A UAV and a Remote Control Helicopter/Aircraft are to totally different arena's. I know a lot of people in the US like to think they are the same thing, and want them all banned. R/C aircraft are smaller, and are used for line of sight flying. A UAV can be controlled from a satellite, or extremely powerful radio station over long distances nearly anywhere in the World.. I would not venture sending my Trex 450 to go battle the Taliban, or to go across the river to bother people. I have a fairly good Futaba radio and at best I may get 2-3000ft (about half a mile or so) of range out of it before the signal is to weak to control the aircraft.
To the best of my knowledge there are no restrictions as far as weight goes for RC aircraft unless you are talking about taking a 1:1 scale bird, and make it remote control. The only restrictions on my little heli are I obviously cannot use it to bother others, and it must be flown in a responsible manner... Other than that it is open season.
You can get gyro's and such for even the single rotor ones, so you do not need to go to a quadrocopter unless you want to. I have a gyro on my Trex 450 that will hold/track a particular heading. I could add more, but the gyro's are pretty expensive. Not to mention I do not have much room to mount them on my heli. It was hard enough for me to find a place to mount the radio reciever box on it, and run all the servo wires through the frame without putting them in any gear mesh.
