FLUG REVUE June 2003: Materials for the aircraft of the future

EVOLUTION OF MATERIALS

By Matthias Gründer

As in other areas of technology, aircraft manufacturers are constantly searching for new materials that will deliver greater safety, longer service life and reduced weight and costs. Researchers and developers alike are focussing their attention on the mushrooming area of synthetic materials, whose potential for use in aircraft construction is most certainly a long way from being exhausted. However, they are steadily becoming more and more widespread in use. Whereas on the Airbus A300 only the radome and a few minor flaps were made of composite materials, on the A310-200 their usage was extended to include the ailerons, spoilers and air brakes, on the A310-300 the rudder, on the A320 the elevator and landing gear doors, on the A330-300/340-300 the wet box of the vertical stabiliser and on the A340-500/-600 the wing-noses, keel, pressure bulkhead and engine cowling.

Altogether, the proportion of composite materials used on Airbus aircraft has thus risen from eight to 15 percent, including thousands of components that have been and will be installed in the cabin. On the A380 this figure will rise to 20 percent and for aircraft of the next generation, expected to appear in 2010, experts are forecasting that this will double to 40 percent.

It should be noted that these figures apply only to aircraft produced by the European manufacturer, which in this respect is proving a lot more innovative than its US rival. Exhibitors at this year's JEC Composites Show, the biggest specialist trade fair in the world in Paris, all confirmed to FLUG REVUE that in the matter of composites Boeing is a lot more conservative and still places its trust in aluminium alloys.

Naturally the use of new materials is best compared by examining the latest products of the two rivals – the A380 and the 7E7 – the one already under construction, the other still at the design stage. Airbus's view is that – in consultation with the airlines – the ultra-large aircraft should utilise materials that either have already established a proven track record in service on the Airbus fleet or have been demonstrated through large-scale trials. In this way so-called “installed potentials” can be opened up step-by-step. Thus, not only will the wing-noses of the A380 be made of Fortron, a material already proven on the A340, but if possible the same material will later be used on the wing boxes and the main undercarriage bay, which are still constructed out of metal on the prototype. The necessary work is already under way and, following successful certification, implementation will be an example of such an installed potential.

Naturally all the research and development work is not simply an end in itself, and at the end of the day it is the customer who will makes the decision on whether to use the new materials, as suggested by the words “in consultation with the airlines”. During operation aircraft are routinely exposed to a variety of potentially damaging external influences: natural corrosion, mechanical knocks and blows during ground handling and in turbulent air space, enormous temperature differences and lightning bolts. Airlines are happy to pay for materials that will survive all this over a long service life and moreover produce cost savings.

Procurement costs are not the only factor that has to be considered: operating and maintenance costs are extremely significant elements also. The materials manufacturers are therefore working feverishly on further development of their products. What they are aiming for, according to Gunther Reitzel, head of the Fortron product line at Ticona GmbH, is components that can be repaired there and then, “like bicycle parts”, using simple repair sets without any need for a costly sojourn in the hangar. If they were to succeed, the specialists would be guaranteed to be always one step ahead of the competition.

One material that is being used for the first time on the A380 is GLARE (Glass fibre Reinforced Aluminium), a laminated material made of three layers of 0.5mm thick aluminium in between which are two layers of glass fibre. This has not only successfully undergone basic life cycle and stress testing in the laboratory but has also already completed field tests in which a section of the fuselage skin on a German Air Force Airbus was replaced by a GLARE skin panel. The flight trials were so successful that on the A380 the upper skin both fore and aft of the wing will be made of GLARE and work is already under way on new applications for the fuselage side skin, with possible variation of the number and strength of both metal and glass fibre layers, and also of the direction of the fibres.

To date it has only been possible to use GLARE on the upper skin due to the fact that tensile stress is experienced there during the cruise, whereas the lower skin is subject to compressive stress. GLARE cannot buckle, but it can be heavily stretched: tensile tests have shown that the glass fibres can tolerate levels of tensile stress which would cause aluminium panels to crack totally. This residual strength, the point in time at which cracking occurs and the speed at which cracks spread (crack bridging) together constitute the damage tolerance, and the airlines are especially interested in this as it will affect their maintenance and repair costs.

Moreover tests on the wonder-laminate have demonstrated that where mechanical damage is caused to the external skin by corrosion, fire or lightning strike, it is arrested at the first glass fibre layer below the outer panel. Another advantage is that GLARE skins can be bonded through gradual overlapping of individual layers, allowing large parts to be manufactured. The size of the skins in turn is limited only by the size of the autoclaves required for hardening. Amongst other things this means that enormous savings can be made in the number of rivet holes required – no less than 2.5 million, for example, on the A340 –, each of which has the potential to damage the material. All in all the allowable fatigue loads on components are about 20 percent higher than those specified by Boeing for the 777 fuselage skin.

Of course the Americans should not be criticised simply because they continue to rely on aluminium. All the same, the alloy possibilities of this light metal are still far from being fully exhausted, yet the manufacturers, led by Alcoa, continue to supply metal which complies no more and no less with the specification they are given. But even at Boeing they are not immune to the pressure to reduce weight.

Nanotechnology too, still in its early days, has a valuable contribution to make to the large structural and skin components. Eurocopter has achieved some first, very promising results from the application of a nanopaint coating to the tail booms, which are particularly vulnerable to accumulations of dirt and are now very easy to clean using the new material. At the Technical University of Dresden some specimen parts treated with nanopaint were subjected to saltwater corrosion testing. The trials were terminated after 3,000 hours as the specialists were unable to detect any damage. By contrast, standard products begin to show signs of damage after only 500 hours. It would appear that nano-coated materials will attain a longevity never seen before.

Meanwhile NTC GmbH of Tholey, Saarland is preparing for the next major success: an anti-ice coating made from a layer of paint only five ì thick, which adheres permanently not only to metals but also to glass and is so successful at getting water to drip off it that it is not possible for ice to build up. All in all, the prospects are looking good for the aircraft of the future.

From page 76 of FLUG REVUE 6/2003

Last updated 12 May 2003

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