Aerospace & Defence
aircraft fleet by 10 per cent would add Euro100M/year to its bottom line. Airline operators are facing fierce commercial
pressure and this is being transmitted to the aircraft manufacturers and designers. There is now a degree of immediacy in demands for weight and cost reduction, which coincides with a raft of proven weight reduction technology in the military UAV sector. Due to the long lead times and regulatory constraints of the airframe itself, this pressure is likely to be transmitted onto the component and system manufacturers. It remains to be seen whether they are able to rise to the challenge of transferring and delivering this military UAV technology across to the civil aerospace sector.
Weight reduction
Many of the more obvious candidates for weight reduction - such as airframe and landing gear - have been trimmed to such an extent that there is nothing more to work at. Attention is now turning towards less obvious areas - not least of which is actuation and control systems, which can account for up 15 per cent of an aircraft’s weight. In a move to meet these new demands, there has been a marked trend away from hydraulically powered actuation and control to electrical power. The use of electromechanical systems as a
lightweight alternative to hydraulics was identified as far back as 1979 by NASA. More recently, the UK government funded an £11m study called ELGEAR (Electric Landing Gear Extension and Retraction) for UK industry to develop electrical actuation technology. The transition to electrical actuation and control is
still underway for the civil aerospace sector but UAVs in the military sector are already using electrical actuation and control combined with plastic and printed electronics to minimise weight and costs.
Printed electronics
Printed electronics technology typically uses conductive, semi-conductive or insulating inks printed on to thin, flexible substrates such as Kapton, polyester or polyimide. The term ‘plastic’ electronics is also used because many of the substrates are plastic or organic. The technology is most usually associated with high volume applications such as RFID tags, photovoltaic cells and printed circuits for consumer electronics. Until recently, it has not been generally considered for aerospace applications, but now that the advantages can be seen so clearly, the technology’s range of potential aerospace applications has expanded dramatically. Printed and plastic electronics technology’s most
obvious application is as a replacement for cabling. More interestingly, it is also a candidate for high functionality elements such as motor encoders, servo
feedback devices and motor control circuits. The technology’s extreme lightness and flexibility offers some unusual features and benefits to UAV designers who are able to avoid the cost, weight and mechanical-electrical constraints of traditional cable harnesses, electronic enclosures and connectors. In some instances the flexible electrical laminates may be simply embodied as layers within composite structures. This is already being used by some Formula 1 racing cars. One area where printed electronics is already
proving its worth is in sensors and motor control systems for servo actuation and feedback. Such systems are common in flight surface controls, intake ducts, brakes, throttles, undercarriage controls, as well as business and first class seating. Traditionally, these systems use an electric motor,
motor encoder, gearbox and a transformer-based servo feedback device such as a linearly variable differential transformer (LVDT). Whilst such transformer-based feedback devices offer precision and reliability, they are often as bulky and heavy as the motor. A 300mm stroke LVDT might be a 25mm diameter cylinder, 500mm long and weigh 1kg, whereas its printed alternative is 3mm high, 330mm long and weighs 25g.
Duplex or triplex redundancy
The advantages of the printed approach increase still further when one considers that many aircraft systems require duplex or triplex redundancy. Whereas a duplex or triplex LVDT roughly doubles or triples the original weight and volume, a printed device simply uses more layers of printed tracks to form an isolated second or third electrical system. In a simplex system, the weight reduction is typically >95 per cent whereas in a duplex or triplex system the weight reduction is >99 per cent. On an individual device the net effect is modest, however, when one considers that more than 50 such devices may be used in an aircraft the total effect begins to be very significant. There is a technology cluster in Cambridge UK,
starting from the Cavendish Laboratories in the 1990s, where some of these technologies were developed by companies such as Cambridge Display Technologies and Plastic Logic at the turn of the Millennium. They are now being commercialised successfully into printed devices that are in production by companies such as Zettlex in Cambridge who produce rotary, linear and 2D sensors as part of printed actuator control systems. Their position and speed sensors are used in fixed and rotary wing military aircraft, as well as a raft of other applications in the industrial, medical, oil and gas sectors.
While at first sight such devices might seem flimsy
and delicate, it should be remembered that the printed forms are rarely used in their ‘naked’ state. More
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