Here’s the thing about airplanes: In order to function at peak efficiency their wings have to be completely smooth. In engineering they call it optimal laminar flow--meaning air can move over the wings without any disruption. But there’s a big problem in achieving optimal flow when you take airplane wings out of an engineer’s wind tunnel and put them into use outdoors. Actually, it’s not a big problem. It’s a bug problem.
It’s kind of hard to believe, but even the smallest of bumps on a wing can mess up laminar flow. All those accumulated bug guts eventually mess up an airplane’s fuel efficiency by increasing drag. It’s a problem that folks have been attempting to solve for more than 60 years. The good news is NASA is on it. Their Langley-based bug team is working on finding the optimal material for repelling bug innards.
But why is this problem taking so long to solve? According to Mia Siochi, who heads up the team, when she was first tackling a similar problem more than 25 years ago their focus was on surface tension. They’d look at materials like those you use to create anti-stain surfaces on carpet or Teflon. “These materials let water bead on the surface. For things to stick they have to spread,” she says.
When a bug goes splat, its body goes through chemistry that thickens its fluids.
But the problem is that bug guts aren’t nearly as simple as water. Turns out, there’s some interesting chemistry that happens inside a bug when it’s about to die. “When the aircraft hits the bugs it’s going at around 150 miles an hour. That’s high impact dynamics. The components of the bug and the blood, it’s a lot of water, but there are a lot of biological components there too. The bug doesn’t know it’s been catastrophically destroyed. So it’s trying to heal. It goes through chemistry that thickens its liquids,” she says.
To counteract this problem, the team is now looking at more modern ideas. Specifically, superhydrophobic chemistry and biologically-inspired surfaces. By combining chemicals that repel water with surfaces that are textured on a microscopic level (like a lotus leaf) they have begun to have success.
To test how their new surfaces work, the team has reverse-engineered a vacuum pump to shoot instead of suck. It was a bit of a challenge because the bug has be alive until it hits the surface. If it gets smashed on the side of the gun on its way out the chemistry will be different once it hits its final destination. Once it smushes, they measure the characteristics of the bug residue--how big is the area that it spreads and how high it is?
According to Siochi: “This is uncharted territory in some ways. When we started we actually used bigger bugs. We thought: what’s alive and easy to get? It’s crickets. We started using a fan and a big opening and we’d drop the crickets in. But when you shoot too many you have bug splat on top of bug splat. And then we went to feeding a single bug in at a time.
We’re materials people. The aerodynamics expert told us our bug was too big. You wouldn’t be hitting a cricket with a plane. So we decided we had to go smaller, but how small? When I was doing the test 25 years ago we mounted samples to a car and drove around. This was at Virginia Tech and a professor of entomology there could look at the splats and tell which bugs were which. So for this project we went back to that table and tried to figure out what was the largest population of bugs that hit the car. So we got flightless fruit flies. And then we had to learn how to propagate them.”
The team has managed to identify some more promising materials from the 100 they tested with the bug gun. This year they started doing flight tests with the materials to see how they hold up in the sky. “The flight tests are very expensive so we have to screen down to the surfaces that are most promising. Here at Langley we’ve mounted surfaces on the edge of the wing and we fly low to make sure we catch bugs,” says Siochi.
By mounting coated and uncoated surfaces next to each other and counting the number of smushed bugs each has at the end of the flight they think they’ve been able to identify a good combination of chemistry and texture. “We found a couple of surfaces that look promising in having much fewer bug splats then the control,” she says.
There are still several factors for them to take into account before the bug-repellant surfaces are ready for prime-time. First, it has to be easy to apply. Right now they’re using a spray-on method. The resulting surface also has to be durable and handle countless take-offs, landings, and long-haul flights.
That said, there has already been a huge amount of interest from the airline industry and, surprisingly, the automotive industry as well. According to Siochi, “if it doesn’t prevent the sticking we can at least make it easier to remove bugs. Some of these surfaces are pretty easy to clean. We did a test in the windtunnel just for checking out how well we can clean it. The ones that make sense for practical application you can just wipe with a wet towel.”