Plastic aircraft flying high


Ground-breaking research by a UL team is attracting international acclaim, writes Dick Ahlstrom

The nuts and bolts of how to build an aircraft can get very tricky, particularly as large elements of models planned by manufacturers Airbus and Boeing are made of plastics.

Particular challenges arise when trying to fix together wings, tail structures and fuselages made of these special composite plastics, explains the University of Limerick's chair of aeronautical engineering, Prof Michael McCarthy.

They are composites made of epoxy resins mixed with carbon fibres and they perform differently under stress than metals. Having to drill holes for rivets or bolts also affects these plastics, he says.

Limerick's Materials and Surface Science Institute (MSSI) has become expert in the performance of these composites in aircraft. The institute involves the work of 211 researchers including academics, post graduates and post doctoral researchers and is strongly interdisciplinary, including engineers, physicists and chemists, McCarthy says.

"We are the only programme in the country doing aeronautical engineering," he states.

The aeroengineering group recently received a double accolade to add to its list of credits. One of its research papers on the bolting of composite materials was voted the best paper of the year in the international journal, Composite Structures.

Soon after, a conference presentation by McCarthy on related work done by the UL group received a UK Institution of Mechanical Engineers (IMechE) safety in aerospace award for the team's research paper, which was declared best at conference.

"It is hard enough to get published in a peer reviewed journal, so being selected the best paper of the year for 2005 was a real thrill," says McCarthy.

The Composite Structures paper was written by McCarthy with Conor McCarthy, Vincent Lawlor and Walter Stanley, and the win was announced earlier this year. The journal's reviewers were asked to name their three best papers of the 240 published that year and the UL research study won the day.

It talks about one of the great challenges associated with the use of composite plastics in aircraft, the integrity of joints, McCarthy explains.

"Once you drill holes in the material you disturb the fibres in the composite," he says. "They are kind of brittle materials and when you get stress in a brittle material they are not able to yield before they fail."

For all that, the carbon fibre-epoxy resin composites are at least as strong as steel and aluminium alloys and much lighter than either, hence the manufacturers' interest in their use. Large structures such as the fuselage and wings are assembled from many pieces that must be capable of disassembly for inspection and repair.

Thousands of holes must be drilled in the composites, but this weakens the structure. It is therefore "critical" that engineers understand how these joints will perform when in use, says McCarthy.

The UL group led an EU study into the issue and published 18 papers associated with it. They analysed the stresses imposed on test joints using specially designed bolts that had integrated strain gauges.

They also developed a mathematically based way to streamline joint testing, says McCarthy. Current methods are empirical and require thousands of tests for a new aircraft design.

"Our work is addressed at improving the accuracy of numerical models for this complex problem, so that less tests are required, resulting in major cost and time savings during the development of new aircraft," he states.

The timing couldn't have been better for the MSSI. The Airbus A380 "Superjumbo" due to come into regular service soon, already comprises 23 per cent composite materials.

And Airbus's A350 which is under development to compete against Boeing's 787 "Dreamliner" will have a fully composite wing. Boeing expects that the 787 will be up to 50 per cent composites including the wings and fuselage.