THE STORY SO FAR: Wilbur and Orville are ready to add an engine and propellers to a glider, to build a true flying machine. After all the problems they have tackled, this should be the easy part.

Even before she climbed the stairs to her brothers’ workshop, Katharine could hear the quarreling. She knew that her brothers often got into scraps over how to solve a problem. She also knew that they considered their arguments a way to work out problems, that each brother’s thinking sharpened the other’s. Still, they weren’t always pleasant to hear. Katharine opened the door to the workshop and asked, “Engine troubles?”Will and Orv whirled around; neither had heard her coming in. “We wrote to ten manufacturers,” Wilbur said. “We asked for prices for a gasoline engine, 180 pounds or less, capable of eight to nine horsepower. Well, there’s no such thing, and there’s no one who wants to build us one.”“So, fine,” Orville said. “We’ve built engines. We built the one that runs this shop. We’ll build our own engine.”“But the propeller—” said Wilbur.“Let me see if I remember,” Katharine said. “You’re looking at how a screw propeller works on a ship. How it works in water. You’ll take what you’ve learned about air pressure and substitute air for water. You’ll make a screw propeller that works in the air. Voila.”Orville sat down. “Right. Voila. We saved it for last because it seemed so simple.”“It turns out,” added Wilbur “that even though marine screws have been in use for a hundred years—”“One hundred years,” said Orville.“—there’s no theoretical understanding of how they work,” finished Will.“It’s all observation and trial and error,” said Orville. “We can’t fly with trial and error.”“We have to figure it out ourselves,” Will continued. “Turns out that a screw propeller is not a screw at all. It doesn’t drill through the air. It operates like a wing, a spinning wing. As it spins, the shape of the blade reduces the air pressure in front of it. The low pressure in front of the blade and the high pressure behind it push it forward, and create thrust.”“But it’s good that it’s like a wing, right?” said Katharine. “You two are wing experts by now.”Orville shook his head. “It’s more complicated than that. Nothing about a propeller, or the air in which it acts, stands still. Not for a moment. The thrust depends upon the speed and angle at which the blade strikes the air. The angle depends upon the speed at which the propeller is turning, and the speed at which the machine is traveling forward, and the speed at which the air is slipping backward.”Orville talked faster now. “The slip of the air backward depends upon the thrust exerted by the propeller, and the amount of the air acted upon. So you’ve got the machine moving forward, the air moving backward, the propeller moving sideways. Change any one of these things, and all the rest changes, too.”Orville slumped in his chair. The propeller would be as daunting as any problem the brothers would face.All through the winter the brothers struggled. How much thrust would the machine need in order to fly? How big would the propellers have to be, and what shape? How fast would they have to spin? The brothers bent all of their extraordinary abilities as engineers toward this problem. They argued and experimented. They made hundreds of tests and calculations. Finally, in the spring, a theory took shape. By summer Orville could look at the issue in a better mood.“Isn’t it astonishing,” he told Wilbur, “that all these secrets have been preserved for so many years, just so we could discover them!”By then the engine was coming together, as well. The brothers built it with Charlie Taylor, their assistant at the shop.“It’s simple, but it will work,” Wilbur said.There were still problems, though. That spring Wilbur and Orville had filed their first applications for a patent. They filed twice, actually. Both times the U.S. Patent Office made short work of their requests. For years the office had been receiving patent applications for jury-rigged “flying machines,” so many that in the early 1890s, the office set a new policy: no flying machine applications would be considered, not unless the inventor could show that a machine had actually flown.“They want to see it fly,” said Orville. “Well, we want to see it fly, too.”“We’ll leave the patent for later,” said Wilbur. “Let’s get this machine into the air.”Two years earlier, speaking in Chicago, Wilbur had said, “The inability to balance and steer still confronts students of the flying problem. . . . When this one feature has been worked out, the age of flying machines will have arrived, for all other difficulties are of minor importance.”Wilbur and Orville had since discovered that assumption after assumption about those other difficulties was wrong. They had corrected each wrong assumption as they went. Their drive, their stubbornness, and their genius as engineers had seen them through problems that others had declared unsolvable. Now, the brothers believed, they had met every challenge.They would know for sure after one more trip to the Outer Banks. Early on the morning of September 23 they boarded a southeast-bound train, as they had three times before. Each of the earlier trips had been exciting. This was something more.As they settled into their seats Orville turned to Wilbur. “This is it,” Orville said. Wilbur just nodded, and looked ahead. The train jerked into motion and rumbled toward Kitty Hawk.<i>(To be continued.)</i>

<b>Coming Wednesday - First Flight</b>Text and illustrations copyright &Copy; 2003 by Brian FlocaSponsored in part by Inventing Flight, Dayton, OhioReprinted by permission of Breakfast Serials, Inc.www.breakfastserials.com

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