RESEARCH FOR FLUTTERING WINGS

Kerosene accounts for more than a quarter of the cost of a flight. Lighter, slimmer wings can help to further reduce fuel consumption in aviation.

Researchers from the Technical University of Munich (TUM) and the German Aerospace Center (DLR) have now succeeded in developing new technologies for lighter, but extremely stable wings put this in context, the greater the wingspan and the lower the weight of the wings, the less drag they generate in the air, meaning they are more energy-efficient than conventional designs. However, there is an aerodynamic problem. Due to wind resistance and gusts of wind during flight, long, slender, light wings vibrate more and more until they flutter like flags.

“The flutter phenomenon leads to material fatigue,” says Sebastian Köberle, research assistant at the Department of Aerospace Systems at TUM. “This can even lead to the wing breaking off.” Although each wing begins to flutter at a certain speed, shorter and thicker surfaces would have a higher structural rigidity and thus stability. “To build wings that are just as stable with a greater span would therefore also add more weight,” says Köberle. And that would mean losing out on the advantages.

As part of the European FLEXOP project, international scientists are thus working on new technologies to control the flutter of lighter and longer wings. Despite all the computer-aided simulation possibilities that researchers now have at their disposal in their institutes and laboratories, practical tests are essential. The responsibilities of the Munich researchers in the project team include designing and conducting flight tests at the special-purpose airport in Oberpfaffenhofen.

First, they built a three-and-a-half-meter-long flight model – a demonstrator – with a large wingspan of seven meters and integrated the systems developed by their European partners, which are mainly sensors. For the initial flights, the demonstrator was equipped with conventional reference wings in order to fly automatically predetermined routes. Experts from the TUM developed manuals and comprehensive checklists for the flight tests.

There are strict safety regulations for operating the autonomous model: The aircraft, which is driven by a powerful mini-turbine, must be visible from the ground at all times so that the researchers can intervene with a remote control in an emergency. All flight maneuvers can therefore only be carried out within a narrow radius of about one kilometer. “After all, the flight demonstrator flies so fast that even the new wings would theoretically have to flutter,” Köberle explains. “At such high speeds we have to be sure that nothing goes wrong.”

“IF THE WING IS BENT BY THE AERODYNAMIC FORCES, IT TWISTS AT THE SAME TIME AND AVOIDS THE WIND PRESSURE, SO TO SPEAK.”

In mid-November 2019, everything was finally ready. For the first time, the Munich scientists fitted a newly developed pair of wings to the demonstrator instead of the conventional ones. These new wings are aerolastically optimized and made of carbon fiber, developed by the DLR in collaboration with Delft University of Technology (TU Delft), another research partner. By specially aligning the fibers in the design of the wing, the researchers were able to influence the way they bend and their torsional behavior – hence the term ‘aerolastic’. Wolf-Reiner Krüger from the DLR Institute for Aeroelasticity in Göttingen explains the wing’s behavior in flight as follows: “If the wing is bent by the aerodynamic forces, it twists at the same time and avoids the wind pressure, so to speak”. His research colleague Köberle was satisfied after the maiden flight: “So far, everything has worked out as we expected. Now it’s time to make a start on evaluating the data.”

Meanwhile, the demonstrator is already on the starting blocks for the next round of test flights, for which it will have new wings fitted. The developers at TUM call them “super-efficient flutter wings” made of glass fibers. If flutter occurs at high speeds, flaps are extended at the outer edge of the wings to act as dampers. “This built-in active flap control increases the possibility for a significantly lighter design,” reports Gertjan Looye, who works at the DLR Institute of System Dynamics and Control Technology in Oberpfaffenhofen.

The aim of the project is clear. The flight demonstrator will not be the only aircraft to lift off with the two innovative wing types. In the near future, the promising results will also be transferred to the design of large freight and passenger aircraft.

THE FLEXOP PROJECT

As part of the European research project FLEXOP (Flutter Free Flight Envelope Expansion for Economical Performance Improvement), new methods are being developed and validated for the design of active and passive systems for flutter damping on very light, flexible wing structures. Within the scope of the European Union’s “Horizon 2020” research and innovation program, industry and research partners from six different countries are working on control algorithms, actuators and design optimization. Their findings are being tested on an unmanned flight demonstrator with a seven-meter wingspan and turbine drive.

FLEXOP partners include the Hungarian Academy of Sciences, Airbus Group Innovation, Airbus Group Limited, aircraft component manufacturer FACC, Integrated Aerospace Sciences Corporation (INASCO), Delft University of Technology, the German Aerospace Center (DLR), the Technical University of Munich, Bristol University and RWTH Aachen University.

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Text by Behrend Oldenburg
Photos: DLR, TU München (Fabian Vogel)

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