Analysis
shark1_0

Shark-skinned

Airbus tried unsuccessfully in the 1990s to emulate the aerodynamic efficiencies of sharkskin, but technological developments and increasing fuel costs have spurred the OEM to try again. Mario Pierobon describes the Clean Sky project undertaken by Airbus and Lufthansa Technik.

As one of the world’s most deadly predators, sharks have evolved to move through water very efficiently. One of the key evolutionary attributes sharks have is their peculiar skin. Although sharkskin looks smooth to the naked eye, it is in fact rough upon contact. If you examine sharkskin under a microscope, you can see it is made up of many small, sharp tooth-like scales which are called denticles. These denticles canalise the water flowing over them and reduce drag. Academic researchers have found that they can slip through the water at speeds of up to 60mph.

Scientists have been studying the peculiarities of the streamlined skin of sharks for about 30 years and several different applications have been found. One example was the high-tech swimsuits developed to enable athletes to move faster through water in the late 2000s. Swimmers wearing such suits broke 29 world records in the first five days of the world swimming championships in 2009, resulting in Fina, the International Swimming Federation, banning them from competition.

In the aviation sector, attempts to test the peculiarities of sharkskin on aircraft surfaces first started in the 1990s in a bid to improve the aerodynamics of large passenger aircraft. The long, extended surfaces of commercial aircraft generate significant turbulence as the air is forced around. This turbulent air flow runs perpendicular to the direction of travel and creates drag. Experts investigating this issue for Airbus concluded that adding a microstructure to an aircraft’s surface that mimicked the properties of the denticles in sharkskin would hamper the perpendicular flow by channelling the air along the body of the aircraft and, therefore, reduce drag.

The airframer tested aircraft with sheets of plastic imitation sharkskin bonded to the exterior. This technology had some major limitations, however. The foil Airbus used was rather heavy and the added weight cancelled out the fuel-efficiencies generated by improving the aircraft’s aerodynamics. It was also difficult to stick to the curved surfaces of the jet without creasing and wrinkling and, therefore, ruining the effect. Another difficulty is that aircraft have to be stripped of their paint and recoated every five years and this is not possible with the foils applied.

 

Rediscovering denticles

Pushed by increases in oil prices and improvements in paint technology, new attempts have been made to generate sharkskin-like effects on aircraft surfaces. Lufthansa Technik and Airbus have tested aircraft paint with a textured surface in a joint project under the European Union’s Clean Sky initiative. The aviation firms worked on the project with coatings manufacturer Mankiewicz and the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) in Germany.

Launched in 2008, the Clean Sky initiative is an aeronautical research programme through which the EU provides grants to industry for the development of technologies that will improve the environmental performance of air transport, resulting in less noisy and more fuel-efficient aircraft.

The Airbus-led two-year research project, which started in 2011, aimed to translate the aerodynamic benefits of denticles into a technically feasible coating for aircraft, while investigating the durability of the surface in real-life flying operations and assessing whether an economically viable service life was possible.

The coating technique required “riblets”, which create a texture similar to denticles, to be applied by pressing a stamp into a layer of fresh paint. The riblets are just 50–60 microns wide and 20–30 microns deep, around the height of a cross section of human air (see diagram). The coating had been successfully trialled in 2009 on a Beluga aircraft and the new project enabled Airbus and its partners to test the coating over a much longer time period and on a passenger aircraft.

To obtain reliable measurement results, 10 patches of riblets each 10cm x 10cm in size were applied to the fuselage and wings of two Lufthansa A340-300 aircraft in the summer of 2011. The application areas chosen were the upper wing, and upper and lower horizontal tail and fuselage surfaces. Over two years, the aircraft completed 12,000 flight hours and the skins were regularly inspected and impressions taken to evaluate the degradation of the microstructured coating.

The tests confirmed that the surfaces treated with the experimental textured coating technique withstood the environmental conditions typical during flight operations. Furthermore, Lufthansa Technik confirmed in October 2012 that the sharkskin-inspired coating could cut fuel consumption. In publishing the results of its initial flights, the MRO firm stated that applying a microstructure to the surface of an aircraft could reduce fuel consumption by more than one per cent. The results were so encouraging that the project partners unanimously agreed to continue running the flight tests for several years.

A new coating technique

A special lacquer system was developed by the project partners to ensure the riblets could be created on the surface of the aircraft and then withstand the high stresses that are encountered in flight. The new coating had to be highly resistant to abrasion and erosion and stable under ultraviolet (UV) radiation.

To apply the microstructured lacquer coating to the aircraft, a novel procedure was developed to ensure that the riblets were accurately rendered on the fuselage. The “simultaneous stamp hardening method” sees the lacquer applied using a silicon film mold that bears the inverse of the riblet structure. The lacquer is then cured under UV lights before the film is removed, leaving the riblet texture on the surface of the aircraft.

The coating in which the riblet texture is stamped is a new product developed by Mankiewicz. The coating is transparent, enabling it to be applied on top of any customised colours. An additional advantage is a reduction of the content of volatile organic compounds (VOCs) in comparison with traditional aircraft coatings, an important factor in the long-term environmental credentials of the lacquer. While conventional paints evaporate solvents during the drying process, the new riblet clearcoat dries so fast under UV light the paint requires a much lower solvent (VOC) content than standard exterior paint.

One of the technical challenges faced by Mankiewicz’s research and  development department in creating the new lacquer was how to ensure the coating remained soft long enough to impress the riblets onto the surface during application and then harden quickly enough to freeze the texture. Another was to ensure this new coating maintained its texture during years of use, resisting the impact of dust, sand, hail, erosion and de-icing fluid. The coating also needed to be flexible enough to endure the expansion and contraction of the fuselage and the intense UV radiation it would be exposed to during flight.

The final paint was formulated to ensure the surface cured within seconds under UV light, compared with the hours that conventional clear coats take to dry in ambient conditions. The new clear coat undergoes a radiation-induced polymerisation under the UV light, ensuring the speedy curing process needed and that the coating meets the tough durability requirements of the aviation industry.

Automating the application

Unlike standard paints, the new riblet coating cannot be sprayed onto an aircraft and an embossing tool is needed to provide the crucial sharkskin-like texture. A prototype tool was developed by Fraunhofer IFAM to apply the riblets in a single process. A silicon mold was created containing a negative impression of the riblets and a system of rollers developed to press the mold into the surface of the aircraft. The coating is applied to the mold and then applied to the fuselage skin.

Several application trials have been performed using the prototype device confirming the feasibility of the technology. According to Airbus the system enabled the continuous application of the riblet coating at a speed of two metres per minute. The OEM estimates that the technology could achieve a coating speed of 60m2 per hour.

If the riblet application process was to be automated in future, guiding systems will be necessary, confirms Airbus. The development of such a guiding system will have to consider the architecture and infrastructure of the paint hangar. Furthermore, to ensure a high-quality riblet application, a positioning system with tight tolerances will be needed, as well as geometry inspection tools, according to Airbus researchers.

While the application of the textured lacquer is different to traditional aircraft painting techniques, no special treatment is needed on the surface of an aircraft prior to applying the riblet coating. The cleaned topcoat or basecoat can be reactivated with the standard abrading and cleaning processes. Large steps and gaps of the structure must, however, be masked to ensure a sufficient adhesion to the coating layer underneath. With automated guiding systems in place, Airbus estimates that it would take one to two days to apply the riblet coating to an entire aircraft, depending on the coated area and the aircraft type.

Environment impact

Alongside ensuring that the new coating contains no hazardous materials and fewer VOCs, the project partners say the new technique offers other environmental benefits, including reducing waste and energy use. The application of the riblet coating directly to the basecoat surface of the aircraft and the fact that the coating is pre-cured, means that no overspray is necessary, unlike standard painting processes that use a basecoat and a clear coat. This reduces the amount of waste generated during the painting process.

Energy consumption is also lower than traditional painting processes, despite the use of a UV-lamp, because once the riblet coating is cured it is not tacky.

This means that the aircraft could leave the paint hangar directly after the application, much more quickly than after conventional clear coat applications which take several hours to dry, saving energy consumption from ventilation and air conditioning, for example. These additional environmental benefits will, of course, sit alongside the key benefit of the new lacquer; that of cutting fuel burn and carbon emissions.

While still in testing stage, the new sharkskin-inspired riblet coating and its application have, so far, proven to be a viable technology and a potential one per cent reduction in the fuel consumption of commercial jets is not to be sniffed at. It’s early days yet, but it seems that textured aircraft coatings could definitely play a role in improving fuel efficiency in future.

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