Shark Skin Inspires Aircraft Coatings
Shark skin-inspired coatings research is underway on dry land and could soon take flight, as German airline giant Lufthansa Technik AG prepares to test the new paint system on two jets.
The novel lacquer is being applied to sections of two Airbus planes to test the durability of an aircraft surface coating that mimics shark skin under real-life flying conditions.
By altering the microstructure of the surface of an airplane, the aerodynamics can be improved, lowering the amount of energy and fuel needed, researchers said.
Lufthansa Technik AG has participated in a multifunctional coating research project since mid-2011, along with its partners, Airbus Operations; the Fraunhofer Institute for Manufacturing Technology and Advanced Materials in Bremen, Germany; and coating manufacturer Mankiewicz in Hamburg.
Testing for the new coatings is expected to last into the summer. The experiment is being conducted as part of Europe's Clean Sky research project.
Shark Skin Phenomenon
The skin of a shark is covered with tiny V-shaped scales called dermal denticles, which are covered in grooves, that decrease drag and turbulence, allowing the shark to swim faster.
|Trevor Sewell / Electron Microscope Unit, University of Cape Town|
Shark skin is covered in tiny scales called dermal denticles that allow them to swim faster by decreasing drag and turbulence.
The circumferential grooves in the scales of sharks, or riblets, are only a few micrometers in size but optimize the flow behavior of some shark species.
But the phenomenon of streamlined shark skin is nothing new, and has been known for about 30 years, according to Lufthansa.
Special Coatings, New Application Method
Using a special lacquer system, eight 10 x 10 cm patches were attached to the fuselage and wings of two Lufthansa Airbus A340-300s.
The lacquer system, hardened by ultraviolet light, contains only a small amount of volatile solvent, and can withstand in-flight stresses. Additionally, the new lacquer system is dirt-repellent, UV-stable, and highly abrasion- and erosion-resistant, which Lufthansa attributed to nanotechnology.
A novel procedure called the "simultaneous stamp hardening method" was developed by Fraunhofer Institute to apply the coating to the patches by accurately transfering the microstructure to the lacquer.
The lacquer is then applied using a silicon film resembling the inverse of the shark skin riblet structure, followed by ultraviolet light curing before removing the film.
Applying the coating is a challenge, as it has to stay soft long enough to impress the riblets onto the surface, but must harden quickly to freeze the texture. With only a small amount of volatile solvents, the paint has been formulated to cure within seconds under ultraviolet light.
The coating aims to mimic the grooves on shark skin scales and are applied using a new method with silicon film cured by ultraviolet light.
Shorter Life, but Faster Dry Time
The coating also has to maintain its texture over years of use by resisting the impact of dust, sand, hail, or de-icing fluid. Plus it must be flexible enough to endure the expansion and contraction and intense UV radiation of a flight cycle.
The head of Mankiewicz's aviation department, Andreas Oseenkopf, thinks the riblets need to remain intact for at least five years, versus the eight-year lifespan of typical base coat/clear coat systems used on aircraft.
Usually, each layer of a coating system provides protection, requiring longer drying times. The new coating uses a clear topcoat that performs all of the protective functions, and underlying coatings are merely for colorization and are formulated for faster drying times.
This new formulation would also allow the surface properties of the clear topcoat to be modified without changing the underlying coatings.
Speeding up the drying time, combined with the fuel savings, would compensate for the shorter lifecycle, according to the researchers.
Earlier tests of the technology involved gluing foiled plastic sheets of imitation shark skin to the aircraft's exterior; however, the test failed to provide any benefit because the material was too heavy.
"[But] the foil had major disadvantages," said Volkmar Stenzel, the project's head at the Fraunhofer Institute. "It was rather heavy and the added weight cancelled out the amount of fuel that could be saved.
"Also, it was difficult to stick the foil to curved surfaces without creasing and wrinkling."
Volkmar also explained that the aircraft has to be stripped and recoated every five years, which would not be possible with the foils.
Denis Darracq, head of research and flight physics technology at Airbus, said, "An airline must not have to clean its aircraft after every flight. The paint needs to last for several years."
He estimated that fuel consumption could be reduced by about one percent if the new paint was applied to 40 to 70 percent of an aircraft.
At Lufthansa's fuel burn rate of 90,000 tons of jet fuel per year, that one percent would yield savings of around $94 million at current prices.
The hope is that information gained from the research will show how resilient these structures are under the environmental issues encountered during flight and whether a viable service life can be demonstrated.
If the tests have positive results, larger patches will be tested, after which a highly automated application procedure will be developed.
After development, Lufthansa says there will then be a highly accurate, fast and cost-effective procedure to allow the entire structure to be lacquered as if with a normal surface lacquer.