The mechanics of a plane are very difficult to understand, especially for Orville and Wilbur. There had been no existing knowlege or writings on the mechanics of how an airplane works, so they had to break the ice.
Before actually attempting to fly, the Wright Brothers had spent a great, great deal of time studying how their multiple gliders worked. Wilbur even took notes while gliding once! They made many gliders, and none of them seemed to give them what they wanted to learn. But in 1902, the two designed a glider with two sets of wings, and a tail. This gave them a longer glide, and it seemed that steering was better.
Studying the wind became their chief study at this point. They made the wings curved so that they would be able to catch more wind, and elevate the glider. It turns out that this glider was more than a glider... but still less than a plane. It required very quick running to get it going, but once it was in the air, it flew like a Boeing 747 would. The only problem was that it couldn't stay up for very long. So they needed a motor.
They couldn't simply plop a motor onto a glider. It was hard enough getting it started already! First they had to get a motor that was light but powerful. Then they had to design propellors that would push had enough to move the plane around in the air. Last, they had to design a body that would not vibrate to the engine and propellors.
All this was accomplished. The Wright Brothers had done what most thought impossible. The engine weighed only 17 lbs, and produced a much better than expected 12 horsepower. They had a difficult time dsigning the propellors, but once they figured it out, they decided to use two propellors mving in opposite directions to neutralize any twisting difficulty, while still pushing the plane. The propellors were attatched to the motor with a sprocket-and-chain transmission, much like the kind on a bicycle. The motor rested on the lower left wing, so that it would not fall on the pilot in case of a headlong crash. To balance the weight of the motor, the lower left wing was 4 inches longer than the others.
When it was time to take off, the two decided to place it on a railway to help it roll straight throught the sand. The Flyer rolled 40 feet, then took to the air.