This year, they would take turns flying, giving Orville a chance to fly for the first time. Always aware of safety, they made short, low glides from the slope of Big Hill to practice working the controls and learn how to steer right, left, up, or down.

On most of flights the glider performed well, but every so often it would spin out of control when the pilot tried to level off after a turn. They were at a loss to know just what was causing the problem. First, they altered the wing tips and went back to Big Hill for more testing. Again, the glider spun out of control during turns. Next they turned the attention to the 6-foot double vane tail rudders, which were rigidly fixed in place. They had installed the tail to help stabilize the glider during turns, but it was not working. Lying in bed one sleepless night, Orville figured out what the problem was. It was the fixed tail.

They removed the double tail and replaced it with a single-vane, 5-foot movable rudder. After this change, the Wright brothers finally had complete control of their glider. With the wings and tail coordinated, the glider would always make smooth banked turns. They stayed until late October practicing at every opportunity.

Back in Dayton, Wilbur and Orville turned their attention to the final part of the project: finding an engine and building propellers. The engine needed to be lightweight, yet powerful enough to produce sufficient trust to drive a flying machine thorough the air. In addition, the aircraft itself would need to be strong enough to carry the necessary weight and withstand the vibrations of the motor and propellers. Finally they needed to design and build propellers.

Wilbur wrote to several gasoline engine manufacturers, asking if they would supply an engine that could produce at least 8 horsepower, yet weighed less than 200 pounds. No company was willing to build them an engine, so the brothers decided they would build their own. Working in their usual methodical fashion, with the help of their mechanic, Charlie Taylor from the bicycle shop, they had a motor ready for testing in just six weeks. It was lightweight and efficient, weighed only 179 pounds, but generated over 13 horsepower.

Designing the propellers turned out to be a far more difficult problem. Orville reasoned the machine was moving forward, the air was flying backwards, the propellers were turning sideways, and nothing was standing still. It seemed impossible to find a starting point from which to begin.

After five months of study, discussion, experiments, and five notebooks filled with formulas, diagrams, computation, and tables of data, Orville and Wilbur finally understood that an aerial propeller worked the same way as a rotary wing. The same laws of physics that produced lift upward, when a curved wing sliced through the air, also produced forward thrust when a curved propeller blade rotated through the air. Once they had grasped this idea, they were able to design propeller blades with the right diameter, pitch and area for their aircraft. The Wright brothers were the first to understand this basic premise of aeronautics.