by Shawn Carlson; ( from Scientific American, 010907)
A well-known parable warns against the dangers of letting excitement get the best of you. As the story goes, Daedalus, a brilliant Greek artisan, crafted two magnificent pairs of wings out of feathers and wax for himself and his brash young son, Icarus. Once airborne, Icarus became so enraptured by the thrill of flight that he ignored his father's warnings to stay close to the sea. When he soared too high, the sun melted his wings, and Icarus plummeted to his death. The parable has survived for 3,000 years, probably because it is so easy for us to put ourselves in Icarus's place. After all, who has not dreamed of flying like a bird?
For a lucky few, that dream is edging closer to reality. And although it may not be quite birdlike, human-powered flight has nonetheless arrived. Relying on ultralightweight yet incredibly strong space-age materials, modern-day pilot-powered craft fly with wingspans of 32 meters yet weigh only 34 kilograms. On small airfields all over the world, a handful of dedicated aeronautical engineers are eking ever better performance from their minimalist machines. Record flights have distanced over 115 kilometers (70 miles) and lasted as long as four hours. Though meager by the standards of commercial planes whose engines can deliver more than 10 million watts, these achievements are remarkable given that the engine-pilot can sustain only enough power to illuminate a few lightbulbs. This new generation of flying machines, moreover, has transcended the Icarus-like hubris that undermined earlier design and sometimes brought disaster on pilots.
Human-powered flight has been centuries in coming. All the earliest aeronautical innovators, including Leonardo da Vinci, focused exclusively on human power because no other source was available. When, in 1903, the Wright brothers proved the potential of internal-combustion engines in aviation with their success at Kitty Hawk, N.C., engineers flocked to the challenge of building machine-powered airplanes. The lure of human-powered flight suddenly lost its luster.
A few well-heeled individuals, however, refused to give up the dream. The decades after Kitty Hawk saw several cash prizes offered for human-powered aircraft that could achieve modest feats of range and aerodynamic control. In 1933 Polytechnische Gesellschaft, a group in Frankfurt, offered 5,000 marks for the first human-powered airplane that could fly around two markers 500 meters apart. They upped the ante to 10,000 marks two years later. The offer did catch the attention of seasoned designers, but the circumstances were not then ripe for success. Similar prizes were offered in the U.S.S.R. and Italy, but all went unclaimed.
Then, in 1959, a visionary British industrialist named Henry Kremer offered £5,000 for the first human-powered aircraft that could demonstrate the same degree of aerodynamic control as the early Wright fliers: trace a figure eight around two markers four fifths of a kilometer apart. This prize did lead to major advances, although it took another 18 years and a 10-fold increase in prize money before the necessary technology and engineering genius would come together.
They finally did in August 1977, when a 24-year-old cyclist, Bryan L. Allen, pedaled the revolutionary Gossamer Condor around the prescribed course and into history. Today the Condor is housed at the Smithsonian Institution's National Air and Space Museum, but to get there the Condor's design team, headed by famed technologist Paul B. MacCready, Jr., had to solve some vexing problems.
The first problem was power. The best athletes can deliver only about 400 watts for long periods. To fly on such little power, the Condor needed a wingspan of 29 meters, wider than a DC-9, yet it could weigh no more than a hang glider a third that size. Ailerons, movable flaps on the rear of the wings, proved too inconvenient to steer the plane. To solve the steering problem, the team rigged the wings to twist during turns (a trick also used by the Wright brothers) and mounted a small tiltable stabilizing wing a few meters in front of the pilot.
To enter a left turn, the pilot twisted the wings to increase lift on the left wing and decrease it on the right--the opposite of what is done with the ailerons on an ordinary airplane. But with the unusually shaped Gossamer Condor, practically just a flying wing, the greater drag on the left wing yawed the craft (that is, turned it sideways) to the left and simultaneously rolled the left wing down and the right wing up to bring the plane into a coordinated left turn. The small stabilizing wing in front acted like a hawk's tail feathers, forcing against the air to limit the degree of yaw.
Neither Kremer nor MacCready was through yet. Kremer knew that it was Louis Blériot's 1909 flight across the English Channel that ignited Europe's passion for planes. Hoping to generate similar excitement for human-powered aircraft, Kremer soon voiced his intention to sponsor a much bigger prize for the first pilot-powered airplane to duplicate Blériot's feat. This announcement set MacCready's team racing to develop a distance flier. The vehicle, dubbed the Gossamer Albatross, was a lean and elegant clone of the Condor; it incorporated advanced composite materials andimproved streamlining but no new ideas. MacCready's team developed the flier so rapidly that by the time Kremer's £100,000 competition officially opened, the Albatross had already been flying for six months. On June 12, 1979, Allen once again flew a MacCready aircraft into history. Battling a head wind that added an unanticipated hour to the flight, he powered his way across the 35-kilometer channel in 169 minutes.
Unfortunately, Allen's Channel crossing did not spark the renaissance in human-powered flight for which Kremer had hoped. So in 1983 Kremer offered another prize, this time for speed--£20,000 for the first flight to complete a triangular 1,500-meter course in under three minutes. The winner would have to average 32 kilometers per hour. This time the MacCready team's entry was trumped by Monarch B, the innovative creation of upstart students at the Massachusetts Institute of Technology.
More than £100,000 in Kremer prize money is still waiting to be won: a £50,000 purse for the first human-powered aircraft to fly a complicated marathon circuit of 40.5 kilometers; £10,000 for a human-powered seaplane; and £50,000 for an aircraft that can fly in minimal amounts of wind, instead of in the dead-still air usually required. These prizes continue to stimulate intense research. To date, nearly 100 human-powered aircraft have been built and flown all over the world.
The current distance record is held by Kanellos Kanellopoulos, who in 1988 nearly completed Icarus's mythic journey by flying in slightly less than four hours the 115 kilometers from the island of Crete to Santorini Island. He flew on average a mere five meters above the water. Like Icarus, Kanellopoulos also fell into the Aegean Sea, when a gust of wind snapped the craft's tail boom. He settled into the surf just 10 meters short of his destination.
Still, a new generation of adventurers is closing in on this record fast. Three hundred volunteer college students and 100 industry professionals worked this past summer in 19 integrated teams to assemble a human-powered plane called Raven that could be the most sophisticated aircraft of its type ever conceived. If things go well, sometime in the winter of 1998 Raven will obliterate Kanellopoulos's records by nearly 45 kilometers and over an hour of flight time.
Paul R. Illian and Heather A. Costantino of Boeing head the team of engineers who volunteered to design this high-tech wonder. Raven's load-bearing structures are precision-machined, high-strength carbon graphite. The skin and propeller are fabricated from a ridged carbon-graphite mat and foam sandwich. Although its wings span an awesome 35 meters and have an area of 33 square meters, Raven tips the scales at a slight 34 kilograms, equivalent to the weight of all the pillows carried by a Boeing 777. Its specially engineered autopilot will steer the rudder and elevators, its only control surfaces. Raven should cruise at an altitude of about six meters at better than 32 kph.
While Raven is going for glory, other aircraft are being built for purely scientific reasons, to better understand how vehicles fly at such extremely slow speeds (physicists prefer to say low Reynolds numbers, which quantify how fluids flow over objects). John McIntyre of the University of Cambridge is a member of a small but highly dedicated group of researchers who are systematically working out every aspect of the aerodynamics of these machines. A longtime veteran of human-powered-flight research, his current plane, Airglow, is instrumented to analyze subtle aspects of the craft's performance. McIntyre flies it every chance he gets. He insists that it is easy to understand why some people are so passionate about human-powered aircraft.
"This is a world of extremes," McIntyre comments. "Things have to be so optimized. We have an airplane that can be picked up in one hand, flies on the power needed to run a lightbulb and has a wingspan of a commercial aircraft."
Applications of such extreme vehicles have nearly arrived. Making use of the expertise gained on human-powered aircraft, MacCready's team has developed the unmanned and solar-powered Pathfinder. In July 1997 Pathfinder set a new altitude record for propeller-driven planes by reaching 21.8 kilometers (71,500 feet). The next solar-powered planes will extend Pathfinder's 30-meter wingspan to an astonishing 67 meters, larger than that of almost any other plane. These vehicles could stretch the altitude record to 30.5 kilometers (100,000 feet) and, at a somewhat lower altitude, serve as a kind of poor-man's satellite, spending months observing both the earth and the sky while roving the stratosphere on solar power by day and battery power by night.
Tantalizing as this possibility may be, 21st-century historians will most likely mark the legacy of human-powered flight more for what it gave us on the ground than in the air. While seeking ways of storing energy on board a human-powered aircraft--by means of a battery charged by the pilot's pedaling--MacCready's team gained insights into making efficient use of very limited battery power. Practical electric cars are one example of the resulting technology. AeroVironment, MacCready's company in Pasadena, Calif., developed the all-electric Impact (now marketed as the EV1) for General Motors in 1989. MacCready traces his company's success in this field in no small part to the experience his team gained while running after his fragile flying machines.
The lure of Icarus will no doubt continue inspiring our future engineers for generations to come. The challenge of edging ever closer to the absolute limits of human performance bridges theory to harsh reality like no classroom can, forcing our engineers to think in revolutionary, and not evolutionary, ways. By struggling to bring us closer to Icarus's ancient dream, these young technologists are honing the skills they need to make a better life for us all.
MAN-POWERED AIRCRAFT. Don Dwiggins. Tab Books, 1979.
GOSSAMER ODYSSEY: THE TRIUMPH OF HUMAN-POWERED FLIGHT. Morton Grosser. Houghton Mifflin, 1981.
HUMAN-POWERED FLIGHT. Mark Drela and John S. Langford in Scientific American, Vol. 253, No. 5, pages 144151; November 1985.
SHAWN CARLSON is founder and executive director of the Society for Amateur Scientists, a nonprofit organization that involves amateurs worldwide in scientific research. He received his Ph.D. in nuclear physics from the University of California, Los Angeles, in 1989 and is an adjunct professor of physics at San Diego State University. Carlson writes the Amateur Scientist column for this magazine and is the creator and principal author of Physics: The Core (Harcourt Brace, 1997) a college-level CD-ROM-based textbook.