Each year, billions of passengers travel the skies in sleek jet aircraft that look far removed from the dragonfly-like craft with which the Wright Brothers broke the flight barrier 100 years ago. Yet the fundamental flight principles are the same, a testament to the legacy left by the two Ohio aviation pioneers as the result of their successful experiments on a wind-swept North Carolina sand dune.

Like the ship, telegraph, automobile and radio, the airplane was yet another technological means to project humanity far beyond its natural physical limits. People had imagined flight for thousands of years, but like so much scientific progress, the advances that turned human flight into a reality followed the industrial revolution.

By the mid-19th century, the English nobleman George Cayley identified the three conditions that had to be mastered for powered flight to occur - lift, forward thrust and control. But it was another half-century before Orville and Wilbur Wright made them work together in the first controlled flight of a powered craft that weighed more than air.

Experts like Peter Jakab of the Smithsonian's National Air and Space Museum in Washington say that the Wright brothers' success was based on their ability to methodically solve these principles by analyzing them separately.

"It was not just genius when they figured it out," he said. "They actually had an approach to invention that largely explains why they were successful, rather than other people who were also working on the problem."

For example, the brothers built upon data from German experimenter Otto Lillienthal to improve the shape of wings for most efficient lift. Another Air and Space Museum curator, Tom Crouch, says they developed an ingenious method that allowed them to test hundreds of small model aircraft with different wing shapes and sizes rather than taking the more time-consuming, expensive approach of building full-sized gliders.

"They took a much smarter approach," explained Mr. Crouch. "They built a wind tunnel. Rather than moving a wing forward through the air, flying, the way you do with a glider in a wind tunnel you position it in one spot and you run air over the top of it."

To solve the problem of continuous thrust, the Wright Brothers developed a propeller that Mr. Crouch says was essentially a twisted, spinning wing powered by gasoline engines.

"The great breakthrough occurred when they said, 'Well, it is much like a wing,' he said. "It is developing lift. Rather than moving forward through the air, it is rotating and the lift becomes the thrust that moves the airplane forward."

But experts say the Wright Brothers' greatest contribution to aviation engineering was conceiving how to keep an aircraft stable in flight. Other experimenters either ignored the problem or believed that power and momentum alone would maintain balance, like skipping a stone at high speed across the water. But the author of a book on the Wright Brothers, James Tobin, says Orville's and Wilbur's background as bicycle builders led them to conclude that the pilot would have to control balance, just as a cyclist does.

"Wilbur believed that there would be enough power available through a fairly conventional internal combustion engine of the day," said. Mr. Tobin. "What he was worried about was this problem of steering and balance. In solving that, they had to figure out a way of moving the wings to control balance once you were in the air."

Earlier inventors tried to do this by having a pilot shift his weight. But the Wright brothers would have the pilot pull wires to slightly warp the wings to change their shape. Later, flaps on the rear edges of wings called ailerons took over this function. Then the brothers added a horizontal rudder in front for pointing the aircraft up or down and a tail rudder for turning left or right.

The result of this pioneer engineering was four successful flights that changed the world - each longer and farther than the one before.

"What the Wright Brothers achieved really set in motion all that followed in terms of aerospace.

"I would argue that aerospace is the defining technology of the 20th century," continued Peter Jakab of the Air and Space Museum. "It changed the way we see ourselves in the world."