Careful when flying under a surface. When you get close, your quadcopter will get sucked up to it. I'm not an aeronautical engineer, but I think what happens is the rotors get much more efficient when the surface blocks the vortex doughnut around the rotor.
Then, Phoebe got a propeller tip stuck in a crack and dangled helplessly blinking her lights, "help! help! help!" while I went to find a ladder to retrieve her.
And, of course, I forgot to stop the video so now I have a big video file that won't play because I turned off the drone before stopping it.
The opposite of Ground Effect? Must be Negative Ground Effect! The closer you get the a ceiling, the less air that's available and so it has an effect of a vacuum on the ceiling. Sounds good, but probably isn't right too. I looked up what the opposite of ground effect was, there was no answer, so I made one up.
If I want a drone (quadcopter) to be flying always under a surface and close to this surface (i.e: under a table 5cm away from the surface), which are the disadvantages this would have in the lift
^^ they don't really point to a term but definitely seen the same thing and makes sense it creates negative air pressure or a partial vacuum above the quad and sucks it up to the ceiling to fill the lower pressure region it makes above itself (only air flowing in from the sides to fill in vs open air have all the air above filling in any vacuum/negative pressure).