Cold Comfort - Nano Anti-Icing
5:06 PM on Mar 23, 2012
This article was originally published as the Leading Edge column in the Mar. 12 issue of Aviation Week & Space Technology.
Icing has been a threat to aviation since the dawn of the aeronautic age and, as manufacturers pursue efficiency improvements measured in fractions of a percent, the power demanded to protect aircraft—whether by bleed air or electrical heating—is coming under attack.
At the same time, unmanned-aircraft developers see the need to operate in all weather conditions, but cannot afford the power and weight penalties associated with traditional anti-icing systems. Nanotechnology may hold the answer, at both ends of the spectrum.
U.S. research and development organization Battelle is mixing conductive carbon nanotubes (CNT) into aircraft paint to produce a lightweight, low-power, anti-icing coating that is easy to apply and repair. General Electric Global Research is developing nanostructured surfaces that delay the formation of ice and reduce its adhesion.
Columbus, Ohio-based Battelle has dispersed nanotubes into a coating solution to make it conductive, so it heats up like a resistor when power is applied to a wing’s surface, says John Ontiveros, operations manager for the program. The coating has been tested in an icing tunnel using a scrap wing from a UAV.
Photos: Battelle (left image shows ice-free heated area in center)
The trick is to suspend the right amount of CNTs in the solution to achieve the resistivity needed in a coating that can be applied by a spray gun like regular paint, he says. The coating “stack-up” consists of primer over the bare metal, the nanotube heater coating, a soy-based protective layer and then the normal top coat of paint.
The coating is only 0.020-0.030-in. thick and four times more power-efficient (in watts/area) than other electrothermal anti-icing systems, Ontiveros says. The lower the resistance, the more efficient the heat generation. “We are down to low single digits, which is pretty awesome.”
Because the heating is on the wing’s surface, not under its skin, there is no thermal lag, according to Ontiveros. The coating heats up rapidly, and power can be cycled quickly between different sections of the leading edge to minimize consumption. Power leads are laid flush to the surface then the coating is sprayed on, minimizing the need to penetrate the wing skin. The coating can also be repaired or replaced easily, he says.
Battelle is talking with aircraft and paint manufacturers, looking at both large and small platforms, while continuing risk-reduction work in areas such as coating application and power management. “We need to look at the full regime of icing,” Ontiveros says, and operating strategies need to be developed based on an aircraft’s flight envelope and available power. He notes, “We are ready to look a full-scale integration and test on an aircraft.”
GE’s Niskayuna, N.Y.-based Global Research Center, meanwhile, is continuing research into superhydrophobic materials with nanotextured surfaces that repel water and prevent ice from forming and adhering. GE’s interest is in anti-icing surfaces and coatings for both aircraft and engines as well as wind turbines, where increased drag caused by ice buildup can significantly reduce the power-generating capacity and efficiency.
Photos: GE Global Research (see bouncing water video here)
Having demonstrated via icing-tunnel tests that microscopically textured aerodynamic surfaces can dramatically reduce ice adhesion, GE says it has now shown that they can significantly delay ice formation. “Compared to standard surfaces . . . nano-enabled anti-icing surfaces would delay ice formation for more than a minute on their own,” says a company representative.
Nanotextured coatings on aerodynamic surfaces and engine blades “could one day reduce and possibly even eliminate the need for existing anti-icing measures,” the company states. But it cautions that further development is required before such surfaces are durable enough to be ready for commercial application.
EADS Innovation Works, the research and technology arm of Airbus’s parent company, is investigating water-repellent coatings to prevent ice adhesion and accretion at a new laboratory-sized cryogenic tunnel in Ottobrunn, Germany. Mississippi State University is providing computational fluid dynamics modeling of supercooled water droplets impacting aerodynamic surfaces. The work is in support of Europe’s Aeromuco research program, which is to include flight tests of ice-resistant coatings.
NASA, meanwhile, has begun modifying a Gulfstream G-II to investigate how ice crystals—formed in high-altitude, warm-weather storms—can enter the core of an engine (see photo), melt and then refreeze, causing loss of power or shutdown. Trial flights are planned for Florida in August, ahead of a test campaign set for January-March 2013 over Darwin, Australia, an area known for storms with high levels of ice crystals