F/A-18F Super Hornet at high angle of attack over a snowy mountain range. Photo Credit: navy-pictures.blogspot.com
Let’s start off with a scenario: You’re flying a sortie for the Navy in a reconnaissance-configured F/A-18F Super Hornet to fly in high over some mountain range in Afghanistan, take some pictures, and come home. You’re flying at 40,000 ft in order to both stay clear of the immensely tall mountain ranges and to remain undetectable and stay out of reach of small arms fire. Unfortunately, at 40,000 ft in this region, icing conditions are very common. Due to the low temperatures in this region (and especially at this elevation) combined with the all-too-often rainy or snowy conditions, you end up having to fly through falling snow or rain (which would be either already ice due to the low temperatures, or supercooled, meaning that the water drops are far below the freezing point, but have not frozen yet due to the absence of a nucleation site – which is essentially a solid surface on which the water can freeze…i.e. dust, other particulate matter, or the wing of an aircraft).
As you begin to fly into the poor weather, the water droplets begin to freeze on impact with the wing, building up a layer of ice on the wing of your aircraft. This ice quickly changes the surface roughness of your aircraft wings, changing the aerodynamics and lowering the speed or angle of attack that you can travel at without stalling (flow separation that leads to loss of lift). Furthermore, say due to the angle of incidence between your flight path and the direction the water droplets are coming from, the ice is forming more heavily on one side/wing of your aircraft than the other (and probably unevenly on each side). You start to feel the aircraft wobbling around in flight and find it much more difficult to control your roll, pitch, and yaw. You are now having issues flying the aircraft at the required speed, are having to fly at a lower angle of attack, and are having trouble controlling the aircraft. You have a decision to make: continue to put yourself, your Radar Intercept Officer (RIO), and your $60,000,000 aircraft in danger or abort the mission. Unfortunately, this situation is not too uncommon when icing presents a real problem to pilots.
F-35 undergoing icing tests.
Photo Credit: AF.mil (Left), globalaviationreport.com (Middle, Right)
With the development of unmanned drones for reconnaissance missions, we are able to push the envelope a little more, but still are wary of putting a very expensive aircraft into poor weather conditions (for both financial losses and the possibility of having a high-tech aircraft downed in enemy territory). This problem also exists for civilian air travel, and in many cases, general aviation pilots are not certified for flight into known icing conditions due to the clear and present danger. So what do we do to reduce the threat posed by icing conditions?
Aircraft wing icing and an expandable rubber boot used to break up the ice. Photo Credit: cloudman23.files.wordpress.com
There are a few traditional ways of dealing with aircraft icing. One is shown above, in which a rubber “boot” is expanded in order to break up the ice on the leading edge of the aircraft; obviously, this is not an optimal solution, as it temporarily changes the profile of the wing, and also does not get rid of all of the ice. Other methods include bleeding hot engine exhaust air over the wing and using “weeping” wings that release antifreeze over the surface of the wing, to name a few. These methods are all quite complex, require a lot of power, can alter the airflow significantly, and may even present environmental dangers (as in the case of the weeping wing).
In order to fill this void of optimal solutions, a company called Batelle has employed its researchers to develop a solution that consists of a carbon nano-tube coating that can be included beneath the topcoat of the aircraft wing and then act as a resistance heater of sorts to warm the wing from inside the wing to prevent or get rid of ice on the wings. This solution employs advanced materials science concepts in order to introduce a more elegant, non-invasive means of dealing with aircraft icing. Furthermore, the coating is controlled through an “intelligent controller [that] monitors the heater performance and applies only the power levels required for the flight conditions”, making it less power-hungry and more adaptable to any given situation! These benefits are realized even further on military Unmanned Aerial Vehicles (UAVs) (their principal application), where large sensor (or other) payload power requirements and desired lightweight dictate that a smaller than ordinary amount of power can be dedicated to de-icing. This solution has the capability to revolutionize the way we go about preventing or dealing with aircraft icing; now only if they could further development this solution for use in the de-icing of pitot tubes (devices used to measure airspeed – to figure out how fast you are flying; pitot tubes are notorious for encountering problems in icing conditions, where a false airspeed reading can lead to disaster).
Read more at: Engineering.com