What's New in Icing?
(continued) Heat is, of course, the ultimate ice protection. Water, supercooled or not, simply can't freeze onto a hot surface. But thermal ice protection is not as simple as it may seem.
The biggest issue with thermal ice protection is prevention of runback ice accumulation. If a surface is warmed above freezing, ice will not form, but the liquid water, or melted snow, can flow back over the unheated portion of the wing or tail and refreeze. On some airfoil shapes that may not be a big issue, but on other, typically more efficient, wings the presence of runback can destroy lift and create very undesirable stall behavior.
The typical solution to the runback ice threat is to vaporize moisture, frozen or not, on contact with the hot leading edge. In a typical jet with hot wings and engine air inlets, the surface is heated to several hundred degrees, well above the boiling point of water. Droplets turn to vapor on contact with the hot surface and have no chance to adhere further aft on an unheated part of the airframe.
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Cessna sprayed yellow dyed water onto the nose of this Citation to test ice formation on the radome. The shape of the radome determines how it will collect ice, which is critical in a jet with aft-mounted engines because ice can break off the nose and be sucked into the engines. Tankers spraying a stream of water are often used to test ice formation on small parts of the airframe.
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As you can imagine, it takes gobs of energy to heat a leading edge to more than 200° with the wind chill of cold air blowing by at hundreds of knots. Some early piston transports had dedicated fuel-fired furnaces to produce the hot air to heat leading edges, but it wasn't until the turbine engine with its supply of very hot compressed air came along that hot wings became truly effective.
To heat surfaces in a turbine airplane, very hot air is tapped from the compressor section of the engine before the air enters the burner. The compressed air is many hundreds of degrees, so it has the capability to heat the surfaces. However, the air bled from the compressor is not available to be mixed with fuel and burned to produce thrust, so engine efficiency suffers greatly when the anti-ice heat is selected. This is a particular problem for small turbine engines that have little compressed air to spare. That's why many small jets and turboprops use pneumatic boots instead of heated leading edges for ice protection. In these airplanes bleed air from the engine compressor is used to inflate the boots instead of requiring dedicated air pumps as in a piston airplane.
Another option for heating a surface is electrical power, which works great for small items such as pitot tubes, static ports, windshields and even a smaller airframe surface such as a horizontal tail. But it takes a lot of electrical power to create enough heat to do the job, so its use has been limited until the Boeing 787, which is to be an all electric airplane. The generators will be so massive on the Dreamliner that electrical power will not only heat the surfaces for ice protection, it will also be used to pump up the cabin, power the controls, and even operate the wheel brakes. Boeing believes that using engine power to turn generators will be more fuel efficient than the lost thrust of tapping compressed air to heat the wings and pressurize the cabin.
Electro expulsive deice technology holds some promise for effective protection with low power demands. A version of the technology is being used to deice the horizontal tail on the Hawker 4000 and the Beechcraft Premier I. Electric actuators are used to "ping" the leading edge from inside the structure. The amplitude of the rap, if you want to think of it that way, on the leading edge is small, but the frequency is high, so the very small deflection of the surface effectively shatters any ice that has formed. A large amount of electrical power is needed to actuate the system, but the powered demand period is very short so the energy can be stored in a capacitor, for example. The situation is not unlike a spark plug where very high voltage is needed to create the spark, but the requirement is for only a very brief interval.
Another area of technological advance is in ice detection. That may not be much of an issue during daylight where, on most airplanes, pilots can see some part of the airframe that is an efficient ice collector. But in large airplanes, or for any airplane at night, it can be very difficult to know if ice is forming, and how much, and how quickly, so automatic ice detectors are an important safety tool.
Among the earliest automatic ice detectors is a probe that has a small serrated wheel that turns continuously. If icing is present it will form almost immediately on the sharp teeth of the wheel. As the wheel turns, a fixed wiper knocks off any ice that has formed. It takes more power for the wheel to knock ice off on the wiper than it does to turn freely without ice, so that increase in power demand is what creates the warning to the pilots that ice is detected.
A newer type of ice detector has a small probe that is continuously vibrated at its natural frequency. The probe is very small and thin so it is naturally an efficient ice collector, and ice will form on the probe before it does on larger surfaces such as the leading edges. When ice forms on the probe it changes the mass of the probe and thus its natural frequency. The change in frequency indicates icing and an alert is sent to the crew. Periodically the probe is heated to melt accumulated ice, and then it quickly cools and resumes its task of collecting ice if it is present.
An even newer technology in development by Safe Flight Instruments uses light to detect the presence of ice. A light source shines on a carefully shaped ice collector and a sensor detects if ice has begun to block its view of the light. Collected ice is removed so that the sensor can remain alert of continued icing conditions, or to detect a new encounter.
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