Super-efficient wings take off

Wings with longer spans and lower weight generate less drag – and are therefore more energy efficient. More efficient lift could reduce kerosene consumption and thus reduce emissions and costs. The limiting factor for the construction of such wings is the aerodynamic phenomenon of flutter. Wing oscillations become stronger and stronger due to drag and wind gusts – much like a flag flying in a strong wind. "Flutter causes material fatigue and can even lead to the failure of the wing attachment to the fuselage," explains Sebastian Köberle, a researcher at the TUM Institute of Aircraft Design. Although any wing will begin to flutter at sufficiently high speed, shorter and thicker wings have greater structural stiffness, and hence greater stability. Building wings with longer spans that are just as stable and stiff would make them much heavier. In the European Flutter Free FLight Envelope eXpansion for ecOnomical Performance improvement (FLEXOP) project, researchers from six countries are working on new technologies to control flutter while allowing wings to be made lighter.

Wings avoid wind

The TUM researchers are responsible for the design and execution of the flight tests that demonstrate the actual behaviour of the two novel wings developed by the project – the aeroelastic wing and the flutter wing. The TUM team first built the three-and-a-half-metre-long and seven-metre-wide flight demonstrator and integrated the various systems provided by the European partners. A particularly light wing, which has now been flown for the first time, is an aeroelastically optimised wing constructed from carbon-fibre reinforced composites. It was developed by DLR in Göttingen, in collaboration with Delft University of Technology. The researchers were able to influence its bending and torsional behaviour through a special alignment of the fibres during the construction of the wing. “When the wing is bent by aerodynamic forces, it rotates simultaneously and thereby reduces airflow-induced loads,” says Wolf-Reiner Krüger of the DLR Institute of Aeroelasticity in Göttingen.

With the help of the reference wings, the TUM researchers worked in advance to have the flight demonstrator automatically fly predefined flight test patterns. They devised optimum settings and developed manuals and checklists for the flight tests. "The flight demonstrator has to fly fast enough with the new wings that they would theoretically have to flutter," explains Köberle. “We have to be sure that nothing goes wrong at such high speeds."

"The aircraft must remain visible from the ground, so that the researchers can intervene at any time. This means that the flight manoeuvres are flown within one kilometre of the ground control station. The extensive test flights followed completion of complex preliminary work. "Everything worked out as we imagined it would," says Köberle. "Now we will begin evaluating the data."

Active damper control for the 'flutter wing'

Another super-efficient wing developed in the project is the 'flutter wing'. This is a TUM design and is made of fibreglass. If fluttering occurs, the outermost flaps are extended. They act like dampers. "The active flap control developed at DLR considerably increases the possibilities for a much lighter design," says Gertjan Looye of the DLR Institute of System Dynamics and Control in Oberpfaffenhofen, which manages DLR's share of the project. A second flight control system is being developed by the Computer and Automation Research Institute of the Hungarian Academy of Sciences (MTA SZTAKI). Project Manager Bálint Vanek of MTA SZTAKI adds: "Such a wing would make it possible to transport 20 percent more cargo or to reduce the required fuel by seven percent." The technology is particularly complex, so tests on this wing will take place at a later date.

Both variants of the super-efficient wing have already been evaluated during static vibration tests conducted at the DLR site in Göttingen.

From demonstrator to passenger aircraft