For all the government conspiracy militia nuts out there, I’ve got some good news and some bad news. The good news is that there is no such thing as silent, stealth black helicopters. The bad news is that, thanks to Eurocopter’s noise-canceling Blue Edge rotor blades, there soon will be.
The extremely loud noise made by helicopter blades results primarily from the blades chopping through eddies in their own wakes, a phenomenon known as blade-vortex interaction. By changing the shape of the rotor blades, Eurocopter manages to pair down the blade-vortex interaction so thoroughly that the sound only reaches the whisper volume of 3 or 4 decibels.
To hear just how drastic the difference between a normal rotor and the Blue Edge rotor is, check out this video.