If you've ever wondered why helicopters make the noise they do, this photo has the answer. For the first time, the way the rotor blades produce sound has been caught on camera.
As a helicopter flies, a vortex forms at the tip of each of the blades, because there is a difference in pressure between the regions above and below them. The noise is caused by blades hitting the vortices created by their neighbours.
A research team led by Markus Raffel of the German Aerospace Center in Göttingen has developed a technique to take pictures of these vortices.
The method uses the same phenomenon that causes air above road surfaces to shimmer on a hot day. Light rays are bent as they travel through patches of air with different densities, and this can be seen clearly against suitable backgrounds.
A series of experiments using different natural backgrounds revealed that the most successful one for spotting vortices was a quarry in the Harz region of Germany. In this confined space, the helicopter had to be photographed rocking just 10 metres above the ground.
The researchers hope their work will help to make helicopters quieter and less prone to vibration.
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