RE: flying at an airport
As far as I know there is no "ruling" banning RC aircraft within 3 mi of any airport, but there may be local laws concerning specific fields. Many airports are owned by the cities or counties they serve, and make up their own rules as necessary. The feds don't usually get involved. As for most airports having radar, well, that's not true. Most airports in the US don't even have control towers, let alone radar. I'm the airport FBO owner here at Oroville Municipal in Oroville, CA and we not only allow RC and control line flying, we encourage it. A really great land/water flying site is operated by the Oroville Air Corps just about two miles away. No problems ever. About 20 miles north of here is Chico muni, and they have a tower, but no radar. They have an RC field right outside the airport fence, in clear view of the control tower. They certainly have altitude and safety rules, but have peacefully coexisted this way for years.
Hag