I can't find the 'right' peer reviewed sciencey article that would keep the likes of peterC005 happy, but I've always been a big fan of empirical evidence over manufactured science.
I know its only a couple of pictures and there's no scale but they look pretty big and that air looks pretty turbulent a long way from the bird chopper itself. Without going out on too much of limb, I'd be prepared to avoid the air behind those fans by a lot more than 5 blade diameters.
