ORIGINAL: mark_d

One thing about fans that is different from prop planes is that you can just change the prop to a different pitch or diameter to alter the amp draw, but you can't with an EDF. Usually you need to start with the model and fan size. You can determine the best batteries - size and cell count- to fit the airframe. The overall weight can be roughtly calculated from that point and the wattage per unit weight from that. You have to research to find out the Kv the motor will need to hit the wattage output for the fan you intend to use. The weight of the motor will also give you a good indication of the maximum power it will accept in before releasing the magic smoke out. You may have to reitterate a few times to get the final design but it usually works for me.

This fancalc site has a few of the higher end fans listed: http://www.s4a.ch/eflight/fancalc_e.htm