Another two months went by before a second block would be delivered from Pittsburgh. This engine worked fine and as a bonus delivered an unexpected 12 horsepower.
Meantime, the design of the propellers had become a still bigger challenge. “I think the hardest job Will and Orv had was with the propellers,” Charlie later said. “I don’t believe they ever were given enough credit for that development.”
The problem became more complex the more the brothers studied it. Much to their surprise, they could find no existing data on air propellers. They had assumed they could go by whatever rule-of-thumb marine engineers used for the propellers on boats, and accordingly drew on the resources of the Dayton library only to find that after a hundred years in use the exact action of a screw propeller was still obscure. Once more they were left no choice but to solve the problem themselves. “Our minds,” said Orville, “became so obsessed with it that we could do little other work.”
They began to see the propeller as an airplane wing traveling in a spiral course, and that if they could calculate the effect of a wing traveling a straight course, why could they not calculate the effect of one traveling in a spiral course?
But on further consideration [Orville would explain], it is hard to find even a point from which to make a start; for nothing about a propeller, or the medium in which it acts, stands still for a moment. The thrust depends upon the speed and the angle at which the blade strikes the air; the angle at which the blade strikes the air depends on the speed at which the propeller is turning, the speed the machine is traveling forward, and the speed at which the air is slipping backward; the slip of the air backward depends on the thrust exerted by the propeller, and the amount of air acted upon. When any one of these change, it changes all the rest, as they are all interdependent on one another.
After several months of study and discussion they had come to understand that the thrust generated by a standing propeller was no indication of the thrust when in motion, and that the only realistic way to test the efficiency of a propeller would be to try it out on the flying machine.
During these months their “discussions” became as intense as they had ever been. Heated words flew, filling hours of their days and nights, often at the tops of their voices. “If you don’t stop arguing, I’ll leave home,” a nearly hysterical Katharine cried out at one point.
According to Charlie Taylor, they were never really mad at each other. One morning after one of their “hottest” exchanges, he had only just opened the shop at seven o’clock as usual when Orville came in saying he “guessed he’d been wrong and they ought to do it Will’s way.” Shortly after, Wilbur arrived to announce he had been thinking it over and “perhaps Orv was right.” The point was, said Charlie, “when they were through . . . they knew where they were and could go ahead with the job.”
The new Flyer, as they called it, would have two propellers positioned between the two wings just to the rear of the operator. One would turn clockwise, the other, counterclockwise, so the spinning, or gyroscopic action, of the one would balance that of the other. Making the propellers with the proper diameter, pitch, and surface area proved no great problem.
Each had a diameter of 8 and a half feet and were made of three spruce laminations glued together and shaped by hand with a hatchet and spoke shaver, or “drawknife,” as used by wheelwrights. That they were different from any propellers ever built before was certain, and the last major problem had been resolved.
Again, the machine would ride on skids, not wheels. The operator would again lie prone at the controls in the middle of the lower wing. The motor and a radiator would be positioned directly beside him on the right. A little one-gallon gas tank hung overhead on a strut to his left. The drive chains for the propellers were specially made by the Indianapolis Chain Company, and Roebling wire would be used for the trusses between the wings—wire made by the Roeblings who built the Brooklyn Bridge.
On March 23, the brothers applied for a patent on their flying machine, its wing-warping system, and rudder.