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Commercial aircraft are a bit like ships of the sky they are a world unto themselves, almost isolated from the outside. Miles above the surface, travelling at hundreds of miles per hour, they exist outside the range of most communications types. world. All communication that does occur, passes through the part known as the 'radar dome'. This is the rounded nose of the aircraft, and all radio wave communication whether for pilot systems or passenger connectivity, passes back and forth through these systems. No other part of the aircraft is capable of sending or receiving during flight. So, if there is a problem here, the aircraft is completely cut off.
In our increasingly interconnected, smart-sensing world, that is a recipe for disaster. So, it is absolutely vital that this dome be manufactured to the highest precision possible. Unfortunately, that is often not high enough.
The dome is made of moulded fibreglass, and as with all fibreglass, it is all too easy for errors to creep in during production - tiny foreign particles, drops of water or air bubbles can all too easily penetrate the films involved during production. Air bubbles in particular are a common bane of moulded manufacturing. Trapped air is impossible to avoid during any molding process, and although steps are taken to limit it as much as possible, the only thing to do is hope all the bubbles work themselves out before the material dries. Often impossible to detect, they alter the way signals flow through the material, introducing random noise and distorting communications. Particles of dust or drops of water work the same way, altering the signals, decreasing reception quality, introducing noise, reducing available bandwidth. Unless part of the imperfection intersects with the surface where it an be seen, it typically goes undetected until the dome is assembled and in use by then, it is too late to fix without grounding the plane.
Worse, these imperfections in the material, when heated and cooled at the extremes occurring during normal flight operations, will over time, start to crack inside the material. When these cracks reach the edges, moisture starts to seep in, greatly compounding the problem. It is not unheard of for the plane's communication ability to fail entirely complete loss of signal due to what was originally, an undetected air bubble in the material.
So, it is then not surprising that methods of detecting these tiny imperfections during manufacture are highly sought-after.
Enter the German Fraunhofer Institute. As part of the Dotnac project, researchers at that institutes Physical Measurement Techniques department are working with partners in industry and research to develop a new testing system which uses the terahertz waveband to scan right through the fibreglass dome which is up to five centimetres thick and identify any flaws based on the way they interact with the radio waves. In other words, use the same property of the imperfections that causes problems, as a means to find them.
The main challenge facing researchers was to find out which terahertz frequencies they would have to use to bombard the material to achieve the most effective results for the various imperfections. Higher frequencies create better resolution, while lower frequencies have less difficulty penetrating the material. So, by using a constantly modulating range of frequencies a composite picture is built up.
So far, only a single prototype detector exists. It sits in a rolling cabinet containing a microwave source and a frequency mixer to multiply the frequencies involved up to the terahertz range. The computer in the bottom of the cabinet continually adjusting the frequency as results filter back for each.
The module emits the terahertz waves toward the radar dome. The fibreglass of the dome partially absorbs the signal, and partially reflects it back to the unit where it is analysed. Any imperfections will deflect the returning waves from their expected receiver point, creating a tiny hole in the data the same as the flaw in the material. `
By using multiple frequencies, multiple depths into the material can be reached, until most of the waves are passing through rather than reflected back. This enables the entire piece to be scanned in a single sitting, with the computer itself responsible for frequency shifting to minimise human error. The end result is a detection system that can detect the tiniest flaws in manufacture, whilst the unit is still on the factory floor, long before it is installed in an aircraft.
The net result is a connection between the aircraft and the wider world that is robust, with the highest bandwidth and lowest signal to noise ratio possible, regardless of what it is used ofr and one that will not fail and lose connection in the most terrible of weather, because there are no imperfections to serve as weak points and stress fractures.
Seeing inside the nose of an aircraft
DOTNAC Project (Development and Optimisation of THz NDT on Aeronautics Composite Multi-layered Structures)