The Latest in Deicing Techniques
By Tracy Martin
In-flight airframe icing has been a problem since the 1930s, when planes could fly fast enough and high enough for ice to form on the leading edges of wings and other surfaces. Even when there was no noticeable ice on an aircraft before takeoff, airmen discovered how quickly icing could occur in flight, with potentially dangerous effects.
Although the nominal freezing point of water is 32 degrees Fahrenheit (0 degrees Centigrade), water in the atmosphere does not always freeze and condense at that temperature. It often remains suspended as super-cooled liquid particles. But if the surface temperature of an aircraft is below zero, atmospheric moisture can quickly turn to ice as an immediate or secondary consequence of contact between the two. While deicing fluids can be sprayed on the aircraft before takeoff, the ability of such solvents to prevent ice from forming once in flight is limited.
Ice buildup on an aircraft’s wing causes a disruption in airflow over the wing’s surface, and a smooth wing surface is critical for maintaining aerodynamic lift and control. Even a very thin layer of ice (as little as .04 inches or 1mm) can be enough to destabilize aircraft in flight.
To reduce the effects of ice buildup, inflatable “boots” were developed and mounted to the leading edges of wings and vertical stabilizers. These early de-icing systems have been around for 80 years and are still in use today. When ice develops, the boots are inflated with compressed air and remove ice by changing the shape of the leading edge. Once the ice breaks up, the boots are deflated, and the wing resumes its normal shape. Use of this type of deicing system generally can be identified by black leading edges on both vintage propeller airliners and modern turboprop aircraft.
Another method of removing wing ice is the use of porous leading edges made from wood. The leading edges are saturated with pressurized alcohol. As the alcohol weeps through the wood, it keeps ice from forming on the surface. Over time, the woods originally used for this purpose have been replaced by porous metals, which are more durable and efficient in distributing the alcohol. The British, who invented this de-icing system, continue its use on Hawker business jets. A similar system also has been fitted to small, American-built, piston-powered and turboprop aircraft.
The process of heating wings was developed along with jet aircraft to deal with in-flight icing. Early examples of this system were used on Boeing’s 707 and the Douglas DC-8. Compressed, heated “bleed” air from the jet engines is passed through ducts, located on the leading edges of the wings and tail, and then is exhausted through holes in the lower surfaces of the wing. The “hot-wing” method of de-icing is commonly used, as it is effective and reliable.
Other Ways to Melt the Ice
While these traditional methods do work, new technology for de-icing aircraft has been developed that uses electricity as the energy source. One example is Boeing’s 787 Dreamliner, which utilizes an electrothermal ice protection scheme. Several heating blankets are bonded to the interior of the protected wing slat’s leading edges. The heating blankets can be energized simultaneously for anti-icing protection during takeoff or sequentially for deicing in-flight.
This method is significantly more efficient than the traditional pneumatic or engine air bleed systems as it requires less energy to operate. At high altitudes, engine air bleed is a relatively expensive source of energy. The engine’s core has to be made slightly larger to accommodate the increased airflow for bleed air, and such systems require many air ducts, all taking up space and adding weight. When compared to pneumatic de-icing systems, electro-thermal systems use approximately half the energy. Moreover, because there are no bleed air exhaust holes, airplane drag and in-cabin noise are reduced.
One disadvantage of Boeing’s electro-thermal heating blanket system is that it still requires a significant amount of electrical energy to operate. This is not an issue with the 787 Dreamliner as it has two, 500-kilowatt-rated starters/generators that provide significantly more electrical power generation than the preceding Boeing models.
What is Newest?
In desert-type locations, ice protection for unmanned aerial vehicles (UAV) is usually not a deterrent to flying. However, as UAVs fly at higher altitudes and in more varying conditions, ice buildup on leading surfaces adds extra weight, disrupts airflow, and reduces lift, potentially limiting or even ending missions.
Researchers from the Battelle Memorial Institute, a technology research and development organization based in Columbus, Ohio, have successfully tested a new in-flight ice protection system for UAVs. Battelle’s HeatCoat technology, which utilizes carbon nanotubes (CNTs), has been successfully tested on engine inlet cowlings and wing components for its de-icing capabilities.
An aero-icing wind tunnel chamber that simulates temperatures as low as –22 degrees Fahrenheit and air speeds of up to 182 knots was used to test the HeatCoat system. According to the institute, the coating successfully performed anti-icing and de-icing functions over a 4-day demonstration and testing period.
What are carbon nanotubes? In material terms, they are allotropes of carbon — think diamonds or graphite. With a diameter of approximately 1/50,000th of a human hair and a length of several millimeters, nanotubes have a nanostructure with a length-to-diameter ratio that exceeds 1 million.
Carbon nanotubes are made of graphene sheets that are obtained from closely attached carbon atoms, also known as graphite. The thickness of a graphene sheet is equivalent to one carbon atom. When rolled into a tube-like structure, it becomes a single-walled carbon nanotube. (There also are multi-walled nanotubes.)With a mechanical strength 200 times greater than steel, graphene is currently the hardest known material.
These cylindrical carbon molecules have novel properties that make them useful in many nanotechnology applications that include electronics, optics, and structural materials. The CNT structures exhibit extraordinary strength, have unique electrical properties, and are efficient conductors of heat. They also can be used as additives to various structural materials. For example, CNTs make up a small portion of the materials used in carbon fiber sports equipment and automobile parts.
Batelle’s HeatCoat ice protection technology was designed for UAVs, but it potentially could be used in larger aircraft,A carbon nanotube coating is sprayed onto aircraft surfaces in several coats, much like paint. The first layer is a primer coating, followed by the heater coating made up of carbon nanotubes, then a barrier coating, and finally an outer top coating. The applied coatings conform to complex curves and thus maintain the wing’s or other component’s aerodynamic performance. A laminate version of the system is also available that uses pre-manufactured carbon nanotube heater panels.
When the UAV is in flight, a ground control station turns on the system, after which it can operate automatically. Electrical power is applied as needed to cause the coating to heat up, much like an electrical resister. An intelligent controller on the aircraft monitors the performance of the heated layer. The power level and the heat being generated is altered dynamically to ensure the minimum power use for the current flight conditions.
“Battelle has made a long-term investment in this technology, because we think it is so promising,” says Ron Gorenflo, HeatCoat Systems Product Manager. “Our recent tests validated improvements we’ve made and prove that we are ready to go from a Technology Readiness Level (TRL) 6 on to a TRL 7, once we identify a key partner to help complete the next step of this process.” (A TRL of 6 means that the prototype tested is near the desired final configuration in terms of performance, weight, and volume.)
Battelle describes its system as being lighter than traditional ice protection systems, having lower power requirements, and being less complex to maintain. While such attributes are necessities for use on UAVs, in which payload and power capabilities are limited, they also mean that this technology holds promise for manned aircraft.
The Future of Carbon Nanotube Technology in Larger Aircraft
UTC Aerospace Systems, a United Technologies Corporation company, has licensed carbon nanotube, heater-based technology from Metis Design, a technical consulting firm that focuses on structural health monitoring and multifunctional materials. Metis Design co-developed the technology with the Department of Aeronautics and Astronautics at the Massachusetts Institute of Technology (MIT).
According to Mauro Atalla, Vice President, Engineering and Technology, Sensors, and Integrated Systems at UTC Aerospace Systems, the aerospace industry has an increasing need for more durable, lightweight, damage-tolerant, and low-power ice protection systems: “Thin layers of carbon nanotubes have several emerging and exciting aerospace applications. This technology strengthens UTC Aerospace Systems capability to deliver the most innovative solutions for aircraft ice protection systems. CNT technology is ideally suited for our ice-protection product line, and we have already seen positive customer feedback from testing conducted at our icing wind tunnel. Metis Design has developed this technology over several years and has demonstrated its feasibility in several projects.”
Others agree that the use of carbon nanotube electrothermal ice protection systems is likely to grow as new aircraft become “more electric,” resulting in a shift away from the energy-inefficient bleed air systems. For example, The Boeing 787 reflects a completely new approach to onboard systems, as virtually everything that has traditionally been powered by bleed-air from the engines has been transitioned to an electric architecture, including auxiliary power unit (APU) and engine start, hydraulic pumps, and even cabin pressurization. The only remaining air bleed system on the 787 is the anti-ice system for the engine inlets.
In both manned and unmanned aircraft, the use of CNT technology undoubtedly will enable the application of lightweight aircraft surface heaters that feature improved thermal inertia and increased damage tolerance. As more electrically-based systems replace traditional air bleed systems on military aircraft, they will benefit from overall weight reduction, more efficient engine configurations, an increase in operating ranges, and improved mission times.
Image #1 – After an overnight snow storm hit the National Capitol Region, a C-17 Globemaster III from the 452nd Air Mobility wing at March Air Reserve Base, CA, sits on the flightline at Andrews Air Force, MD. Airmen spent most of the morning de-icing aircraft and clearing the flgihtline.
Image #2 – Battelle’s HeatCoat technology uses a high-conductivity, carbon nanotube coating that conforms to the existing skin of the aircraft. Intelligent sensors and controls optimize energy use to reduce ice accumulation without compromising flight performance. (Photo courtesy of Battelle)
Image #3 – NASA has patented a method for growing carbon nanotubes and nanofibers in various patterns. This computer graphic shows a single-walled carbon nanotube. Commercial applications include heat exchangers in electrical circuits, thermal protection/cooling systems, and many others. This technology is available for licensing from NASA’s space program to benefit U.S. industry. (Image courtesy of the U.S. National Aeronautic and Space Administration)
Image #4 – Amy Heintz, a senior research scientist at Battelle and inventor of the HeatCoat anti-icing technology, is shown inspecting an installation of the coating applied to a mockup of a wing section. Heintz’s research focuses on such heady phenomenon as dissimilar interfaces; organizing materials to tune adhesion; and electron, phonon, or gas transport. (Photo courtesy of Battelle)
Image #5 – The UTC Aerospace Systems Advanced Icing Wind Tunnel, located in Burnsville, Minnesota, is capable of simulating air speeds up to Mach 0.9, air temperature down to –76 degrees Fahrenheit, and altitudes up to 47,000 feet. UTC’s Carbon nanotube electrothermal ice protection systems are tested in this harsh environment before installation on aircraft. (Photo courtesy of UTC Aerospace Systems)