The most promising theory is that the "stealth hawk" used actuated micro flaps on its blades' trailing edges to reduce noise and vibrations. I would put money o

The most promising theory is that the "stealth hawk" used actuated micro flaps on its blades' trailing edges to reduce noise and vibrations. I would put money on this being true. Flapped blades have been studied in academia for over a decade, it's definitely an engineering problem now and has clear benefits. http://www.helijapan.org/pdf/general/general05/Friedmann/Japan-05VF-notanimated_PART-1.pdf http://www.helijapan.org/pdf/general/general05/Friedmann/Jap...