Helicopters or dual/quadcopters stop rising after reaching some height from the ground. What causes that? And what one should do if he want to prevent this, if he want the helicopter to keep rising up?
In the most simplified view, a helicopter is basically a really powerful fan, which is something that pushes air (and therefore feels a force in the opposite direction). When you get high enough, the air starts thinnning, so there is less to push, so that force becomes too low to hold it up. Imagine the limiting case of outer space: nothing would happen.
There's nothing you can really do to combat this, except run your motors faster and faster as the air gets thinner. Eventually you have to rely on things like rockets that don't need an atmosphere to gain momentum.