Airplane wings have it easy. Most of the time they fly in one direction only - forwards - and both sides fly in the same direction. Not so for helicopter rotors, which work in the same way as wings but with one side moving forwards (the advancing blade) while the other moves backwards (retreating blade). As speed increases, the advancing blade has to worry about its tip going supersonic while the retreating blade has to worry about stalling along most of its length.
German Aerospace Center DLR is experimenting with techniques to soften the stall and reduce the vibration loads on the rotor. One involves blowing air through small holes in the blade leading edge. The other, inspired by the nobbly fins of a humpback whale, uses tiny bumps on the leading edge.
Photos and graphics: DLR
In forward flight, as it moves from the advancing to the retreating side, airflow over a blade slows down and its pitch has to increase to generate enough lift to keep the rotor in balance. At high forward speed and in maneuvers this can result in a dynamic stall, which causes turbulence, loss of lift and high forces on the rotor control links as the stalled blade pitches nose-down.
DLR says its concept acts like an aerodynamic damper. Air blown through holes in the upper leading edge (see above) reduces the turbulence and "substantially reduces" the pitching moment on the blade. "During difficult flight maneuvers, the pilot has the option of engaging this function for brief periods," says DLR's Anthony Gardner. "At this point aerodynamic forces cease to act like sledgehammer blows on the rotor, becoming more akin to taps with a rubber mallet."
The concept is being tested in DLR's transonic windtunnel in Gottingen (above), where a compressed-air system blows air through 42 holes, each 3mm in diameter, on the upper leading edge of a 1-meter section of rotor blade. For an animation of how the blowing concept works, check here.
Another DLR concept has progressed to flight test, after promising windtunnel results, and involves installing tiny rubber bumps, or LEVoGs (leading-edge vortex generators), on the blades. These are 6mm in diameter and weigh just 0.04 grams each, and 186 were glued under each of the four rotor blades of DLR's Bo105 research helicopter for flight tests (below).
Humpback whales "are renowned for their great speed and acrobatic skills," says DLR's Holger Mai, adding this 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," he says.
Pilots noticed a difference in the behavior of the rotor blades on an initial flight of the Bo105 to check the safety of the LEVoGs, DLR says. The next step is an instrumented flight to measure the effects. If successful, DLR hopes existing helicopters could be retrofitted with the devices at low cost, while on new helicopters the contours could be milled into the blades' titanium leading edges.