Originally Posted by foamflyer
Thanks clausxpf, I did start with the duct, and built it so that next I can build the plane for it. It resembles one of the early jets where the motor is beneath the fuselage. So it's better to have a longer intake tube than exhaust tube? And the tail cone should be about 80% the diameter of the fan itself? I'll try it and see. Thanks.
Well, I don't know whether starting with the duct is such a good idea, unless it's already sized according to the requirements of the fan. The fan, or rather its annulus (FSA) is usually the starting point.
I have attached a sketch which shows the "ideal" ratios in respect of the annulus for the various duct diameters (or rather area ratios) at the relevant positions of the ducting. These are not figures cast in stone (as Claus quoted) but can be deviated from up to a certain extent, without sacrificing performance. It depends very much on the fan unit you will use.
You also have to be carefully with the indicated lengths. They are applicable for the WM600 (which I have measured), but I would reduce the length to less than half for the cheaper (and less powerful) fan units on the market; they just don't like the additional drag of long ducting.
Have fun with fans