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In response to my question about quieter helicopters, KeithS mentioned the Eurocopter Blue Edge helicopter blade design.

According to an article I found on them, there appears to be two components at the very least: a boomerang style rotor tip (Blue Edge) and piezoelectric active flaps (Blue Pulse) on the trailing edges.

How do these (as well as other changes) affect the noise level (i.e., what is the aeronautic principle behind their design)? Also, the sound reduction itself seems relatively modest (3-4 decibels), but is the quality or tone of the sound perceptually different such that it seems much quieter?

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EC155 helicopter. Compared with a conventional blade, the Blue Edge has a tip with both forward and rearward sweep. This gives the blade more time to interact with the vortex 'shed' by the rotor, reducing the characteristic impulsive (slap-slap) blade-vortex interaction noise generated by a helicopter. The active trailing edge flap harmonises the frequency thus making it quieter.

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