If U look at the FAR 91.119 Minimum Safe Altitudes
Code of Federal Regulations
Sec. 91.119
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Part 91 GENERAL OPERATING AND FLIGHT RULES
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[TD="width: 50%"]Subpart B--Flight Rules[/TD]
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General
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Sec. 91.119
Minimum safe altitudes: General.
Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a)
Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b)
Over congested areas. Over any congested area of a city, town, or settlement,
or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.
(c)
Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.
[ (d) Helicopters, powered parachutes, and weight-shift-control aircraft. If the operation is conducted without hazard to persons or property on the surface--
(1) A helicopter may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section, provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA; and
(2) A powered parachute or weight-shift-control aircraft may be operated at less than the minimums prescribed in paragraph (c) of this section.
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I asked the Phoenix Flight Standards District Offices (FSDO) What constitutes an "open air assembly of persons" I was told it could be as few as a small gathering at a picnic. This means that if your R/C Field is occupied Full Scale air craft are required to
maintain an altitude of 1000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.