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Fritz Boden and Boleslaw Stasicki from the DLR Institute of Aerodynamics and Flow Technology carried out the experiments at Kunovice in the Czech Republic, using a single-engine Evektor VUT 100 Cobra. The researchers developed a special stereoscopic high-speed camera and installed it in the propeller hub.

The camera was aligned with a propeller blade and rotated in synchrony with the propeller during flight. The camera was thus subject to vibrations and centrifugal forces of up to 20-times Earth’s gravitation. Because of this, it was previously not considered possible to observe propeller deformation during flight, as the required sensitive measurement technology would be destroyed by such stresses.

To overcome this, the Göttingen researchers placed the camera, a miniaturized computer and other electronics inside a metal container to protect them from damage. In this way they were able to take thousands of images of a special pattern of dots placed on the propeller blade. This enabled the smallest deformations to be rendered visible and measured even during extreme flight maneuvers.

The measurement technology, also developed by the DLR researchers, involves using two camera sensors with different angles of view (a stereoscopic camera) to acquire images of the object under investigation. Using specially developed software, the entire surface being observed can be represented in 3D. Until now, it has not been possible to measure the actual shape and position of propeller blades under actual flight conditions. The only option thus far was to attach sensors to the blades. The disadvantage of this is that these sensors only measure individual points, and they affect the airflow. In addition, the wiring required is often problematic.

This new technique also allows researchers on-board the aircraft to start and stop the imaging from the cabin via a wireless network, review the images and, if necessary, change the camera settings. The camera uses an integrated GPS receiver to simultaneously log the position of the aircraft and when the image was taken.