I'm not so certain about that. I'm no expert on ducted fans but I think there is somthing else happening there. Could you take the fan blades and attach them to the front of a plane and spin it as fast as it would in a duct and have the same thrust? I don't think so but then again, I'm not an expert in that area.
ORIGINAL: Trisquire
Turning an 11-7 at 15,000 RPM is certainly no less efficient than turning the ducted fan rotor blades on a jet. I don't know if you've seen any electric ducted fans lately, but they are not lacking in performance.
Tom