Like a shark in the water

Innovative lacquer system to reduce drag

Especially over the last few years, the aerospace industry has succeeded in developing many innovations aimed at reducing the fuel consumption of aircraft and thus sparing the environment. As well as new, light composite materials, low-resistance surfaces are also becoming very important, for the lower the aerodynamic resistance of an aircraft is, the lower the propulsive output required and hence the amount of energy needed. To improve the aerodynamics of modern passenger aircraft still further, the microstructure of the surface can be altered. One possibility for improving the surface structure is to harness the riblet effect in sharks. Riblets are circumferential grooves in the scales a few micrometers in size that optimize the flow behavior of certain shark species. The effect of these microscopically small grooves is to prevent any turbulence flowing at right angles to the direction of flow and so to reduce the resistance.

The phenomenon of the streamlined sharkskin has been known for about 30 years. The challenge of the long-term experiment conducted as part of the EU's Clean Sky research project is to transfer the known biological structures to corresponding technical processes. In the two-year Multifunctional Coating research project, which has been under way since the summer of 2011, Lufthansa Technik has joined forces with Airbus and the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) to investigate the durability of these surfaces in real-life flying operations. The idea is that the insights gained will show how resilient these structures are under the environmental influences encountered during flying operations and whether an economically viable service life can be demonstrated.

To obtain reliable measurement results, eight patches each 10 x 10 centimeters in size were attached to the fuselage and wings of two Lufthansa Airbus A340's. For this purpose a special lacquer system that is hardened by ultraviolet light, contains only a minute amount of volatile solvent and can withstand the high stresses encountered in flight was developed. The new lacquer system is also dirt-repellent, UV-stable and, thanks to nanotechnology, highly abrasion- and erosion-resistant. To apply the microstructured lacquer coating to the patches, a novel procedure that accurately transfers the microstructure to the lacquer, known as the "simultaneous stamp hardening method", was developed. As in the case of negative pressure, the lacquer is applied using a silicon film that bears the inverse of the riblet structure and is cured under ultraviolet light before the film is removed.

If the tests currently under way confirm the positive results, the next stage in the research project will be to test larger patches before a highly automated application procedure is then developed. At the end of the development chain there should then be a highly accurate, fast and cost-effective procedure available that allows the entire structure to be lacquered as if with a normal surface lacquer. On the basis of the results to date it looks as if it will be possible to reduce fuel consumption by over one percent. In this way the project will significantly enhance the positive image of Lufthansa Technik with regard to technology lead, durability and environmental sustainability.