The prospects for constructing a flying machine appeared to be much more favorable after the invention of the internal-combustion engine late in the 19th century. Several people before the Wrights had worked on the problem, and their contemporary Samuel P. Langley, secretary of the Smithsonian had reached the point late in 1903 of testing a powered, man-carrying airplane. In two widely publicized tests, in which the craft was launched by catapult from a houseboat on the Potomac his plane failed to fly and dived into the river. The progress of the Wrights was not publicly known, but they were feeling the competition.

Orville and Wilbur had begun work on their Flyer in the fall of 1902. Their plan was to install an engine and propellers on an improved version of the 1902 glider. They assumed that they could adapt an automobile engine and that the marine technical literature would provide the basis for designing efficient propellers.

After trying unsuccessfully to obtain a suitable engine from manufacturers they decided to design and build their own. This was not a particularly difficult decision for them, because they had already built a gasoline engine to power machinery in their bicycle shop. By March, 1903, they had what they sought: an adequate engine that developed 12 horsepower and weighed 15 pounds per horsepower. They had not tried to create an exceptional engine, and their result was far inferior to the engine built for Langley by his engineering assistant Charles Manly.

Then the brothers were surprised to find that the available propeller theory provided only one useful result: for a given output of power the best efficiency is obtained by passing the largest possible amount of air through the propeller. (This principle accounted for the rather large diameter of their two propellers: 8.5 feet.) The most significant consequence of their discovery was that they were forced to work out their own methods for designing propellers.

No theory existed for the detailed design of any kind of propeller. The Wrights employed their knowledge of airfoils to develop what later became known as blade-element theory, based on the idea that a small segment of a propeller blade is treated as a section of wing. Since the propeller is rotating and the airplane is moving, each such little wing is exposed to a different velocity and therefore generates different lift and drag forces. The thrust of the propeller and the power absorbed by it are found by summing the lift and drag contributions from all such blade elements.

The Wrights' analysis showed them how to design propellers with an efficiency of 70 percent or better at a time when the propellers of other investigators had an efficiency of less than 50 percent. With the brothers' superior propellers they were able to power the 1903 Flyer with their relatively modest engine. Their work on propeller design was their greatest analytical accomplishment, but for them its only value lay in the practical consequences. They never published their ideas and analyses, and modern propeller theory is traceable to other sources.

WING-WARPING SYSTEM devised by the Wright brothers as a means of executing turns is depicted. At the top is a front view of the wings; each of the eight bays between struts is fitted with truss wires, whereas only four of the bays at the rear have them. Thus the outer edges of the rear wings could be moved (bottom) by the wires attached to the hip cradle. The same system of wires also actuated the cranks that moved the vertical taiL In order to make a turn, say to the right, the pilot would warp the right wings up and the left wings down.

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