Researchers have succeeded for the first time pictured the main cause of what makes helicopters so noisy while in the air.
Images © DLR
Scientists from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) based at Göttingen and Braunschweig, conducted at times daring in-flight experiments – most recently in a quarry in the Harz region. The knowledge they acquired will open up opportunities to make future helicopters much quieter.
Markus Raffel, Head of the Helicopter Department at DLR Göttingen, said:
“Almost everything heard from a helicopter is aerodynamic noise. A large proportion of that is caused by what are known as blade tip vortices,”
Vortices produce noise
As the air flows around the blade tip to reduce the pressure difference, a concentrated vortex is created behind the blade tip. “The noise comes from the vortex behind a rotor blade interacting with the following rotor blade,” explains André Bauknecht, leader of the current experiments. These vortices are not just responsible for the typically ‘carpet beater’ noise; they also produce vibration within the helicopter and make the ride less comfortable for passengers.
The researchers employed a well-known phenomenon to conduct their tests. On hot summer days, the air above some sections of a road starts to shimmer. This is due to fluctuations in the air density causing refraction of light, which is visible as striations when perceived against a suitable background. Raffel and his colleagues exploited this insight to develop what they call the Background Oriented Schlieren Method (BOS), a system now used in aerodynamic test facilities throughout the world.
Last year, the DLR scientists conducted a series of unique and at times daring experiments to test the new measurement method in flight. The Göttingen-based researchers employed a novel technique, using natural background areas to visualise the rotor vortices. “A suitable background must be as finely textured and uniform as possible – so we investigated where nature can offer this kind of surface,” says Bauknecht.
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