http://www.zdnet.com/article/silent-rotor-blades-could-lead-to-true-stealth-helicopters/
According to this, the noise comes from blade encountering vortexes shed off the preceding blade.
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.Without freedom of speech we wouldn't know who all the idiots are. -- anonymous poster
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.Because much of what is in the published literature is nonsense,
and much of what isn’t nonsense is not in the scientific literature.
-- Sabine Hossenfelder
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