Nanotechnology Shapes New Future for Aerospace Materials
Developments in fields such a bio-mimetics and nanotechnology are opening doors for new low-weight aircraft design. Dr Gareth Evans reports.
Conventional aerospace vehicles all suffer from the same fundamental limitation - they are essentially designed around a single mission, be that reconnaissance, interdiction, aerial combat or spaceflight. Inevitably this dictates a particular craft geometry that is ideally optimised for that given primary role, but leaves the vehicle's structural form in a distinctly sub-optimal configuration for other potential mission segments. This represents an obvious operational constraint. With the rise of morphing metal technology, however, all that may be about to change.
Throughout aeronautical history, a number of solutions have been envisaged, some of them never progressing beyond the status of notional concept while others, such as variable sweep wings, have formed paradigm shifts that are now both universally accepted and widely adopted.
Today's innovation in this space is pushing the boundaries still further, drawing on the sort of 'living material' technology that seems to border on the realm of science-fiction, to bring compact hybrid actuators – self-healing, self-sensing and self-moulding alloys – to bear on the issue.
Wings made from such materials, which can 'feel' changes in pressure or temperature, or transform from liquid to solid when a magnetic field or electric current is applied to them, could unfurl, bend and shape themselves on-the-fly to adapt to evolving demands. Their appeal is clear.
Existing control systems intentionally restrict the operational envelope of modern high-performance aircraft to prevent them entering a post-stall environment during dynamic manoeuvring. Morphing aerial structures capable of changing shape in real time could control air-flow predictably and avoid the aerodynamic destabilisation of control surfaces over a greatly expanded range of flight conditions, potentially opening the door to truly multi-role vehicles, without compromise. As Dan Goldin – NASA's former administrator – put it, “the seemingly effortless flight of birds provides the inspiration for new aircraft utilising wings that reconfigure in flight”.
More prosaically, from a military point of view, the ability to modify wing shape and vehicle morphology throughout an operational deployment would, of course, enable missions to be undertaken that far exceed existing technological capabilities. Perhaps even more importantly, it would also empower a single aircraft to be successfully tasked with multiple and disparate missions, which currently demand a range of separate individual aerial vehicles operating as part of a larger, coordinated system.
Previously, theoretical incarnations of such aero-morphing designs have always been significantly restricted by the weight of the actuators required to bring about in-flight shape-shifting. Now, however, developments in fields such as bio-mimetics and nanotechnology allow the construction of low-weight 'designer' materials, opening the door to reconfigurable smart components that could ultimately revolutionise the whole concept of flight.
Research into the smart alloys and polymers necessary to make aero-morphing a practical reality has been going on for many years – starting as far back as the 1980s – and at a number of institutions across the globe, including the University of Adelaide, Imperial College and Cornell University. In Europe, elements were found in programmes such as the aircraft wing advanced technology operations (AWIATOR) and active aeroelastic aircraft structure (3AS), while in the US, NASA and the Defense Advanced Research Projects Agency (DARPA) have been the principal protagonists in driving the concept.
NASA's eponymous Morphing Project was run from the organisation's Langley Research Center from the mid-1990s, with the core goal of identifying enabling technologies rather than a complete functional prototype – despite famously producing a virtual reality model that caught the public eye in 2001. Tasked with developing and assessing advanced methodologies and integrated component concepts to facilitate efficient multi-point geometric adaptability, the project undertook fundamental research into smart materials, adaptive structures and micro-flow control. Although it formally closed in 2004, much of the work continues in collaboration with industry and other governmental agencies.
DARPA's Morphing Aircraft Structures (MAS) programme developed along a slightly different angle, providing and integrating the requisite technologies to enable the design, construction and demonstration of aerodynamically efficient, shape-changing wings.
This was to culminate in the fabrication and successful wind tunnel testing of a completed wing to prove the principal of flight-traceable morphing, and set the stage for the next phase of the ongoing project.
Changing the shape of war
Unmanned aerial vehicles (UAVs) are one area that could provide a particularly good opportunity for the swift roll-out of morphing developments, and a range of DARPA-supported initiatives are underway involving Lockheed Martin, Hypercom/NextGen and Raytheon Missile Systems.
Experience in Iraq and Afghanistan has firmly established the operational value of hunter-killer class UAVs, such as the Predator, particularly when the fuselage platform has been teamed with an effective weapons system such as the Hellfire missile. It has also highlighted the inherent vulnerability of such a slow-moving target to hostile ground fire. The envisaged next generation of UAV, equipped with morphing technology, will circumvent such limitations, entering its operational environment at high speed, shifting its aerodynamic geometry to allow it to loiter once on target and then changing back again to move on to its next engagement.
In addition to providing the vehicle with unprecedented speed and manoeuvrability, further developments in self-sensing and AI could potentially see it capable of defending itself, at least in part, autonomously against ground forces and even air-to-air threats.
Eventually the aim is to produce truly adaptive structures that can shift in real-time as a response to changing environments and operational needs, and by altering surface geometry, substantially vary an aircraft's aerodynamic performance to suit, and apply it to manned craft. To achieve this will require the integration of sensing and actuating devices within the aerial structure itself, along with the concomitant level of embedded intelligence to allow the morph to be properly controlled. Clearly, reliability is set to be a major issue in this.
Progressing the concept remains reliant on enabling technologies that are still emerging in areas such as artificial intelligence, nanotechnology and bio-mimetics as well as the necessary transduction materials, such as piezo-ceramics, magneto-strictives and shape-remembering alloys that can both sense and self-actuate. Innovation is not a linear business and different disciplines do not advance conveniently at the same pace; it may take some time to get all the necessary pieces in place. NASA has suggested that it could be as much as 20 years before the most appropriate assemblage of component technologies will be fully identified and harnessed, and there are few voices dissenting from that timeframe.
Until then at least, the "birds” that Goldin described will retain their unparalleled mastery of the air, but if morphing research does ultimately go according to plan, those days could be numbered. Feathers may yet finally lose their edge.