OK .. how do you determine what size of EDF and motor combo for a particular plane. I know that in the prop world ... approximately 100 watts for each pound of airplane weight. There must be some rule of thumb to size the EDF to the plane .. Help

Start by deciding what you want the plane to be capable of. Will it be bungee launched, hand launched or use landing gear.

Once you've decided on that, make an estimate of the ready to fly weight, including the fan and battery.

Then decide if you want vertical performance. If so, the thrust must be greater than the weight of the plane.

So if the AUW of your plane is 5 lbs and you want true vertical performance then the fan must produce at least 5.5 lbs of thrust. With that value in mind, your next task is to find a suitable fan/motor/ESC/battery combination

Or you can take the easy way out and use watts/lb. For good "jet" like performance figure on 175 watts.lb. While the resulting power/weight ratio may not permit true vertical, it will provide ample power to take off from a smooth runway.

The other proviso is that a larger diameter fan gets you more thrust per watt... so if your design is such that it can accommodate a bigger fan, and you value thrust over top speed... go that route.

200 to 500 watts per pound in 70mm single, twin and 90mm single, planes 2.5 to 6 pounds.
I use 40-50C packs. You don't have to use WOT all the time, but it's nice to have.
My Rafale 70mm twin is 5.25 pounds and 2.25 kilowatts. 428 watts per pound.
My T-50 was 750 watts for roughly 3 pounds, but I've increased it to 1200 watts, for Speed.
The Rafe is fiberglass, the T-50 foam. I'm not going to be doing much maneuvering with the T-50, she'll break apart. Also the Rafe, which I have not flown, is set up for speed. At 2.25 KW and a 4.0 aH 14.8 volt battery (40-50C) the flight time Wide Open is 1.6 minutes at the outside.

Decide on a good fan unit. Decide on an RPM for the fan, the WeMoTec is 45,000 max. Then you can get the motor watts/kv rating and battery voltage.

The decision for the fan is based on the available thrust, and you have to scour the web for the figures, from YouTube, Extreme RC.au and Efflux RC etc.
www.pw-rc.com ( Philip Workshop ) is a good source for data too.

The available thrust from a given fan unit, will compare to the weight of the plane. Higher weight plane, and wing loading, the greater thrust to weight ratio.
Remember that scale jets are high wing loading, and may be a handfull for a beginner. So, foamies, with light wing loading a easier at first. EJF has a trainer too. 70mm. www.ejf.com