They were not the first people to fly, or even the first to fly motorized aircraft.
But in less than four years, Wilbur and Orville Wright solved the baffling puzzle of airplane flight.
In 1903, the two young men from Dayton, Ohio, became the first to succeed in making a manned, sustained and controlled flight in a heavier-than-air craft, an achievement that had eluded eminent researchers and clever inventors for a century.
Early and contemporary experimenters became mired in dead-end technologies, focusing too narrowly, bypassing fundamental aspects of flight, or trying to fly in one large indigestible gulp.
Many aviation experimenters believed they could create an airplane that was aerodynamically stable, like a well-balanced boat floating serenely on a pond, a craft that would join the clouds without much human intervention.
Enthusiasts and researchers had built models, man-carrying gliders and even proof-of-concept demonstrators that had flown to a greater or lesser degree, some with bits and pieces of control.
Only the Wright brothers, however, recognized that a successful flying machine would represent a system of systems, all of which had to work well together.
And the brothers came with the intellectual power, mechanical acumen and physical skill to launch that recognition into flight, though not without their own demoralizing fits and starts.
"If you do something right, you may not know why it's right," said Wright brothers expert Rick Young of Richmond. "They started with about all you could have wrong."
Taking considerable personal and professional risk, "They gave us a systematic way not only of building airplanes, but of problem solving," said Wright expert Ken Hyde of Warrenton.
Producing a practical flying machine was a problem with several tightly connected parts, defying earthbound knowledge, intuition and common sense. Some scientists and engineers viewed the problem of flight as unsolvable.
Each aspect demanded its own solution -- the failures of other would-be aeronauts showed that -- or the airplane would not fly:
*The airplane needed effective wings to lift it.
*The aircraft had to be strong enough to withstand the forces acting on it.
*The plane's engine had to be both light and powerful, and its propellers efficient enough, to sustain flight.
*The airplane must be controllable in the unexplored ocean of air.
*Its pilot had to know how to fly it.
The Wrights methodically went about the business not just of learning to fly, or how to build a flying machine, but of creating the science of aeronautics.
After exhausting their local libraries' offerings, they wrote the Smithsonian Institution in 1899 for all the information available on aviation experiments.
Starting from that base of data, the brothers embarked on a well-conceived theoretical and experimental program. They calculated the dimensions their flying machines should be to lift a given weight, what the stresses on them would be, and what kind of performance they could expect from their craft.
Then they built them and flew them. As they did that they measured speeds, forces, times and distances. They took notes on their work and made hundreds of photographs of their aircraft in flight and at rest.
This development program allowed them to first crawl, then walk, and finally run into the world of flight.
"From the beginning," wrote British aviation historian Christopher Chant, ". . . the brothers saw that the real problem lay in controlling the aeroplane . . . once it had risen into the air."
We humans think of ourselves as three-dimensional beings, but compared to the world of the sky, we are really two-dimensional.
We view the world from 5 feet or 6 feet above the ground, and move only right and left, forward and back, rarely leaning more than a few degrees away from straight upright.
Acquainted through bicycling with the idea of balancing unstable vehicles, the Wrights realized that they would need to control their aircraft in three dimensions: moving up and down, forward and -- occasionally -- backward, left and right, rolling, pitching and yawing in the air.
In 1899, the brothers began working with a dual-winged kite of 5-foot span, learning the critical importance of lateral control -- banking or tipping one wing down and one wing up -- to turn the craft.
They figured out -- at least partially from their observations of soaring birds -- that they needed to change the balance of lift along the wing's span to cause their kite to turn, an insight that had not come to other would-be aeronauts. To bank their aircraft, the Wrights warped their wings.
While it worked for roll control, the wing-warping concept later fell into disuse as the aviation world settled on handier ailerons, flaps along a wing's trailing tips, to provide roll control.
Their method of pitch control came in 1900 when the Wright brothers added what they called a "forward rudder," what engineers now term a "canard elevator," to the front of a larger version of their 1899 craft. The elevator controlled the angle at which the machine met the oncoming air, in turn regulating its speed and lift.
Mounting it forward would produce close to instantaneous response in pitch control and acted as an energy-absorbing structure in a crash.
Wilbur Wright performed what would now be called a stress analysis on the glider, building it to carry five times his weight, and, he said, "testing every piece."
Using a 17-foot-long wing, they tested the craft as a kite and as an unmanned glider at Kitty Hawk, N.C. Finally Wilbur climbed aboard and flew it, achieving flights 15-20 seconds long and 300 to 400 feet long.
Though they validated the operation of their system of roll control, the 1900 glider "appeared sadly deficient in lifting power as compared with the calculated lift," Wilbur wrote.
They returned to Dayton, determined to fix that deficiency.
To make more lift, they almost doubled the area of the wing for their 1901 glider, using a wingspan of 22 feet that made it the largest glider to fly to that time.
Once again, the glider did not perform as well as their calculations had predicted.
Those calculations were based on pressure tables developed by the German engineer Otto Lilienthal predicting the lift that could be expected for a given speed and wing area.
But the 1901 glider's lift was only a third Lilienthal's tables predicted and almost twice the expected drag. The glider also was almost uncontrollable in pitch movement.
On the return trip to Dayton, a discouraged Wilbur told his younger brother "man would not fly for 50 years."
To unravel the lift puzzle, the brothers "pioneered the practice of aerospace engineering as we know it today," said Dr. Peter L. Jakab, chairman of the aeronautics division at the National Air and Space Museum and curator of the museum's exhibition on the centennial of the Wright brothers' flight.
In two months, the Wrights built a wind tunnel and, studying dozens of wing models, gathered the aeronautical engineering data used to build the world's first successful airplane, "just the way modern engineers do," Jakab said.
"That's what sets them apart from mere bicycle mechanics."
Using their newfound data, they designed the 1902 glider with two striking features.
Their wind-tunnel data demonstrated the aerodynamic advantage of long, narrow wings. Such wings have a long tip-to-tip span compared to their front-to-back dimension.
The new 32-foot-long wing flew -- by the standards of the time -- beautifully, and the handsome glider's longest flight lasted 26 seconds and covered 623 feet across the ground.
The 1902 glider made another contribution to flight control: a vertical fin to overcome the disturbing phenomenon of adverse yaw.
Sometimes when the pilot tried to bank, and thus turn, the 1901 glider, the craft would slew -- yaw -- its nose the other way. The extra lift on the up-moving wing tip was accompanied inevitably by extra drag. The drag due to lift pulled back on the wing tip, yawing the aircraft away from the direction of the turn.
The vertical tail, the Wrights thought, would resist this yawing motion and the glider thus turn easily. In fact, the new tail did not work that way.
It actually aggravated the problem after the glider began slipping sideways through the air, setting up a corkscrewing motion centered on the lowered wing. The Wrights called this "well-digging," but pilots came to call this condition a spin, one of the great killers of flight.
Orville came up with the idea of enabling the pilot to pivot the tail to stop the adverse yaw from beginning. Then Wilbur suggested interconnecting this movable rudder with the wing-warping control so the glider would automatically make coordinated turns.
The Wrights now had a reliable structure. They had an efficient wing. They had solved the problem of three-axis control. And they had taught themselves how to fly.
They just needed a motor.
Actually, they needed an integrated aviation propulsion system: a powerful, light-weight engine driving an efficient propeller.
The brothers divided up the problem: Orville took the engine, Wilbur the propeller.
Orville worked with their talented bike-shop machinist, Charles Taylor, to produce an aluminum block, four-cylinder in-line engine that weighed about 200 pounds. It produced about 12 horsepower: not much, but enough.
Gasoline engines, while relatively new, were well-understood technology at the beginning of the 20th century. Propeller theory was a black hole of errors and misinformation.
"Wilbur was the first person to recognize ... that a propeller is nothing more than a twisted wing," said historian and aeronautical engineer John D. Anderson with the National Air and Space Museum.
Using information from their wind-tunnel tests, Wilbur Wright had invented "blade-element theory," the idea that at each point along its span, a propeller meets the air at a different angle and speed.
Wilbur understood that to realize the most thrust out of a propeller, its blades must be twisted to a high angle at the slow-moving root with the twist decreased gradually out to the fast-spinning tip.
"The propeller is probably the most significant contribution they gave to us," said Ken Hyde, "and the least credited."
And, he said, "the propeller is the most important single thing that nobody [else] had."
Hyde is building the Wright Flyer reproduction that will be used to try to re-create the brothers' feat at Kitty Hawk, on Dec. 17.
The Wrights' propeller design was 81 percent efficient, making it extraordinarily effective at converting the engine's power into usable thrust through the air.
After a century of engineering development, the efficiency of aircraft propellers has been pushed to 85 percent to 90 percent, experts say.
Using the general design configuration they had developed from their careful flight tests, the Wrights calculated that their airplane would need a wingspan of about 40 feet and a wing area of about 500 square feet, traveling at 23 mph, to lift the pilot, engine and airframe into flight.
They had planned on it weighing about 630 pounds when it flew, but like almost every other aircraft in history, its weight crept up, producing an empty weight of 605 pounds and a eventual flying weight of about 750 pounds.
With the weight growth, the Wright brothers estimated the plane would need 100 pounds of thrust to overcome its drag and take flight.
Their little engine was up to the challenge: Their tests showed it would produce 132 pounds of thrust.
The 1903 airplane put out that thrust through two 8 1/2-foot propellers mounted behind the wings and connected to the engine, centrally located on the bottom wing, by a chain-and-sprocket transmission.
In another bit of insightful engineering, the Wrights arranged the chain drive so the props turned in opposite directions, canceling the otherwise-destabilizing effect of their torque.
Following the aircraft-design rule of "simplicate and add more lightness," the Wrights dispensed with the weight and complexity of wheels for takeoff and landing. Instead, the airplane took off from a trolley running on a rail track and landed on simple skids.
Their total costs were about $1,000.
Dec. 17, 1903, was raw, cold and windy on the Outer Banks. An earlier coin toss made Orville the pilot in command. Five local folks gathered to watch and lend a hand.
The brothers shook hands and Orville climbed onto the plane.
"With a short dash down the runway, the machine lifted into the air and was flying," Orville wrote later, traveling 120 feet against a 27-mph wind.
"It was only a flight of 12 seconds, and it was an uncertain, wavy, creeping sort of flight at best," he said. "But it was a real flight at last."
"They did it! They did it!" one of the witnesses yelled. "Damned if they didn't fly!"
The Wrights made three more flights on Dec. 17, each longer than the one before, the longest 852 feet in 59 seconds, before a gust of wind wrecked the plane.
Its unstable handling characteristics probably make it the most difficult airplane to fly ever built, said Hyde, a retired airline pilot.
"This [was] a lousy airplane," said engineer and Virginia aviation historian Norman Crabill. "But for its time, it was the best airplane in the world."
In truth, said Young, "The Wright Flyer was the greatest aircraft ever built."
The Wright brothers had built and flown an airplane after just three and a half years of study, experimentation and flight testing. Wilbur was 36 years old, Orville 32.
In 1906, O.& W. Wright were awarded U.S. Patent Number 891,393, not for the 1903 Flyer, but for the 1902 glider. The engine that made it an airplane was an add-on.
"It is possible to fly without motors," Wilbur Wright said, "but not without knowledge and skill."
Peter Bacqué is the Richmond Times-Dispatch's aviation writer. He holds an airline transport pilot's license and is an active flight instructor in airplanes and gliders. In a quarter century as a pilot, Bacqué has logged more than 5,000 hours of flight time in aircraft from balloons to helicopters to F-16s.
