The History: The propeller experiments in December and February had been static tests. The propeller was mounted to the motor which remained stationary, as if it were a large fan. Of course on their flying machine, the propeller would turn and the airplane would move forward.

The Wrights recognized that when their flying machine moved forward more air would move over the blades. (When the propellers were operated in static mode, this could not happen.) Consequently, the amount of thrust generated in the static tests would be different when the propellers operated on a moving flying machine. What would the difference in thrust be? The Wrights needed to know the answer to this question to design the propellers efficiently.

The Wrights decided to place two propellers on the Flyer. Two propellers could push a greater mass of air creating more thrust. How much thrust would these propellers have to generate? How much air would the propellers have to move? How much horsepower would the engine have to generate to spin the propellers with the machine moving forward? Before trying to design and build another propeller, the Wrights began to mathematically estimate the answers to these questions on paper.

On March 6, 1903, Wilbur recorded their operating assumptions and mathematical estimates of how the propellers and engine would need to perform.

Some of these operating assumptions included:

• Speed of the Machine = 24 miles/hour
• Thrust = 90 pounds
• 2 Screws [propellers] = 8.5 feet X 8 inches
• Area of Blades = 5.4 sq. ft.
• Revolutions per minute = 330

Based on their mathematical estimates the brothers could now design the propellers to produce the proper amount of thrust for the moving flying machine.