... But now, I find UAVs, particularly the commercial UAV quadcopters that online stores like Amazon uses to deliver products to their customer's house, very useful and cool.
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Also, do you need any kind of pilot certificate and medical to fly a UAV, especially quadcopter UAVs?
Amazon does NOT use UAV's to deliver. It was a publicity stunt. They are looking into it for the future they say, but the technology is not up to snuff.
I do actually mess around a little with robotic multicopters. Take a look at
this site if you're interested, it's pretty cool stuff. However, a 4-5 lbs multicopter only has an endurance of 5-6 minutes right now, and that's with minimal load. That's certainly long enough for stupid people to get themselves (or others) injured and seriously hurt, and long enough to do the whole "careless and reckless" thing in criminal court (or liable in civil court), but we're still way far away from aircraft that can deliver goods. THAT will be the point at which these things really could accidentally kill people. That's why the FAA only regulates UAV's above a certain weight, 40 lbs or something?
Otherwise, the FAA only has guidelines that apply to Radio-controlled model aircraft. These are neither prohibited, nor protective, e.g. you can't be fined by the FAA, but complying with them may not prevent another agency from charging you with a civil or criminal violation. Those guidelines are pretty common sense:
Don't fly near airports or controlled airspace
Don't fly above 400 ft
Don't fly out of line-of-sight
Don't fly near people or buildings
etc.
For reference, with robotic or autonomous aircraft, I keep hands on the controls in LOS for safety. If I ever use remote cameras, I'll need a spotter to keep LOS while I'm looking at the monitor.
They're another fun and relatively inexpensive way to stay in touch with aviation. I just hope people don't keep doing stupid things that make will make it no longer fun...