The Wrights knew that rough estimates, which might be near enough for a motor-boat, would not do for an airplane. On a boat a propeller having only a fraction of one per cent of the desired efficiency could move the boat a little; but on an airplane, unless the propeller had the full amount of thrust needed, it would be worthless, for it couldn’t lift the plane into the air at all! In short, the Wrights had to have a propeller that would do exactly what was expected of it. And they had neither the time nor money to carry on a long series of experiments with different kinds of propellers until they could hit on one suitable. They couldn’t afford to make mistakes except on paper. They must somehow learn enough about how propellers acted, and why, to enable them to make accurate calculations. It was apparent to the Wrights that a propeller was simply an airfoil traveling in a spiral course. As they could calculate the effect of an airfoil traveling in a straight course, why should they not be able to calculate the effect in a spiral course? At first thought that did not seem too difficult, but they soon found that they had let themselves into a tough job. Since nothing about a propeller, or the medium in which it acts, would be standing still, it was not easy to find even a point from which to make a start. The more they studied it, the more complex the problem became. “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 upon 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 upon the thrust exerted by the propeller, and the amount of air acted upon.” It was not exactly as simple as some of the problems in the school arithmetic--to determine how many sheep a man had or how many leaps a hound must make to overtake a hare. In trying to work out a theory about the action of screw propellers, Wilbur and Orville got into many arguments. Right here it may be noted that this habit the brothers had of arguing technical points was one of the reasons why they were able to accomplish all they finally did in a relatively short time. Neither was a “yes” man to the other. But in their arguments about propellers a peculiar thing happened. “Often,” Orville later reported, “after an hour or so of heated argument, we would discover that we were as far from agreement as when we started, but that _each had changed to the other’s original position_.” Many months passed before the intricacies of the problem began to untangle themselves. The Wrights finally got a better understanding of the action of screw propellers than anyone had ever had before. The time came when they felt sure of their ability to design propellers of exactly the right diameter, pitch, and area for their needs.