Ice is delightful or deadly, depending on where it is. This is especially true for aircraft. Tinkling in a glass of orange juice while flying at cruising altitudes, it is quite refreshing. But, if you look out and see it forming on the wings, beware. Temperatures are low enough at flying altitudes for ice to condense on the wings of an airplane, which is basically a giant heterogeneous nucleation site flying through a giant puff of miniscule water droplets looking for a reason to freeze—and wreak havoc.
According to a press release from the Fraunhofer Institutes, ice on wings can increase aerodynamic drag by up to 40 percent. Ice adds weight, too, which can lead to a loss of up to 30 percent of an airplane’s lift. At best, these effects increase the fuel consumption significantly. At worst, they can disrupt the airplane’s aerodynamics and controllability enough to cause crashes.
Ice formation on other parts of aircraft also can lead to disaster. The tragic crash in 2009 of Air France flight 447 into the Atlantic Ocean was caused, in part, by ice formation on small devices called pitot tubes that provide airspeed information to the pilots. A Nova television episode describes how the air in the storm that the plane was flying though could have been supersaturated with water vapor that would have condensed and iced-over the tubes almost instantly.
Ice management comes down to two approaches: de-icing, where ice is removed from the surface, and anti-icing, where ice is prevented from forming.
Presently, inflight de-icing methods involve blasting ice off the wings by inflating rubber boots that crack the ice off. Alternatively, to prevent icing, heat is diverted from the engines through hollow chambers in the wings to keep them too warm for ice buildup. Both methods are reported to have “exorbitant energy requirements,” and are difficult or impossible to integrate with fiber composite components, which are finding more and more applications in aircraft.
Two of the Fraunhofer Institutes in Germany are researching new, materials-based approaches to ice management that, it is hoped, will be less expensive and more effective.
Researchers at the Institute for Structural Durability and System Reliability LBF in Darmstadt are developing a promising “heat up the wings” method that integrates “nanomaterials into the wing materials that generate an electrically conductive layer and heat the wings,” according to Martin Lehmann, deputy head of department at LBF. The press releases says the electro-conductive layer is part of the material, and therefore “protected by the overlying fabric.” While they are not disclosing any details about the materials, the press release does mention that the system is nonmetallic, which “improves lightening protection and avoids the weak points that the metal would form,” reducing fatigue.
Early test results are promising. At ground level, the system heated the wings to 120˚C, well over the freezing point of water. The team also ran icing tests in a wind tunnel-one where wings were allowed to ice over in the wind tunnel and then the system was turned on (de-icing), and in the other, the system was activated on ice-free wings exposed to spray in the wind tunnel (anti-icing).
In both tests, the system performed well. Leahmann says that simulations allowed the researchers to optimize the heat output and minimize the energy consumption. Next, the researchers are looking to test the system in more realistic environments.
Two other ice management technologies are in development at the FI for Manufacturing Technology and Advanced Materials IFAM in Bremen.
An anti-icing approach starts with the premise that “where there is no water, there can be no ice.” One consequence of using heat to de-ice the leading edge of the wing is that “runback” ice can form on the unheated trailing edge. IFAM is developing hydrophobic coatings to repel the water and avoid ice formation. Stephan Sell, IFAM scientist, says in the press release, “We can achieve that by blending certain additives into the paint, such as fluorinated compounds. The main challenge is figuring out how to produce water-repellant coatings so that they remain stable for several years-resisting the effects of UV radiation and high erosion stresses.”
Also at IFAM, researchers are investigating a de-icing system that would take advantage of the unique properties of shape memory materials, which change their shape-that is, volume-in response to temperature change or to electrical current. The idea is to activate the volume change to blast the ice off the wing. Sell says, “We expect energy savings from this to reach up to 80 percent compared to conventional heating methods.”
If you happen to have plans to be in Berlin Sept. 11-16, you might want to stop by the ILA Berlin Air Show, where LBF and IFAM will have their technologies on display. Visit them in Hall 3, Booth 3221.