Helicopters have one of the most recognizable sounds in aviation. The signature "whop whop" or deep thumping sound announces its arrival long before it appears in the sky. It's a sound so iconic, it's ...

The noise is barely audible at first, just a low rumble somewhere in the distance. Within seconds it rises to a growl, then swells to a roar, a predator lurking just overhead. The sound makes windows ...