Most small quadcopters use rigid rotors with some fixed pitch. In principle, I can imagine it might be possible for such a rigid-prop quadcopter to hover upside-down, but that apparently requires reversing the direction of rotation of all 4 motors.
(This is very different from the way some "standard" single-rotor model helicopters can hover upside-down by continuing to spin that rotor in "the same direction", but moving the swash plate to give negative blade pitch).
Is it possible for a rigid-prop quadcopter to hover upside-down?
When I build a quadcopter so it can switch from flying upright to flying upside-down and back again in mid-flight, what do I do differently than a normal quadcopter designed to always fly right-side-up?