The physiology of human-powered flight.

The physiology of human-powered flight 

When bicyclist-cum-pilot Kanellos Kanellopoulos pedaled an aircraft 74 miles across the Aegean Sea in April 1988, breaking the previous record for self-powered time aloft, the engineers who designed the craft garnered a hefty dose of praise (SN: 4/30/88, p.277).

Less publicized, however, was a stunning success story about the application of theoretical physiology to a practical task. Out of the limelight, a team of metabolic mechanics put in long hours figuring how to keep Kanellopoulos' human-body engine perfectly fueled and tuned during the strenuous four-hour trip.

The so-called Daedalus 88 flight provided a wealth of information about the physiological adjustments required for human-powered flight, reports Ethan R. Nadel, a physiologist at Yale University who helped perform the computations that went into keeping the pilot airborne. Most critical was the need to sustain a constant energy output that would keep the plane moving at the 15- to 17-mph airspeed required to stay at altitude. With the plane designed to fly only 12 to 15 feet above the sea, even a brief loss of energy could spell disaster.

Using measurements taken from ground-based bicyclists, Nadel and his colleagues calculated the amount of adenosine triphosphate--the ultimate energy source in skeletal muscle--required to produce the 3 to 3.2 watts of mechanical output per kilogram of pilot weight that the plane was designed to use. Burning that much energy, they figured, would generate enough heat to raise the pilot's body temperature about 1 [degree]C every five minutes unless the heat was dissipated. The human body is a water-cooled engine, so in order to radiate that heat it was necessary to keep the pilot properly hydrated by replacing the estimated 900 milliliters of water he'd lose every hour from sweat and respiration, the physiologists predicted.

Moreover, they calculated that the pilot would run out of glycogen--the stored form of glucose in the body--after three hours, necessitating in-flight glucose supplements. From estimates that the flight would deplete 1.5 grams of the pilot's glucose per minute, they had Kanellopoulos consume 250 milliliters of a 9 percent glucose solution every 15 minutes. They added to this beverage a carefully balanced salt solution to increase fluid retention and maintain plasma volume, improving cardiac output.

In the end, Nadel says, although the flight fell about 10 meters short of its intended destination, it was an unqualified success from the physiological point of view. With the pilot's heart rate never exceeding a healthy 135 beats per minute durint the 3-hour, 54-minute effort, the experiment confirmed the practical value of estimates the researchers had derived from less lofty experiments.