This was my first UAV.
It was the simplest, cheapest UAV possible: a magnet. It couldn't fly very high, it couldn't follow waypoints, it couldn't carry much of a payload, it was even more unstable than a helicopter, but it could fly as long as PG&E didn't have a rolling blackout.
The guidance system was as simple as can be. Just pulse the throttle when too low or high. Scale the duty cycle based on the time between boundary conditions for lead compensation. A side magnet could move it sideways.
Later on, discovered wings. Wings had huge advantages over a magnet.
But, after the spare parts consumed $500 of rent doing nothing, decided there would be no other use for them & it was time to rebuild the magnet UAV. The hall effect sensor no longer works. It'll take some time to track one of those down.