The rough skin of sharks has long been known for the low friction of its myriad tiny, sharp-edged scales. Airbus tested aircraft in the 1990s which were partially covered with a foil that mimicked the animals' surface texture. But despite the promise of aerodynamic improvement, the technology has yet to pass the experimental stage.
Lack of durability has been one problem. Another is the fact that while a sharkskin texture can be applied to aircraft or ships, the underlying physics of surface drag is extremely complicated; if nothing else, a shark is far more slippery than a similarly-textured aircraft if only because it is subtly flexible.
However, evolution in paint technology has combined with rising fuel prices to revive interest in transferring the shark's slippery surface texture to aircraft.
Lufthansa Technik (LHT) is testing aircraft paint with a similarly grooved surface in a joint project under Europe's Clean Sky initiative, together with Airbus, Germany's Fraunhofer research institute and Hamburg-based coating manufacturer Mankiewicz.
US coatings specialist PPG is also working on drag reduction surfaces, using paint- and foil-based approaches. Mark Cancilla, global director of PPG's aerospace division, says patterned surfaces can lead to "very significant" reductions in the airframe's drag coefficient, thus lowering specific fuel consumption (SFC).
However, the problem has been to keep the aircraft skin clean without additional washing, so progress depends on developing dirt-repellent coatings. Cancilla says such drag-reduction surfaces could come on to the market during the next four years.
Ultra-smooth surfaces reduce friction at slow speed but at high speed the riblets are more effective, despite increasing an aircraft's surface area because they cut drag by reducing turbulence perpendicular to the airflow.
Fraunhofer's institute for manufacturing technology and advanced materials has devised a process it calls the "simultaneous stamp hardening method", whereby the nanometer-range grooves are embossed into the freshly applied wet paint using a silicon film with inverse riblets, which is later removed.
DRYING ON DEMAND
Application is a challenge. Any practical coating must stay soft long enough to allow the riblets to be impressed into its surface, but then harden rapidly to freeze the delicate texture. This has been achieved with a paint containing only a small amount of volatile solvents, formulated for curing within seconds under ultraviolet light.
Aside from repelling dirt, the coating must also maintain its surface texture during use by remaining resistant over a long time to the abrasion and erosion resulting from the physical impact of dust, sand or hail, or the chemical action of de-icing fluid. And, as with regular aircraft exterior paint, it must be flexible to endure the fuselage's expansion and contraction during flight cycles and withstand intense UV radiation without weathering.
Close up image of the riblet structure of anti-drag coating
Andreas Ossenkopf, head of Mankiewicz's aviation department, thinks the riblets need to remain intact for at least five years, compared with the eight-year lifespan of the modern base coat/clear coat paint systems typically used on aircraft, and on which the low-drag coating is based.
Unlike normal paint systems, the low-drag system's clear topcoat provides all environmental protection, while underlying layers merely provide colourisation and are formulated for fast drying times. In conventional systems, each colour layer fulfils all protective functions, leading to longer drying times - so the low-drag coating can compensate for its shorter lifespan not only by saving fuel but by speeding up the application process.
The new chemistries also allow for modification of the surface properties of the clear topcoat without regard to the formulation of the underlying colour coats. This flexibility could be exploited to create multifunctional surfaces.
LHT has been testing riblet paint on two of its parent carrier's Airbus A340s since mid-2011, in a trial scheduled to run until summer 2013. Eight 10 x 10cm (4 x 4in) test patches have been placed on the fuselage and wings of each aircraft to assess the coating's durability in regular flight operations.
Ensuring the stamp process can be applied on an industrial scale without prohibitive production costs and turnaround times is another major hurdle. Ossenkopf points out that the riblet paint has so far only been used in small sections in selected areas. While it will not be necessary to apply it to the entire fuselage and wings, he says the aerodynamically relevant sections still constitute a large area that will be "challenging" to cover.
LHT aims for a "highly automated [paint] application process" and might conduct subsequent trials with larger test patches. But the German maintenance, repair and overhaul provider says preliminary evaluations of the riblet paint show potential SFC savings of "more than 1%". If cost of the paint and application process can be kept low enough, even this small gain should prove attractive.
If the grooved lacquer finds application on tomorrow's airliners, however, airline marketing departments will have to gloss over a cosmetic hitch of the "green" coating. A rough surface may save fuel, but these slippery aircraft will have lost their shine.