Marshfield, Mass. — It looks like a motorized lawn chair with oversized kite wings on tricycle wheels. But the latest fad in flying -- ''ultralight'' aircraft -- is really taking off.
''It's real excitement -- a beautiful feeling,'' says ''Sonny'' Oxner, the first person in Marshfield, Mass., to own and fly an ultralight. ''There's absolutely nothing around you. You look down from 500 feet and see everything. It has put the fun back in flying.''
Ultralights are a 1974 hybrid developed by Wisconsin engineer John Moody, who crossed a hang glider with a go-cart engine. He used the engine to climb, then turned it off and soared back to earth.
Today, most ''ultralightists'' leave the motor running for the entire flight. The design and engine now resemble a small plane with tiny seat and open cockpit. And it is now built with a more powerful engine.
But some critics warn that these motorcycles of the wild blue yonder need taming. No pilot's license is required to operate an ultralight, and while this is certainly part of the appeal, it's also a potential danger. Experts point out the importance of basic training and flight knowledge.
John Lasko, vice-president of sales for ultralight manufacturer Eipper Aircraft, takes his aircraft up 4,000 feet above California's coastline, shuts off the motor, and glides -- keeping one eye on the spectacular orange and red sunset, and the other looking for any neighboring Boeing 747s.
''I've raced motorcycles and I ski,'' Mr. Lasko says. ''But ultralight flying is the most thrilling sport I've ever done.''
''Ultralights are the rebirth of aviation,'' says Henry Ogrodzinski of the Experimental Aircraft Association. ''They have taken us back to the '20s and '30 s, when flying was simpler and more affordable.'' And with a per-hour fuel cost of $3, most of the estimated 10,000 to 12,000 owners of ultralights would probably agree.
With a primary purpose of recreation, this air force of ultralight enthusiasts dabbles in odd jobs like helping farmers spray crops or lifting photographer and camera for a little aerial shooting. They are also being tested for military use in the United States and overseas.
Mr. Oxner pilots his ''Weedhopper'' ultralight from a sand and gravel pit in Marshfield. One hundred feet of runway is more than enough for takeoff and landing. The fiery-red 28-foot wing span is wide enough to keep the craft aloft and cruising at 35 miles per hour. It lands at about 20 m.p.h. A dual rudder and elevator control system steers all 160 pounds.
Sonny bought his Weedhopper as a kit a year-and-a-half ago when there were about 25 ultralight manufacturers. Today there are three times that many, and as one ultralight official says, ''a new one popping up every week.'' Most models run from about $4,000 to $6,000.
The Federal Aviation Administration, in keeping with President Reagan's concern about overregulation, does not require operational certification for pilots or registration of the aircraft. However, regulations to be made public in the next two or three weeks will limit ultralights to:
* Weighing less than 254 pounds.
* Single-seat machines for solo flights.
* Carrying five gallons of fuel.
* A top speed of 55 knots at full power.
Paul Thomas, editor of ''Ultralight Flyer,'' says most fatal accidents are because of ''foolishness'' such as low aerobatics or exceeding the aircraft's limitations -- limitations that may not even be fully understood yet.
Unofficial estimates place the 1981 death toll from ultralight accidents at about 20. A recent Sport Aviation Association report said that about 50 percent of all reported ultralight mishaps are the result of pilot inexperience.
''And that's why we encourage ground school,'' says Mr. Ogrodzinski. ''Most accidents occur during the first few flights. There are regulations and information an ultralight pilot simply must know.''
The first time Sonny Oxner flew his ultralight, he was 50 feet in the air and the engine stopped dead.
''I didn't panic because I knew what to do,'' he says. ''But someone without proper training might not. People think they can just get in them and go. Well, you can't. You've got to have a knowledge of what your doing. These can be very dangerous without the basic knowledge of flying.''
Complaints of renegade ultralight pilots buzzing freeways and beaches have been made, but are rare. A few pilots have complained ultralights get in the way at airports.