Aircraft manufacturers are keen to reduce weight whenever they can. There are good reasons for this wrestling over every gram: the lighter the aircraft, the less fuel it consumes, the more passengers it can accommodate and the better its carbon footprint. Consequently, engine manufacturers are constantly optimizing their designs: fuselage parts are increasingly being made from expensive, carbon-fiber-reinforced plastics; for the interior design, solid metals are being replaced by fiber-reinforced composites. And then the painters come along and spray almost a ton of paint on an Airbus A380. Is that really necessary?
“Absolutely,” explains Maike Timm, production manager for aircraft painting services at Lufthansa Technik. “Without the protective layer of paint, the aircraft components would get damaged very quickly—whether they are made from metal or plastic, it makes no difference.” In everyday flight service, the surfaces are exposed to huge strains: ice crystals, dust particles, ash and grains of sand pelt into the materials at speeds of 1,000 kilometers an hour. On top of this, there is UV radiation and temperature swings of -55 to +100 degrees Celsius. Unless the surfaces are sealed, de-icing agents, kerosene and lubrication oil residues cause metals to quickly corrode and destroy the material compounds of fiber-reinforced plastics. For cost reasons, the first passenger aircraft took to the skies unpainted—there was a war on in Europe. But the aluminum alloys were soon tarnished and had to be frequently repolished. Today not even the Junkers Ju 52 flies without a protective paint layer—even if it is not apparent at first sight. The outer shell sports a coat of metallic paint.
Protection coat by coat
Without paint, nothing works in aviation. However, the cans of paint you find in your local home improvement center are not up to the job—aircraft paint has to be extremely thin, scratchproof, dirt-resistant and environmentally friendly. Summing up the situation, Timm says: “Thanks to new developments, we can now work very economically. Almost a ton of paint for the Airbus A380 may sound like a lot, but it’s spread over a surface of nearly 4,000 square meters. The four to five coats of paint that must be applied have a combined thickness of only a fraction of a millimeter.”
Perfect fit A Lufthansa Technik employee working on a window band on D-AIRX, an Airbus A321-131, which was painted in early 1950s style to celebrate 50 years of Lufthansa in 2013.
Perfect fit A Lufthansa Technik employee working on a window band on D-AIRX, an Airbus A321-131, which was painted in early 1950s style to celebrate 50 years of Lufthansa in 2013.
Top coat The aircraft gains color with the third coat of paint—as here at Airbus.
Top coat The aircraft gains color with the third coat of paint—as here at Airbus.
Decades of experience are contained within these layered systems. In the simplest case, two different components are required: painters first apply an anti-corrosion primer to the cleaned and sanded surfaces; then the paint itself can be sprayed onto the primer. At this stage, there are also one-coat and two-coat systems available. The paint manufacturer AkzoNobel has developed a two-coat system composed of a base coat and a clear coat. “The base coat/clear coat system has various advantages,” explains Thomas Böttcher, sales manager for aircraft paints at AkzoNobel Aerospace Coatings. “The clear coat protects against UV radiation and increases the gloss and paint retention. In addition, the surfaces are so smooth that the aircraft is less susceptible to dirt and has to be washed on average only half as often. And, lasting six to eight years, the lifetimes of the two-coat paintwork are significantly longer than most other paint systems, which last an average of only five years.”
“Without the protective layer of paint, the aircraft components would get damaged very quickly—whether they are made from metal or plastic, it makes no difference.“
Detailed work on aircraft wing Painting at Airbus paint shop in Toulouse.
Further intermediate layers allow additional functions to be integrated, such as selective paint removability. This function is created by means of a thin separating layer applied to the anti-corrosion primer. If the aircraft needs a new paintjob after a few years, the old paint can be chemically stripped along the separating layer, leaving the anti-corrosion coating beneath intact. “In this way, it is possible to cut the times needed for removing old paint and applying new paint by ten percent,” emphasizes Böttcher.
Colorful planes
After the 2016 Olympic Games in Rio, the German Olympic team flew home in a “Plane of Champions,” a Lufthansa Boeing 747-8 specially designed for the event. However, the aircraft was not repainted for the occasion, just covered with adhesive colored films. The aircraft underneath still has the classic white design.
In principle, aircraft can be painted in any color. However, white has the advantage that it does not heat up. Although a black airplane could use a thinner coat of paint because dark colors provide better coverage, it would also absorb much more sunlight. The interior would then have to be constantly cooled.
Even colorful and complex designs can be used. That said, multi-colored paintwork is expensive, because every color has to be applied in a separate operation, and parts that are meant to become or remain a different color have to be masked off by hand. To minimize the amount of work this entails, engineers are currently developing robots that can apply the various colored paints directly to the aircraft.
Using surfaces to save fuel
In the future, paints will even help save fuel—for example, by having ultra-smooth surfaces over which the wind glides with scarcely any resistance. Engineers at Lufthansa Technik are currently experimenting with polishing agents containing nanoparticles that smooth out all surface irregularities.
Another approach involves providing the surfaces with a structure that reduces drag. This is how sharks move so easily through the water: microscopically thin grooves in their scales running parallel to the direction of the current reduce resistance. Researchers in the EU’s Clean Sky project have explored how this riblet effect can be used for aviation. Engineers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM developed a technique by means of which fine grooves can be applied to an aircraft. The simultaneous stamp hardening method makes it possible to imprint microstructures using a paint-coated silicon film that bears a negative of the riblet structure. By means of simultaneous UV irradiation, the paint sets, and then the silicon film can be taken off. “The paint offers several advantages,” explains Fraunhofer researcher Dr. Volkmar Stenzel. “It’s UV-resistant, it’s good at withstanding mechanical strains, it doesn’t create any additional weight and it can be applied easily to non-flat surfaces.”
“For example, work is being conducted into special resins that make paints dirt-, water- and ice-repellent. Such paints would dramatically reduce the times and costs needed to clean and de-ice aircraft.“
Paint from a gun Among other techniques, Airbus uses electrostatic paint spraying systems, which generate less spray mist. Here the fuselage the first A350XWB for Finnair is painted in Toulouse.
As part of the Clean Sky project, Lufthansa Technik and Airbus engineers tested the resistance of 10x10 centimeter microstructured areas of paint under real conditions in standard flight operation. Currently the researchers are looking for solutions to apply the required structures in a fully automated process, and are investigating whether the cost is worth it. According to projections, it can save around one percent of the fuel used in aviation—with an annual kerosene consumption of some 300 million tons in civil aviation, it certainly is worth it.
Reduced drag is just one of many expectations the aviation industry has for the aircraft paints of tomorrow. “The trend is toward the integration of extra functions,” reports Fraunhofer expert Stenzel. “For example, work is being conducted into special resins that make paints dirt-, water- and ice-repellent. Such paints would dramatically reduce the times and costs needed to clean and de-ice aircraft.”
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