Morphing for efficient aircraft
Aircraft may soon be able to respond to varying external conditions using ‘smart morphing technologies’ that increase fuel efficiency, improve maneuverability and lower landing speeds.
A team of researchers at the University of Liverpool, UK, are investigating ways to alter the internal aircraft structure in order to achieve these goals, as part of a wider European collaboration, the SMorph project.
‘Civil and military aircraft are currently designed to have optimal aerodynamic performance characteristics at a single point in the flight envelope and fuel condition,’ says Jonathan Cooper, Professor of Aerostructures and Aeroelasticity at the University. ‘However, the fuel loading and distribution changes continuously throughout the flight and aircraft often have to fly at non-optimal conditions due to air traffic control restrictions. The consequent sub-optimal performance is more significant for commercial aircraft where aerodynamic performance, and hence fuel efficiency, as a performance parameter is of far greater importance.’
Morphing technology is not new. High lift flap systems using hydraulics exist on most aircraft to increase the camber and wing chord, improving performance for taking off and landing. But the team at Liverpool has exploited an adaptive internal structure approach that enables the wing to respond to aerodynamic forces by bending and twisting to maintain the best lift-drag ratio during the flight, rather than trying via an actuator, that adds weight and increases fuel consumption.
Cooper and his colleagues have used a rotating spar concept. A wind tunnel prototype has been built that includes aluminium spars whose orientation is controlled via four motors positioned at either end. A thin polyethylene skin provides an aerodynamic surface. The prototypes have been subjected to static loads and vibration testing, which confirms that the spars can be rotated while carrying a significant static load. It took 0.5 seconds for the spars to rotate from 0-90º, meaning that the structure can quickly adapt to achieve the optimum aerodynamic performance.
‘Changes in the position, orientation and stiffness of the internal wing structure via
rotation of the spars can be used to change the bending and torsion stiffness properties, and hence to control the aerodynamic performance, in particular the lift and drag characteristic’, says Cooper.
David Alexander, a retired structural engineer formerly of Rolls-Royce in London and the Warwick Manufacturing Group, both in the UK, sees potential in the work. ‘There are some big players working on this type of technology,’ he explains. ‘Airbus and Boeing are working on developing winglets [small drag-reducing attachments at the tip of an aeroplane’s wing] that will move while an aircraft is in flight. That is a change from today’s winglets, which are fixed in place, and it would mean a big boost in fuel efficiency.
‘However, the two aerospace giants are investigating technologies using electric motor mechanism, and are yet to find an effective way to reduce weight. This concept using rotating spars could therefore be a promising avenue to pursue.’
Cooper says that the approach is now being considered on finite element models of full-scale aircraft using multidisciplinary design optimisation methods to investigate the best ways of including morphing technologies and to quantify the benefits.