When bugs explode against the wings of oncoming airplanes, they create a sticky problem for aerospace engineers.
“A bug doesn’t know that it’s been catastrophically destroyed,” says Emilie J. (Mia) Siochi, a materials scientist with the National Aeronautics & Space Administration. “Its blood starts to thicken as if it’s healing any other injury.”
This bug blood, or hemolymph, clings to an airplane’s wings, disrupting the smooth airflow over them and sapping the aircraft’s fuel efficiency.
NASA scientists are now developing coatings that help aircraft shed or repel bug guts during flight. After screening nearly 200 different coating formulations, the NASA researchers recently flight-tested a handful of promising candidates on a Boeing ecoDemonstrator 757 aircraft in Shreveport, La. The team explored different combinations of polymer chemistry and surface structure and reports that it has created a coating that could reduce the amount of insect insides stuck to the wings by up to 40%.
With further optimization, such coatings could allow planes to use 5% less fuel, Siochi says. Although that may not sound like much, it adds up. “That could be millions of dollars in fuel savings,” Siochi explains. The bump in fuel efficiency would also curb the amount of greenhouse gases emitted by planes, she says.
These coating studies are part of NASA’s Environmentally Responsible Aviation Project, which launched in 2009 to study new technologies to make flying more eco-friendly. But researchers have been trying to reliably debug planes in flight since the middle of the 20th century, Siochi says. Her own interest in the problem goes back to her master’s degree work on splatter-resistant coatings in the 1980s.
NASA’s more recent studies started with an examination of commercially available products covering a range of chemistries, including polyvinyl alcohol, polysiloxanes, and fluorinated polymers.
To test these materials in the lab, researchers developed a pneumatic launcher to fire living bugs at a sample coating. They first used crickets as ammunition, but a physicist colleague urged them to switch to fruit flies, which would be more representative of what planes hit during takeoff and landing.
Of the off-the-shelf coatings, Siochi says, the most promising was a commercial fluorocarbon that’s usually used to prevent printed electronic circuits from getting gunked up—though presumably not by eviscerating insects at 150 mph. Despite its promise, the fluorocarbon simply couldn’t slough off enough bug juice to maintain optimum airflow over plane wings.
So the NASA team crafted its own coatings to solve the problem, which is a rather daunting one considering the diversity of bug gut chemistry. When insects collide with an aluminum plane wing, a bug’s exoskeleton cracks open and bounces off. “You’re basically left with sugars, fats, and proteins” on the wing, says Lynn S. Kimsey, an entomologist at the University of California, Davis, who helped support NASA’s flight tests this spring. “Sugars are easy to get off. Fats and proteins are a different story.”
Once a bug’s hemolymph is “activated” by an impact, its lipid-encased hemocyte cells and phenoloxidase enzymes become tacky and adhere to plane wings. But wing coatings also have to contend with other hangers-on: pigments from red-eyed hoverflies, clear goop from honeybee stomachs, and yellow yolks from the eggs of female insects.
Developing a coating that can deal with all of that and more is a challenge, but NASA has the polymer chemists who can do it, Siochi says. Right now, the agency isn’t disclosing the precise composition of the coatings it tested in Shreveport, but a 2013 publication gives some ideas as to what researchers have investigated previously (Prog. Org. Coat. 2013, DOI: 10.1016/j.porgcoat.2012.08.009).
For example, the team looked at glycol-modified surfaces to try and minimize protein adhesion, as well as a hydroxyl-functionalized methacrylate that frustrates hemocyte accumulation. NASA is patenting its newly tested coatings and has announced that more information is forthcoming.
But polymer chemistry isn’t the only factor that contributes to a coating’s nonstickiness. By incorporating structures such as silica nanoparticles into the coatings, the NASA researchers worked to give the wings rough surfaces like those found on many superhydrophobic materials. The trick is to make a surface with protrusions large enough to block bugs’ guts from sticking but small enough that they themselves don’t disrupt airflow, Siochi says.
The team appears to have found a balance, but it is still unclear whether the coatings will be a viable solution to the bug problem. The leading edge of an airplane wing is a harsh environment. Dust and rain can erode the surface coatings during flight, Siochi says. It remains to be seen how much a coating’s upkeep offsets the fuel savings it affords.
Still, the ramifications of this work are broad: Bug guts stick to a lot of things other than plane wings, points out Kimsey of UC Davis. “Think about all the people who whine about cleaning bugs off their radiator grilles.”