German researchers have managed to visualise the causes of helicopter noise for the first time, paving the way for future quieter helicopters.
During a challenging in-flight experiment conducted recently in an abandoned quarry in the Alps, the team from the German Aerospace Centre (DLR), have flown a Swiss Air Force Cougar helicopter, making the pilots manoeuvre at times dangerously close to the rocks.
Using ten advanced cameras mounted at different angles, the scientists have managed to capture images of the vortices that form around the blades, for the first time enabling their 3D visualisation. These vortices, forming behind the blades of the helicopter’s rotor are known to be the main cause of the helicopter noise.
"Almost everything heard from a helicopter is aerodynamic noise. A large proportion of that is caused by what are known as blade tip vortices," said Markus Raffel, Head of the Helicopter Department at DLR Goettingen.
Blade tip vortices form at the outermost end of a rotor blade as a result of a zone of reduced pressure forming above the blade and a region of increased pressure below the blade.
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 Andre 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 method used to visualise these vortices relies on the same phenomenon that makes air appear shivering above roads during hot summer days. This phenomenon is a result of fluctuations in the air density that cause refraction of light, creating observable patterns when perceived against suitable background. "A suitable background must be as finely textured and uniform as possible – so we investigated where nature can offer this kind of surface," Bauknecht explained why the experiment was conducted in a quarry.
The DLR researchers have previously developed a method called the Background Oriented Schlieren Method (BOS), which is now used in aerodynamic test facilities throughout the world.
The next round of flight tests will involve fitting helicopters with measurement equipment, enabling a direct comparison between the vortices recorded and the control inputs from the pilot.
The team believes it could be possible to reduce the noise produced by the helicopters in the future by slightly changing the shape of the rotor blades and modifying rotor controls. Such changes would contribute to reducing the blade tip vortices and their interaction with the blades trailing behind.
"This would help helicopter manufacturers to compare various rotor blades under realistic conditions and to select the quieter option," Bauknecht concluded.