Ok lets assume you build a 3 foot tall incubator with a thermostat at the top an eggs half way down. Cold air would settle at the bottom an hot air at the top. If the air outside is 80*F you can assume the air at the bottom will be 80 to. If you want the eggs half way up to be 100 then your thermostat would have to be set at 120. Sorta like this.
We could add a fan to fix it but it wont. A fan at the top will move the air at the top but the egg shelf is a restriction an the air below the first egg shelf will not move an still form layers. So just a fan will only work if you have only one layer of eggs. But if you put a false back in it. Where there is two backs with a chamber between them. Then make a fan at the top blow air in to that chamber an drill holes at the bottom to let that air back out in to the main part of the incubator under the egg shelves you will then be forcing the air to circulate threw the egg shelves where it can't layer anywhere. You can also hide all the heater, thermostat, turner motor, wiring, linkages an whatever else in side that false back chamber. Leaving room to have shelves from top to bottom.
so then all the air in the bator will be the same temp. i got you now, so just mount the fan blowing into the false back, & drill holes in the bottom of the false back & the air will keep circulating & stay the same temp. right? thanks for your help!!!!!