An experimental, single-engine airplane, designed for enhanced safety, crashed yesterday during a demonstration flight over the Chesapeake Bay, killing the aircraft's designer and two employes of the National Aeronautics and Space Administration's Langley Research Center.

The propeller-driven, four-seater plane, outfitted with an unusual wing near its nose, plunged into the bay shortly before 11:25 a.m. near Gwynn Island, below the mouth of the Rappahannock River about 30 miles north of Hampton, Va. A NASA spokesman said the cause of the crash was not known.

Among those killed was George Mead, an aeronautical engineer in his early 30s who designed the novel general aviation craft for Piper Advanced Technology Inc., a Kansas company established last year to build, test and market the plane. The firm, based in Wichita, was started by a son of the founder of the well-known Piper Aircraft Corp. but has no corporate relationship to the older enterprise, a company official said.

The others who died in the crash were identified as Wendell Kelley, 41, of Newport News, Va., a NASA research pilot, and Paul F. Coy, 23, of Hampton, Va., a NASA engineer. The two men were aboard to "see how the plane handled," Langley spokesman Maurice Parker said.

A spokesman for the National Transportation Safety Board, which is expected to start investigating the crash today, said initial reports indicated the plane may have broken up in flight.

Dennis Harms, one of the craft's builders, said in a telephone interview from Wichita that the plane destroyed yesterday was the only existing prototype, designated PAT-1. The company has not yet decided whether to rebuild the aircraft, estimated to cost $75,000 to $100,000, and resume testing, he said.

The small wing near the plane's nose, known as a canard, was designed as a safety feature, Harms said. It was intended to prevent stall spin, an often fatal aviation phenomenon that may occur when a plane climbs too steeply, loses its upward lifting force and pitches back, spinning. There was no indication of stall spin in yesterday's accident.

Harms said the 160 horsepower plane -- previously flown, chiefly by Mead, for 70 to 80 hours -- was also designed for higher speeds than normally achieved by light planes without loss of fuel efficiency or cabin space. Its cruising speed, he said, was 165 miles an hour.

NASA researchers were primarily interested in the aircraft's canard feature, a subject of study for possible use in passenger and military planes, officials said.

NASA spokesman Parker said the plane took off from Langley at 10:40 a.m. yesterday on its second day of demonstration flights for officials of the aeronautics agency. No distress call was received from the craft, whose crash was reported to a county sheriff in eastern Virginia. The Coast Guard recovered the three bodies and towed the plane ashore in two pieces, he said.