraft from six to twelve years,” says Alain Tropis. “For customers this brings a massive value in terms of maintenance costs reduction.”
All the structural weight reductions permitted by composite materials translate into reductions of the fuel burned during the entire operational life of the aircraft. As they are corrosion free, composites reduce the quantity of chemical products used to protect the structural components. Overall, light composite structures are a significant factor in the reduction of air transport’s environmental impact.
Loyal to its environmentally-friendly full lifecycle approach, Airbus has launched a number of initiatives to find ways to recycle the composite parts at end-of-life of the aircraft in the same way it did so for its first generation of metallic airliners. However, the first airplanes with a significant CFRP content will not retire for decades.
The development of new materials and of the associated technologies is a very long process. “If I look at what we are applying on the A350 today, that is more or less what we were starting to develop when I joined Airbus in 1983,” notes Alain Tropis with a smile. That means that it takes around 20 years to develop new material technologies. That is the reason why Airbus has maintained and is still maintaining a strong R&T effort.
One of the R&T streams is to develop materials integrating more functions. For example, carbon fibre composites that will be good not only at carrying loads but also at dissipating lightning strikes or even materials that could be self-healing. To this end, work is conducted on the use of different fibres and resins, and on nano-particules.
In parallel, much research is ongoing on parts-number reduction through physical integration. The development of larger integrated components would mean fewer parts to assemble and consequently less risk of damage and corrosion and a more robust structure.
Cost remains a key driver. It seems unlikely that the price of composite materials - as well as the price of advanced metallic materials - will lower dramatically. It is why Airbus is working on improving the buy-to-fly ratio that aims to make more efficient use of the materials.
