Most aircraft of this type have a propeller diameter of 2m, hence area =
3.141m2.
Using this as a ballpark figure.
Take two ducted fans at diameter 0.6m each, this gives an area of 2(0.28) =
0.56m2.
Therefore the disc loading with two ducted fans of this size is HUGE.
*
Working backwards from our ballpark of 3.141m2 gives us a diameter for EACH
of two ducted fans
of 1.414m, giving a total diameter for both fans placed side by side of
2.828m which
is greater than our design width.
*
Also, tip losses are proportional to circumference and hence the two fan
design is less
efficient from this viewpoint.
*
JW