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Quiet choppers

April 11, 2011

Researchers from the German Aerospace Centre (DLR) and NASA are collaborating on a project which paves the way to silence the din so synonymous with helicopters.

A female scientist in a holds something up to a green light
Monitoring helicopter behavior in simulated testsImage: DLR

There's something endearing about calling a helicopter a chopper, but less endearing by far is the deafening sound which spawned the nickname.

To stand next to, let alone sit in, a chopper is to be exposed levels of noise ear-splitting enough to motivate researchers from the German Aerospace Centre, better known as the DLR, and the American space agency, NASA, to try and find a possible solution.

Over the last few months, German and American scientists have been collaborating in the US and Germany to improve the intense sound that helicopters make.

Contrary to popular belief, almost all helicopter noise is aerodynamic rather than mechanical, their research focuses on the rotor and the interaction between the blades and the vortices they create when the aircraft is in flight.

"The key to noise reduction is the detailed knowledge of aerodynamics," said the head of the helicopter department at the DLR site in the city of Göttingen, Markus Raffel, in an interview with Deutsche Welle.

Quick capture

A stand holding a test rotor. Scientists watch on
The rotor test stand in actionImage: DLR

In using a rotor test stand, which they have on loan from the University of Aachen, and state-of-the-art cameras, which can film at speeds of up to 4,000 frames per second, the trans-Atlantic team is able to study the vortices up close.

"Before we were only able to make snapshots of vortices, but the development of high speed cameras and lasers means we can look at the flow features and follow them and try to understand them," added Karen Mulleners, a DLR researcher, in an inteview with Deutsche Welle.

Thus far, she says the experiments have proved highly successful, and that she and her colleagues are gaining an understanding of how the vortices, which are not the same in each cycle, are formed.

The next step

A helicopter flies over the sea. Around its rotors are visible vortices
The scientists make the flow of the vortices visibleImage: US Navy

Once they have established that, they will be able to work on possible ways of weakening them or changing their trajectory to lessen contact with the blade, and thereby make chopper flight easier on the ears.

The current level of noise is not only a source of discomfort for pilots, but given that helicopters are often used for lifting patients and for military purposes, it is also highly impractical.

Raffel says there has been a reduction in noise volume over the past decades, but that with the right initiative, it could be turned down even further.

"We could have helicopters that have half the noise emissions if people would focus on it," Raffel said. "But they are not, because they have a lot of other safety and environmental issues to deal with."

Multiple solutions

A field of arrows depict the instantaneous velocity field in a rotor tip vortex
The instantaneous velocity field in a rotor tip vortexImage: DLR

He says the cooperation with NASA, which both sides had been keen to get off the ground for several years, is a positive step in the right direction. A team from the National Aeronautics and Space Administration has just left Germany after two weeks of experiments.

"We measured highly resolved blade positions, density, gradients and velocity fields," Raffel said, adding that they would publish the results in scientific journals and would use them for further planning.

In the immediate future, they will continue to run tests on the rotor rig for as long as they have it. They will use the data they obtain to try and secure funding for their own test stand, which would allow them to continue optimizing their research.

"It's an ongoing process," Mulleners said. "There is not one solution, but multiple possible solutions which mean we have some work to do over the next couple of years."

Reporter: Tamsin Walker
Editor: Cyrus Farivar