FLUG REVUE March 2006: Composites for airliners are increasing

March 2006

COMPOSITES DOMINATE FUTURE AIRLINER DESIGN

By Matthias Gründer

In Europe, research into the possibility of an all carbon fibre fuselage has been under way for some time, but Boeing's Dreamliner will be the first flying commercial aircraft that incorporates this technology. Has Airbus missed the boat?

“No,” says Professor Elmar Breitbach with an air of insistence. The Director of the Institut für Faserverbundleichtbau und Adaptronik [Institute of fibre carbon lightweight design and smart structures, IFA] at the German Aerospace Centre (DLR) in Braunschweig, one of world's leading specialists in this area, knows what he is talking about. After all, the largest single concentration of European research on the “black fuselage” is at his institute. But to understand properly the nature of the technology rivalry between Airbus and Boeing, we need to turn the clock back a few years.

Already in the 1990s, studies had been carried out at the then Dasa Airbus GmbH, the results of which suggested that a carbon fibre composite (CFC) fuselage the size of an A320 could be made about 30 percent lighter than one built from conventional materials. This in turn implied a fuel saving of around 24 million litres in the course of an aircraft's service life and was sufficient reason for the then Braunschweig Institute of Structural Mechanics to initiate the “Black Fuselage” research project. In close collaboration with a number of other research establishments, pioneering work was performed in this area, yet at the same time the scientists played down any unduly overoptimistic expectations. They reckoned that it would take about ten years for such a monolith fuselage, deliberately glued and stitched together, to be not only built but also adequately tested. Only then would one be able to consider seriously the possibility of introducing it into series production (see also FLUG REVUE 2/2001).

In Professor Breitbach's view, “That prediction was politically and financially correct at the time.” Boeing had not yet given any thought to the possibility of a black fuselage. On the contrary, as recently as the Paris Composites Show in the spring of 2002, exhibitors were complaining about Boeing's conservatism as regards the use of CFC and were heaping praise on Airbus for its inventiveness. According to Breitbach, the world's biggest aluminium manufacturer, Alcoa, was actually preparing itself – with Boeing's help – for the aircraft of the new century, so that, “For us, there was no reason to hurry unduly.”

Then came the 787. After the unsuccessful attempt to arouse interest in the Sonic Cruiser, this new aircraft would have to constitute a technological leap forward, if Boeing was to remain an equal contender at the forefront of commercial aircraft construction. As a result, the Americans announced a completely new aircraft which, to the surprise of the Europeans, was to be built predominantly out of CFC, including the entire fuselage despite this being extremely sensitive. “Of course it was primarily a political challenge to the Europeans. You can see that from the fact that today we are no longer funded exclusively by research grants, but are fully financed by Airbus,” says Breitbach.

“We now have staff permanently in Hamburg, Stade and Bremen. The work has become tougher and it reflects the pressure that is bearing down on Airbus. Nevertheless, our research is not just research for the sake of it, but our sponsor expects us to deliver products capable of meeting industrial quality standards.” Airbus aircraft already contain numerous CFC components, and its own competitor to the 787, the A350, will have even more. Thanks to the excellent European research landscape, specialists here will soon be in a position to seriously defy the Americans as regards the use of CFC.

As Elmar Breitbach points out, “When the Boeing engineers argue that fatigue is irrelevant to their black fuselage, they are talking rubbish.” The fuselage with its many “holes “ – doors, windows, cargo doors and other hatches of various sizes – is the most critical part of an aircraft. If carbon fibre composite components are used here, sooner or later delamination will occur, followed by micro-cracks. The IFA conducted research on this very subject some years ago, both on the computer and in practical tests. “Boeing will find it has a lot of problems here,” Breitbach believes, “but if it does work at some point, then Airbus can always follow suit and avoid all the Americans' mistakes.”

Nevertheless they are not waiting around in Braunschweig for the Boeing engineers to make some mistakes before taking any action themselves. The Europeans have already produced masterpieces in the form of complete vertical tail units or even the rear pressure bulkhead of the A380, and they have every intention of playing in the first division in the future. Amongst other subjects, the research team in Braunschweig is currently working on combining infusion and microwave technologies, an area which holds out the prospect of enormous savings in energy and dramatically shorter cycle times. Up to now the prepregs for carbon fibre components have been shaped by hand, impregnated with resins under vacuum and hardened in autoclaves. The industrial vacuum ovens required for this process are extremely expensive, use a huge amount of energy and take a lot of time to heat up and cool down. Moreover, only components with fixed dimensions can be manufactured in them. “With our new methods, the resin is preheated to the correct viscosity, and after the infusion the microwaves concentrate their energy on the entire component, irrespective of size.” This method will be introduced to the production line as soon as possible, following which it will be possible to manufacture up to 50 components per day.

Furthermore, the strength of the material can be improved by 50 to 60 percent and its rigidity and impact resistance can be virtually doubled through the introduction of nanoparticles. “Future Airbuses will fly with such fuselages, whereas Boeing is still having to employ extra fasteners just in case the fuselages it sticks together turn out to be not strong enough That means drilling even more holes in the fuselage. I can't wait to hear about the test results.” Another possibility is hybrid joints, under which CFC components are permanently joined together using metal flanges which in turn allow them to be mounted at high-load interfaces. But in this area the Braunschweig research team is currently in the midst of a patent infringement case with Boeing, even though the Americans have only been interested in such technologies for one year.

Up to now parts damaged by ground vehicles have been viewed as unrepairable, let alone a complete fuselage. For this reason research is already under way in Braunschweig on the subject of health monitoring. This entails using continuous waves in the shell level to localise damage and remove the old resin by mobile microwave, then fit new fibre placements and harden them again. But at the present moment in time this lies still very much in the future.

“Don't forget,” says Professor Breitbach, “that the Americans are very good at selling. Only the future will show which route is the right one.” A double shell constructed from carbon fibre composites definitely has a future in aircraft construction, but a lot of research and testing remains to be done. Again, it is not that easy to convert existing production of metal fuselages to facilities for the production of a CFC fuselage. One thing is certain, though: the next European single-aisle aircraft will have a black fuselage.

From page 90 of FLUG REVUE 3/2006

Last updated 10 February 2006

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