Researchers at the German Aerospace Center (DLR) in Göttingen may have discovered a way to make helicopters more maneuverable, by reducing the dynamic load on the rotor head control rods.
During rapid forward flight or maneuvering, airflow stalls on the main rotor blade as it retreats (moves backwards), giving rise to a “dynamic stall” and subjecting the rotor head control rods to formidable dynamic loads.
“It is just as though someone is striking the rotor with a sledgehammer,” explained Anthony Gardner, from the DLR’s institute of aerodynamics and flow technology. The rotor blades undergo pitching moments that move the leading edge down and the trailing edge up. The result is restrictions on the maximum speed and the maneuvering capability of helicopters, especially at high altitudes, said Gardner.
The DLR’s newly tested system dampens the phenomenon by forcing air outward through small holes in the rotor blades. This “reduces the amount of harmful turbulence” in the event of a dynamic stall, and aerodynamic forces on the rotor are reduced to “taps with a rubber mallet” rather than sledgehammer blows, according to Gardner. The pilot could have the option of activating the system briefly before engaging in demanding flight maneuvers.
The researchers say this is the first time that blowing air through holes in the main rotor blades has been seen to influence airflow in a controlled, favorable fashion, at least in a wind tunnel.
“It was only recently that we acquired the computational power to calculate what a wind-tunnel model of this kind should look like,” Gardner said. The DLR’s supercomputers have resolved questions about how widely spaced and how large these holes should be to obtain a positive effect.
DLR describes the experiment as the most complex ever performed in the field of dynamic stall control. For these tests, a one-meter (3.3 feet) segment of a rotor blade was installed in the wind tunnel. A compressed air system with a complex set of valves blew air through 42 openings, each measuring three millimeters (0.12 inch) in diameter. Seventy-four sensors measured pressures on the rotor blade up to 6,000 times a second, enabling precise depiction of the airflow.
The next step will be trials with a rotating rotor installed on a new rotor test bench.