Hi, I have just joined, and my 'electric' experience is mainly, but not entirely, confined to EDF. Here, for what it is worth, is what I have found from several years of trying to make them fly better and better. I have only used 90mm fans, and am not keen on 'foamies' because of their lack of durability, and no other reason.
MATCHING. The most important thing is to match the motor to the fan. compared to this the finer points of the fan unit design are insignificant. Several (expensive) mistakes with both fan choice and motor choice have shown this to be true.
FAN DESIGN. It does not matter which make of fan you use. All fans of reasonable quality give a very similar result. Given the correct choice of motor to match the power absorption of the fan, the differences between fans is very small. For example, I have used the Schubeler DS51 Dia 3phase, the Midifan, and the new Midifan Evo and motor for motor, the thrust difference is tiny. Watt for Watt, a couple of ounces, which is less than measurement error.
EFFICIENCY. I have seen here, and in other places, that an EDF, usually smaller than a propeller, is not as efficient. My measurements have shown that an EDF can be more efficient, not less. For example, I have measured, simply, by attaching a fishing spring balance to the tail, and the model standing on smooth concrete, on its wheels, the following -
OS 91 AX 2-stroke on its recommended 16x8 propeller, in an Astro Hog - 8.4 lbs.
Hacker B50-12XL in a standard Midifan (not the new Evo) in a Rob Bulk Grumman Panther (balsa kit) - also 8.4 lbs.
OS claim 2.8 bhp for the 91 AX. And this is, of course, output power, at its shaft. It is not 'input power'. 2.8 bhp is about 2150 Watts.
The input power to the fan motor was measured at about 2300 Watts.
So bearing in mind that with the EDF we can only measure the input power, not the power at the output shaft, and electric motor efficiency is at best 90%, the small EDF is more efficient than the large propeller, not less.
HEAT. Fan motors often get hot. People put finned heat sinks on the motor. This merely disguises the heat - it is still created. A heat sink may cool the motor marginally, but it is only removing the already created motor generated heat, not reducing it. Personally I ignore the motor manufacturers maximum rating. I just increase the cell count until the motor gets 'pretty warm', after several minutes of full power, but not too hot for a lengthy touch. This will be considerable less than the boiling point of water, so the magnets will not be degraded. Works fine, my motors are still 'as new'. And we don't run at full power all the time anyway.
SIZE and WING LOADING. I am beginning to think our usual models may be too small. My scale Panther is about 46 inch span and the wings are more like flippers than wings. My latest effort is the 'Black Horse' Viper Jet ARTF, at 55 inches span. It has large wings, and a much lower wing loading. As most of the drag of an aircraft is 'induced drag', caused by the creation of lift, and the angle of attack in level flight will be less than the Panther, it will be equally fast. Hope so, not completed yet.
FAN ROBUSTNESS. Some of these multi-bladed carbon fans look very delicate. I fly from a 70 years old deteriorating concrete runway, with concrete dust, tiny stones, and considerable wild horse droppings. I have observed erosion of the leading edges on my carbon fan, which has not occurred on the simple filled plastic ones. The plastic ones are more flexible, and may not be damaged by small stones either. Should I build a larger aircraft, I would probably use a BVM fan. Though rather heavy, they look very robust.
Please do not think I am trying to 'teach' anyone. Just reporting my experiences. Many will have been doing all this stuff for much longer than me, others less.