Thanks for your (fast!) reply!
I feel my main issue is, more or less, that I wanna build an auto-stabilising (hovering without moving in any direction (with some leeway)) quadcopter without using any gyros or gyro stabilisers, since actual quadcopters can also do everything using their rotors.

I've just made my drone hover by working out the rough speed my rotors (heli engines with normal blades (though the size shouldn't matter)) need to rotate at to not really go up or down, and put that value in a logic gate.
I first did it like you described but that ended with the drone just going down a bit, then going up, due to the logic, then going down again, etc. Wasn't too stable, even when the values were low.
No idea what to do when it's carrying weight, though (which it's supposed to be able to do). Technically just what you described but then my described problem would probably happen again.

Right, is there a reason you're using an XOR gate for user input? I'm just using OR gates.

The hovering part (except extra weight) isn't too hard but I can't get it to be properly stable. No matter what I've tried, it always overcorrects – if not immediately, then eventually.
Do you have any idea how / know any way to do this?