A glimpse into a "masterpiece of engineering"

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The "Normal Segelapparat" comprises a fabric-covered, wooden skeleton. The video shows the fabric and rope. Later, the willow-wood sticks with the insect holes are shown. They are displayed in blue because of the low material density. (Video: Michael Mosch, Airbus Helicopters, Denis Kiefel, Testia-Airbus, Rainer Stössel, Airbus Group Innovations)

Research news

More than 125 years ago, Otto Lilienthal laid the foundation for modern aviation with his innovative gliding apparatus, the "Normal Segelapparat". Only four specimens of the gliding apparatus have survived to this day, one of them at home in Deutsches Museum in Munich. Computer tomography investigations carried out by researchers at the Technical University of Munich (TUM) in collaboration with Airbus have for the first time provided a glimpse into the inner workings of the construction design. The images will help researchers and conservators at Deutsches Museum during restoration.

"The apparatus is a masterpiece of engineering: built to be extremely light and using flexible materials that are reinforced at critical points." Prof. Christian Große , head of the TUM Chair of Non-destructive Testing , goes into raptures when he talks about Otto Lilienthal’s gliding apparatus.

The "Normal Segelapparat" comprises a fabric-covered, wooden skeleton with a wingspan of nearly seven meters. From today’s perspective, the ingenious construction serves as a basis for further developments in modern aviation: The Wright brothers, for example, took advantage of Lilienthal’s insights to construct the first powered airplane. Four specimens of Liliethal’s invention have survived to this day, one of them at Deutsches Museum. The aviation curator of the museum, Andreas Hempfer, says: "It is a stroke of luck that this glider has survived in an authentic, albeit fragile state. This provides us with unique insights into Lilienthal’s mode of work."

To date, visitors of the Flugwerft Schleissheim aircraft hangar could only venerate the original cross framework of the Lilienthal glider - albeit copies of the gliding apparatus are on display both on the museum island in Munich and in Schleissheim. The research in collaboration with TUM is geared towards conserving the remains of the original glider so that it can once again be put on display at the aerospace exhibit by 2025, at the latest. To this end, Teresa Donner and Laura Lehmacher at the Chair for Restoration, Art Technology and Conservation Science at TUM have compiled a detailed description of the glider’s current state. On behalf of Deutsches Museum, the two students investigated the fabric and wood fragments and worked out suggestions for conserving the glider apparatus in collaboration with the curator Andreas Hempfer, the aircraft conservators Mathias Winkler and Philipp Stengele, as well as the conservation scientists at Deutsches Museum headed by Dr. Marisa Pamplona-Bartsch.

Peering beneath the paint

Obviously, the apparatus should not be damaged during the investigations. That is why the young TUM researchers turned to Christian Große, an expert in non-destructive testing processes. He explains, "In this case, I recommended computer tomography. It can be used for detailed imagery of large objects and various materials - here, we are dealing with wood, textiles and metallic reinforcement elements. In addition, the investigation is entirely non-destructive and contact-free."

However, finding a suitable computer tomograph is not so easy. The aviation industry utilizes such high-tech devices to test fiber reinforced composite components. The engineers at Airbus Helicopters in Donauwörth agreed to investigate three fragments of the historic flight apparatus using the modern technology. The subsequent evaluation of the results was done at Airbus Materials X and Airbus TESTIA in Munich.

Große explains: "The aerospace conglomerate has at its disposal computer tomographs with excellent resolution and extensive experience in the application of non-destructive testing procedures. Using the imagery, we could recognize myriad details without having to destroy the valuable original component parts. The utilized evaluation processes are among the most advanced worldwide."

Planning measures in collaboration

The 3-D imagery revealed bonded joints, nails, layers of lacquer and plenty of holes bored by insects. Some of the wooden struts of the gliding apparatus are completely eaten away and are held together merely by the outer layers of lacquer. The nails are, incidentally, bent and placed so that they distribute the load - which strengthens the structure. "This is a further interesting detail that we only discovered because of the computer tomography," says Große.
The results of the investigation are now helping the researchers and the conservation scientists at Deutsches Museum develop the proper conservation processes. For example, they must find a suitable wood firming agent. However, deploying non-destructive test procedures is not only useful in planning conservation measures. The images from the computer tomographs also give visitors a glimpse inside the objects on display, says Große.

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