Scientists at DLR Göttingen have achieved a world first – showing the deformation of an aircraft propeller blade during flight. They have developed a special camera that can resist the enormous forces exerted during rotation. Their findings could not only help to improve aircraft propellers, but also helicopter rotors and wind turbines.
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. However, the Göttingen researchers have found a solution. They placed the camera, a miniaturised 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 manoeuvres.
Propeller deformation in flight
"We wanted to know precisely how a propeller behaves during flight. This is because the performance of the propeller changes when the blades deform," says Boden. "Our results can help manufacturers find out how the effectiveness and service life of propellers can be improved," adds Stasicki. Both the material and the shape can affect the way propellers deform during flight. The results might also help pilots. "Previously, pilots have set the rotation speed and angle of attack of the propeller by 'feel'," explains Boden. As the effects can now be measured, "a pilot will be able to know what setting will give the lowest fuel consumption or the highest speed without placing an unnecessarily high load on the propeller. This also increases flight safety."
Dot pattern makes forces visible
Not just the camera and its housing have been developed by the DLR researchers in Göttingen – so too has the measurement technology employed. This 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.
"Use of a rotating camera is a new technology that has now matured. It can be used to precisely measure the position and deformation of fast-rotating objects throughout an entire revolution and without actually having to contact the object in question," says Stasicki.