hi clint, for airplane normal mode what do you mean?
the mode1 or mode2 of sticks configuration? or the kind of model (helicopter or airplane) that the TX is setted for?
8FG is a great TX, you can widely configure it, I have to admit that never was minding to set different levels of response for sticks (not for multirotor)
but basicly the function of the idle up is exactly what you say, in SR helicopters, especially the electric ones, some people sets idleup 0 just to make the blades reach the RPM (and avoid to breack gears) then they switch to idle 1 for flyght and then to idle 2 for acrobatic
in 8FG you can change assignemt of switches as you like, probably you just need to take a look at TX documentation
hope this helps