To validate the results of his research, which had been questioned in some quarters, Langley started mounting stuffed birds, obtained from the Smithsonian’s vast archives, to the whirling table and measuring their lift and drift. Often there was not enough lift to raise the carcass.

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Between 1887 and 1891, Langley’s staff constructed more than 100 models powered by twisted strands of rubber. None proved capable of staying airborne for more than six seconds. At that juncture, the team investigated every known alternative- electric motors and batteries, compressed-air motors, hot-water motors and even flywheels. All were too heavy. While carbonic acid (H2CO3) looked promising as a fuel, nagging technical problems dampened enthusiasm for that option. Germany’s Nicholas Otto had recently invented an internal combustion engine, but early examples were big, heavy and underpowered.

Lightness, strength and power were prerequisites for successful Aerodromes, and the steam engine seemed to be Langley’s last hope. Over the next five years, a tremendous effort was expended to optimize the delicate steam power plant. By 1896, the system consisted of an engine, burner, boiler coils, pump, fuel reservoir and associated delivery tubes and fittings. One example, which produced one brake horsepower, weighed only 7 pounds.

Langley, true to form, avoided theoretical considerations and resorted to empirical methods for determining the airframe design. Large rubber-band-powered test models (Nos. 30 and 31) were used to settle the wing configuration issue. Today those two models would be considered conventional-a monoplane and a biplane, each equipped with a Cayley-type cruciform tail. Yet a puzzling question remains: How did the final tandem-wing arrangement emerge from those test models? Whatever the reason, all Aerodrome models sported tandem wings.

Work began on the first Aerodrome in November 1891. (Langley’s numbering system for identifying the Aerodrome models gets confusing-his first model was No. 0, the second No. 1, the third No. 2, etc. Later, No. 4-the 5th model-was modified, becoming No. 6.) According to Langley, Aerodrome No. 0 had ‘a single pair of large wings containing about 50 square feet, and a smaller one in the rear about half as much, or in all some 75 feet of sustaining surface, for a weight which…would not exceed 25 pounds.’ Six months later, that first attempt at a working model was abandoned as hopelessly overweight. Three new Aerodromes (Nos. 1, 2 and 3) were built between June and November 1892. Different engines, burners, boilers, fuels, working fluids and a variety of construction materials were tried in the Aerodromes, all in an effort to save weight.

Determining the ideal size of the Aerodrome was an exercise in trial and error. The second prototype (No. 1), which was much smaller than its predecessor, was designed to operate on compressed air or H2CO3 acting on the piston of a modified steam engine. Its total supporting surface was only 6.5 square feet, but the engine was weak-producing only a fraction of the power required for flight.

The third model (No. 2) was steam-powered. It had a lifting area of 50 square feet and was larger than No. 1 but smaller than No. 0. It also turned out to be overweight and underpowered.

The fourth Aerodrome (No. 3), about the size of No. 2, showed the best power-to-weight ratio thus far, but Langley was not happy. ‘There was…,’ Langley said,’so much that was unsatisfactory about it, that it was deemed best to proceed to another construction before an actual trial was made in the field.’

Design and construction of the fifth Aerodrome (No. 4) began in December 1892. Its relatively small wings totaled 14 square feet, and the weight was a modest 10 pounds. Its power plant and propulsion system lifted 40 percent of the Aerodrome’s weight during static tests-the best performance yet.

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