By Ed Waldrep
FSA (Fan Swept Area) is the area of the opening where the air goes through the fan. In other words, find the area of the entire face of the fan, the subract the area of the motor/spinner/hub, the result is the FSA.
Here's an example
WeMoTec Mini fan 480
Fan Shroud ID 2.715"
Motor tube OD 1.27"
Max Area 5.79 sq. in.
Motor tube area 1.27 sq. in.
Fan swept area 4.52 sq. in[U].
Tailcone diam. 100% 2.4"
Tailcone diam. 95% 2.28"
Tailcone diam. 90% 2.16"
The inlets may look quite small but if you measure and calculate the area you may find plenty of area. 100% of the FSA is the goal, a bit under you'll be ok, a bit larger is OK. You can go way above that and possibly improve static thrust, but too large and the scale looks begin to suffer (look at the huge nacelles on the GWS Me 262 and you'll see what I mean, they're way oversized) and there's increased drag at higher speeds because too much air is going in.
For exhaust aim for about 85 to 90 percent of the FSA.
Another thing to consider is the fan location. For most fighter type edfs, with the battery in the nose under a hatch, the fan needs to be placed with it's face toward the trailing edge of the wing. A fan access hatch on the bottom of the airplane usually looks better than one on the top.
Also, the battery mounting area up front should be made as long as possible to allow room for different sizes, lengths, and weights of batteries to be used. Some users will want to upgrade the power and making a small opening for only one size of battery that can't be moved makes upgrading difficult, a heavier battery would cause problems with balance.
Another factor is inlet ducting....it should be kept far apart until mid wing, then it should come together to meet at the fan. The reasoning is this will allow room between the ducting for a battery to slide farther back to achieve proper balance. A stock size battery may not need to go back that far, but with a power upgrade a heavier battery may be needed. However, you still need to make room for retracts on the outside, so there must be some room between the outside of the fuselage and the ducting, so back from the inlets in the front the ducting can head toward the center of the airplane but not at a really sharp angle, then once you get past the rearmost end of a battery the ducting can make another turn toward the center of the airplane and join in front of the fan. Avoiding sharp turns is important, make gradual turns if possible. With molded foam this is easily achievable.