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.
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