Researchers from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) based at Göttingen and Braunschweig have succeeded in visualizing helicopter rotor blade-tip vortices—the main cause of what makes helicopters so noisy while in the air—from an in-flight helicopter in three dimensions.
Helicopters can perform vertical take-offs and landings thanks to their main rotor, but this is also the reason why they are so noisy in flight. “Almost everything heard from a helicopter is aerodynamic noise. A large proportion of that is caused by what are known as blade tip vortices,” says Markus Raffel, Head of the Helicopter Department at DLR Göttingen.
|Blade tip vortices produced by the main rotor, photographed simultaneously from four different angles. Source: DLR. Click to enlarge.|
Blade tip vortices form at the outermost end of a rotor blade; a zone of reduced pressure forms above the blade and a region of increased pressure forms 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 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 simple technique to visualize density gradients, based on the deviation of light rays due to refractive index changes.
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 visualize the rotor vortices.
During one test, a Swiss Air Force Cougar helicopter maneuvered in front of a rocky background in the Alps. This yielded a successful world premiere—for the first time, images of a helicopter in flight produced a sufficiently precise visualisation of rotor blade vortices to enable the acquisition of scientific knowledge. During other tests, the DLR BO 105 research helicopter flew over fields and meadows near Salzgitter and Braunschweig, while researchers in a microlight aircraft flying above the helicopter acquired images.
The latest experiment took place just recently; the BO 105 descended into a quarry in the Harz region—a hazardous environment for a helicopter. The confined space and maneuvers so close to the ground tested the skill of the DLR test pilots. In places, they were asked to perform rocking maneuvers barely ten meters above the ground while the loose scree littering the excavated rock face served as a backdrop.
|Ten cameras recorded the BO 105 helicopter from various angles in the quarry. Click to enlarge.|
Ten cameras set up at various angles yielded images of such quality that, for the first time, a substantial proportion of an in-flight helicopter’s rotor blade vortices could be visualized in three dimensions.
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.
Altering the blade shape or modifying the rotor controls could enable a reduction in blade tip vortices and their interaction with the blades trailing behind, resulting in quieter flight.