It's true that big, slow things are, all other things being equal, more efficient than small, fast things. Momentum is mv, but kinetic energy is 1/2 mv^2, so th

It's true that big, slow things are, all other things being equal, more efficient than small, fast things. Momentum is mv, but kinetic energy is 1/2 mv^2, so that squared term means it's cheaper to move a lot of mass(m) slowly than a little of it quickly. You can build big, slow ducted fans, but you do hit size limits much earlier than with free props because of the shroud. HOWEVER, efficiency is maximized when outlet velocity matches airspeed (or you have wasted momentum that will dissipate into heat, when your outlet stream mingles with the rest of the atmosphere). For your slow gliders, a small ducted fan with a high outlet velocity will indeed be quite inefficient. But this guy, traveling at hundreds of mph, needs a propulsion unit with a similarly high outlet velocity. A big, slow thing isn't going to cut it for that. A big, fast thing would, but would also blow their power budget. Within their design constraints, this is pretty close to the optimal solution.[1] There's also the small matter of free props being horrifically dangerous :) [1] They've helpfully given us enough information to work out some basic aerodynamics of their fans. At 7.5 Kw each, assuming 90% efficiency each of the motors and ESCs, and eyeballing the duct and hub diameters as 18cm and 8cm each respectively (could be off, here), we should expect an outlet velocity of about 230 mph. The guy reached an airspeed of nearly 190mph, or over 80% of the outlet velocity, so they aren't doing badly at all considering the drag.