The efficiency of a propeller depends on the ratio thrust/area, the smaller this ratio the higher the efficiency. (See 11.7.3 at https://web.mit.edu/16.unified/

The efficiency of a propeller depends on the ratio thrust/area, the smaller this ratio the higher the efficiency. (See 11.7.3 at https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node86.html https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes...) The issue with the ducted fan is that the propeller size is constrained and the weight is higher due to the mechanical structure. A small quadcopter has twice or more propeller area for a portion of the weight.