was certainly a technical performance considerably above the existing average, Manly's goal was that of something so far beyond this average as to have been considered by many impossible. Importantly, the Wrights had their own experience with their shop engine and a good basic general knowledge of the size of engine that would be necessary to meet their requirements.

Engine roughness was of primary concern to them. In the 1902 description of the engine they sent to various manufacturers, they had stated: "... and the engine would be free from vibration." Even though their requirement for a smooth engine was much more urgent than merely to avoid the effect of roughness on the airplane frame, they were faced, before they made their first powered flight, with the basic problem with which the airplane has had to contend for over three-quarters of its present life span: that is, it was necessary to utilize an explosion engine in a structure which, because of weight limitations, had to be made the lightest and hence frailest that could possibly be devised and yet serve its primary purpose. However great the difficulty may have appeared, in the long view, the fault was certainly a relatively minor one in the overall development of the internal combustion engine—that wonderful invention without which their life work would probably never have been so completely successful while they lived, and which, even aside from its partnership with the airplane, has so profoundly affected the nature of the world in which we live.

It seems quite obvious that to the Wrights vibration, or roughness, was predominantly if not entirely caused by the explosion forces, and they were either not completely aware of the effects of the other vibratory forces or they chose to neglect them. Although crankshaft counterweights had been in use as far back as the middle 1800s, the Wrights never incorporated them in any of their engines; and despite the inherent shaking force in the 4-inline arrangement, they continued to use it for many years.

The choice of four cylinders was obviously made in order to get, for smoothness, what in that day was "a lot of small cylinders"; and this was sound judgment. Furthermore, although the majority of automobiles at that time had engines with fewer than four cylinders, for those that did the inline form was standard and well proven, and, in fact, Daimler was then operating engines of this general design at powers several times the minimum the Wrights had determined necessary for their purpose.

What fixed the exact cylinder size, that is, the "square" 4×4-in. form, is not recorded, nor is it obvious by supposition. Baker says it was for high displacement and low weight, but these qualities are also greatly affected by many other factors. The total displacement of just over 200 cu in. was on the generous side, given the horsepower they had determined was necessary, but here again the Wrights were undoubtedly making the conservative allowances afterwards proven habitual, to be justified later by greatly increased power requirements and corresponding outputs. The Mean Effective Pressure (MEP), based on their indicated goal of 8 hp, would be a very modest 36 psi at the speed of 870 rpm at which they first tested the engine, and only 31 psi at the reasonably conservative speed of 1000 rpm. The 4×4-in. dimension would provide a cylinder large enough so that the engine was not penalized in the matter of weight and yet small enough to essentially guarantee its successful operation, as cylinders of considerably larger bore were being utilized in automobiles. That their original choice was an excellent one is rather well supported by the fact that in all the different models and sizes of engines they eventually designed and built, they never found it necessary to go to cylinders very much larger than this.

Figure 3.—First flight engine, 1903, installed in the Kitty Hawk airplane, as exhibited in the Science Museum. (Photo courtesy the Science Museum, London.)

A second basic determination which was made either concurrently or even possibly in advance of that of the general form and size was in the matter of the type of cylinder cooling to adopt. Based on current practice that had proven practical, there were three possibilities, all of which were in use in automobiles: air, water, or a combination of the two. It is an interesting commentary that Fernand Forest's[10] proposed 32-cylinder aircraft engine of 1888 was to be air-cooled, that Santos-Dumont utilized an air-cooled Clement engine in his dirigible flights of 1903, and that the Wrights had chosen air cooling for their shop engine. With the promise of simplicity and elimination of the radiator, water and piping, it would seem, offhand, that this would be the Wrights' choice for their airplane; but they were probably governed by the fact that not only was the water-cooled type predominant in automobile practice, but that the units giving the best and highest performance in general service were all water cooled. In their subsequent practice they never departed from this original decision, although Wilbur Wright's notebook of 1904-1907 contains an undated weight estimate by detailed parts for an 8-cylinder air-cooled engine. Unfortunately, the proposed power output is not recorded, so their conception of the relative weight of the air-cooled form is not disclosed.

One of the most important decisions relating to the powerplant—one which was probably made long before they became committed to the design itself—was a determination of the method of transmission of power to the propeller, or propellers. A lingering impression exists that the utilization of a chain drive for this purpose was a natural inheritance from their bicycle background. No doubt this experience greatly simplified the task of adaptation but a merely cursory examination shows that even if they had never had any connection with bicycles, the chain drive was a logical solution, considering every important element of the problem. The vast majority of automobiles of the time were chain driven, and chains and sprockets capable of handling a wide range of power were completely developed and available. Further, at that time they had no accurate knowledge of desirable or limiting propeller and engine speeds. The chain drive offered a very simple and inexpensive method of providing for a completely flexible range of speed ratios. The other two possibilities were both undesirable: the first, a simple direct-driven single propeller connected to the crankshaft, provided essentially no flexibility whatsoever in experimentally varying engine or propeller speed ratios, it added an out-of-balance engine torque force to the problem of airplane control, and, finally, it dictated that the pilot would be in the propeller slipstream or the airflow to it; the second, drive shafts and gearing for dual propellers, would have been very heavy and expensive, and most probably would have required a long-time development, with every experimental change in speed ratios requiring a complete change in gears. Again, their original choice was so correct that it lasted them through essentially all their active flying years.

The very substantial advantages of the chain drive were not, however, obtained at no cost. Torque variations in the engine would tend to cause a whipping action in the chain, so that it was vulnerable to rough running caused by misfiring cylinders and, with the right timing and