Researchers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) in Göttingen have discovered a way to make helicopters more maneuverable. In a novel wind tunnel experiment, they have been blowing air through holes in the rotor blades to actively influence airflow.
|Computer simulation of airflow. Source: DLR. Click to enlarge.|
Helicopters owe their special ability to vertically take off and land to their main rotor, but this also contributes to aerodynamic disadvantages. The airflow over a backward-moving main rotor blade separating from the aerofoil during fast forward flight or maneuvering, referred to as a dynamic stall, creates turbulence, loss of lift and exerts large forces on the rotor. Drag increases and the rotor head control rods are subjected to large dynamic loads.
This limits the top speed of helicopters at high altitude and their maneuverability. In addition, the resulting vibration compromises passenger comfort. Modern engines would be able to deliver significantly better flight performance were it not for these limitations.
It is just as though someone is striking the rotor with a sledgehammer.—Anthony Gardner, DLR Institute of Aerodynamics and Flow Technology
The concept devised by the researchers in Göttingen acts like an aerodynamic damper on helicopter rotors. Air is forced outwards through small holes in the rotor blades, which reduces the amount of harmful turbulence when stalling occurs. This enables the pitching moments exerted on the rotor blades—that restrict performance—to be substantially reduced.
During difficult flight manoeuvres, the pilot has the option of engaging this function for brief periods. At this point aerodynamic forces cease to act like sledgehammer blows on the rotor, becoming more akin to taps with a rubber mallet.—Anthony Gardner
The idea of actively influencing aircraft aerodynamics by blowing air in this way is not new. Back in the 1940s, researchers in Göttingen were already working on this. However, their successors have successfully demonstrated that the idea works on helicopters, in a wind tunnel experiment under realistic conditions.
The questions of how widely spaced and how large these holes should be to obtain a positive effect have now been resolved with the help of the supercomputers at DLR.
These experiments took place in the Transonic Wind Tunnel in Göttingen, a wind tunnel unique in Germany in terms of its speed range capability. This facility, measuring 50 meters in length and 12 meters in height is able to simulate how aircraft behave when travelling at close to the speed of sound in the transonic range (about 1000 km/h, 621 mph) and beyond —at speeds of over twice the speed of sound or Mach 2.2.
For these tests, a one-meter segment of a rotor blade was installed in the wind tunnel. A compressed air system with a complex set of valves blows air through 42 openings each measuring three millimeters in diameter. Seventy-four sensors measure pressures on the rotor blade up to 6000 times a second; this enables the airflow to be depicted precisely.
In a next step, these results will be checked on a new rotor test bench with a rotating rotor.
Recently, DLR researchers in Göttingen tested a passive method for influencing airflow, drawing their inspiration from the bumps on the pectoral fins of humpback whales.
The great speed and acrobatic skills of humpback whales is due to their unusually large pectoral fins, which have characteristic bumps along the front edge. Research has shown that these bumps cause stalling to occur significantly later underwater and increase buoyancy.
DLR researchers translated the idea of using bumps for delaying the onset of stalling to helicopter rotors, and patented it as Leading-Edge Vortex Generators (LEVoGs).