The high-pitched whine grew steadily louder until, at last, student Frank Scarabino, pedaling furiously, hurtled past spectators at 21 miles per hour - about nine feet off the ground.
Looking more like an oversize child's model than a real airplane, the graceful 61-foot, gull-like wing - made of aluminum tubes, styrofoam, and a transparent Mylar skin - carried the busy pilot in a slender, aerodynamic envelope.
Dubbed Monarch, the craft weighs in at a fragile 74 pounds. That is just light enough for a 160-pound human to use pedal power (and batteries charged by the same means) to fly about a mile.
Thursday morning's demonstration flight, however, only hints at Monarch's full capabilities.
Last Friday at dawn, Mr. Scarabino, wearing a T-shirt, black bicycling shorts , and shoes, boarded the craft for the real test.
A team of students from Massachusetts Institute of Technology (MIT), who designed and built Monarch, hoped the secret flight would win a $30,000 prize from British industrialist Henry Kremer. Mr. Kremer has offered the prize to the first person who, under his own power, flies a triangular 1,500-meter course in less than three minutes.
The Monarch crossed the finish line at Hanscom Field here with a time of 2 minutes and 49 seconds. Witnesses for the Royal Aeronautical Society of London have sent written testimony to Kremer.
Part of the reason for secrecy was the intense competition to break the mark before Californian Paul MacCready, who tried to break it last fall. Mr. MacCready was disqualified because of a rules violation.
Though the low-speed, very-low-altitude approach of human-powered flight seems to have brought aviation back to its beginnings, the aircraft is actually a hybrid of digital electronics and space-age materials like the Mylar film.
''We've been to the moon before we've been able to do this kind of stuff,'' says MIT Assistant Prof. Antonio Elias.
Compared with previous human-powered flights, the Monarch travels about twice as fast as the Gossamer Albatross, whose pilot, Bryan Allen, crossed the English Channel in less than three hours in June 1979.
''This is a new concept in man-powered aircraft, using energy storage which is, in this case, electric,'' says team member John Langford.
The Monarch gets a rolling start on two small wheels, one powered by the pedaling pilot. Thus, the plane is like a bicycle for the first 30 feet or so. When the plane is traveling 5 to 8 m.p.h., the pilot switches on the electric motor to boost power to the propeller.