Reducing fuel consumption, harmful substance emissions and noise are enduring topics when it comes to air travel. New types of tailplane structures, frequently made from plastics and incorporating structure-integrated functions, are making air travel more environmentally friendly as well as safer. It is, nevertheless, important to monitor the durability and reliability of these new structures. Showcasing their newest developments on this front at the SIAE International Aerospace Exhibition in Paris in June was the Fraunhofer Institute for Operational Stability and System Reliability LBF: the model of a heated aircraft wing front edge made from fibre composite and aircraft panels in carbon fibre reinforced plastic (CFRP) with integrated fibre optics.
Leading edge under de-icing condition (photos: Fraunhofer LBS)
In-flight icing has serious consequences. The ice sheet may cause up to 40% increase of the plane’s aerodynamic drag, the aeroplane becomes heavier and can lose up to 30% of its lift.
Researchers at the Fraunhofer Institute for Structural Durability and System Reliability LBF in Darmstadt, Germany have developed, built and tested a thermal ice-protection system based on nano-materials. Conductive nano-materials are embedded in the wing structure for de-icing (melting ice) and anti-icing (preventing ice build-up) purposes. This system offers several advantages: since the electro-conductive layer is integrated into the material, it is protected by the overlying fabric. In addition, no metal parts are integrated, avoiding the weak points that the metal would form compared to the fatigue behaviour, stiffness and thermal expansion of the composite structure. The heat at ground level reaches up to 120°C.
In a second test run, the ice-protection system was activated at the beginning of the test so that no ice sheet could form at all, while the heated wing was sprayed with a fine cloud of water. This approach is referred to as anti-icing. Both test runs were conducted successfully over several days and validated the simulations that were conducted beforehand, also at Fraunhofer LBF. Two different models with heating zones set up slightly differently, as well as an unheated wing for reference, were investigated. With the two heated leading edge segments different operating modes were studied so that the heater keeps the wing reliably free of ice, and yet consumes as little energy as possible.
Nanomaterial based de-icing system (video: Fraunhofer LBF)
Since the system does not feature any metal, it is especially suitable for components made from CFRP. The variable thermal expansion, together with the differences in fatigue behaviour and in the basic structure of the fibre composite that have been a characteristic of metal heating elements up to now were again and again found to account for premature failures and changes in terms of drag/resistance. It has also been possible to achieve advantages in the lightning protection sector. However, the crucial advantage lies in the low weight. In the same way as CNT wiring is set to replace copper wiring in the future, this material can also be used to replace metal heating elements and thus provide savings in both weight and emissions. It is anticipated that the concepts developed at Fraunhofer LBF for the functional integration of nanomaterial in fibre composite will lead in the future to new and improved electrical resistance heating systems in the aerospace industry, the wind power sector, the automotive industry as well as other sectors.
Up to 20t: new dimension in structure monitoring
A range of different sensor systems can be used to monitor the structure of aircraft parts made from plastic. At the Paris fair Fraunhofer scientists were showcasing large fibre composite components – so-called panels with two-channel FO cables for load monitoring and piezo-ceramic sensors or actuators for monitoring structural integrity by means of acoustic methods. Miniaturised hardware is used for monitoring data.
Numerical simulation of temperature distribution
For the first time success has been achieved in closing the complete development chain from the idea through to the test with a load of 20t. In the case of the items showcased at the exhibition, the development chain comprises the layout of the structure using the finite element method (FEM), the detailed design and construction of the drawing using CAD, the design of mould units and fixings as well as the manufacture of prototype panels with fibre optic strain sensors integrated in the structure and sensors for identifying impact damage.
Another innovation as regards the use of structure-integrated sensors for monitoring purposes is an innovative plug concept for the optical fibre developed at Fraunhofer LBF. The new plugs were initially used in a manufacturing process as close to volume production conditions as possible. The panels thus populated underwent destruction-free testing with lock-in thermography, ultra-sound, X-ray technology and, on the destructive testing side, by means of trials aimed at establishing impact as well as fatigue and residual strength.
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