IN THE LATE 1930s Dr. Walter Good of Bethesda was among the first to build and fly a gasoline engine model airplane guided by radio signals transmitted from the ground.

Today there are an estimated 250,000 RC enthusiasts building, flying, sailing, and racing model planes, helicopters, sailboats, hydroplanes, cars and even snowmobiles. The RC hobby is a $200-million-a-year business.

The early RC model airplane flights startled onlookers by demonstration that one indeed could control an unmanned airplane with no strings attached.

Today the internationally recognized altitude record for RC piston-powered models is 26,919 feet, the speed record is 213 m.p.h. and the pending mark for closed-course non stop distance is 424 miles. A radio controlled glider stayed aloft for over 25 hours; a 5 1/2-pound RC model flew, with stops, from Kitty Hawk, N.C., to Oceanside, Cal., directed from the front seat of a car.

But most RC aeromodelers aren't involved in world record attempts. John Worth, exective director of the Academy of Model Aeronautics, said the vast majority fly for relaxation or in local club competition.

There are five categories of competition: pattern (acrobatics), pylon racing, soaring (gilders), helicopters, and scale. In all five the rules encourage competitors to build miniatures of full-sized planes, in scale competition that become a precise and exacting art.

The RC pattern world championships are scheduled for June 29-July 4 in Springfield, Ohio. The National Model Airplane Championships, a dawn-to-dusk frenzy of both RC and non-RC competition, will be held at Riverside, Cal., Aug. 6-14.

In addition, a $45,000 prize will be offered to the top flyer at a pattern invitational meet in October jointly sponsored by a Las Vegas hotel and Model Airplane News.

Costs involved in RC aeromodelling depend on the type of plane and the number of channels on the radio. Some modelers design and build their own planes from scratch, but more than adequate kits are available that anyone can put together.

An RC glider kit and a two- or three-channel radio cost about $160. A kit for a high-powered pylon racing or a pattern (acrobatics) plane and a five-orsix-channel radio may cost $600-$700. Extra radio channels are used to turn wheels while taxiing, apply wheel barkes or even drop miniature parachutes.

Early radio control technology was crude, compared to today's. The first RC control systems guided models right or left and up or down, with no control over the degree of change. Today precise steering is possible by manupulating sensitive control sticks on the hand-held transmitter box.

The transmitter sends out inaudible radio signals that are picked up by the model's antenna and decoded by the receiver in the model. The receiver adjusts wheels, wing flaps, sail, rudder and throttle exactly as called for.

Advances in RC technology have had important applications in areas other than the hobby industry. Defense-related industries make miniature prototypes of aircraft and test-fly them at a fraction of the cost of full-scale planes.

RC technology extends even to model yachts. A three-channel radio is normally used, with one channel controlling the mainsail, one the rudder and a third the jib. In 1970, the American Model Yachting Association was formed and it now has over 1,300 members.

"We have come a long way in a short time," said past AMYA President Bob Harris of Springfield, Va. "We've only been in operation for seven years and we're already organizing an international regatta in San Diego in the summer of 1978."

Competition in the Washington area is sponsored by the Potomac RC Sailing Association, which uses Lake Fairfax and the Pentagon Yacht Basin, among other Northern Virginia locations. Yachts are divided into competition classes based on length, weight and sail size.

Model yachts are almost impossible to damage while sailing; they're noiseless and the only energy they use is rechargeable batteries for the radio equipment.

RC car racing also is popular in the area. The Washington RC Racing Association, formed in 1971, sponsors races from March to November on a track on a Tysons Corner parking lot. These races use one-eighth scale cars about 22 inches long, weighing five or six pounds and powered by .21 cubic inch engines. Cost for a car kit, radio, engine and accessories runs from $250 to $500.

The Washington club is a member of the national organization, ROAR (Radio Operated Auto Racing, Inc.), which has about 1,000 members and has sponsored championships every year since 1970.

Competitive racing - road racing, oval racing and drag racing, is divided into classes based on driver competency, body style, engine size, and scale of the car. As with aeromodeling and yachting, the rules encourage actual models of full-sized cars.

RC power boats like cars, are divided into classes based on size and design. Hulls are marine plywood or fibreglass; power is either electric motor or internal combustion engine. The fastest are model hydroplanes which can easily do 70 m.p.h.

RC power boats are operated locally at the Pentagon Yacht Basin. National championships will be held in Colorado this year, although no date has been set.