Читать книгу The Wright Brothers' Engines and Their Design - Leonard S. Hobbs - Страница 6

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In commencing the design of the first engine, the first important decision arrived at was that of the number and size of the cylinders to be employed and the form in which they would be combined, although it is unlikely that this presented any serious problem. In a similar situation Manly, when he was working on the engine for the Langley Aerodrome,[9] was somewhat perturbed because he did not have access to the most advanced technical knowledge, since the automobile people who were at that time the leaders in the development of the internal combustion engine, tended for competitive reasons to be rather secretive about their latest advancements and designs. But although the standard textbooks may not have been very helpful to him, there were available such volumes as W. Worby Beaumont's Motor Vehicles and Motors which contained in considerable detail descriptions and illustrations of the best of the current automobile engines. The situations of Manly and the Wrights differed, however, in that whereas the Wrights' objective 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 magnitude of normal regular variations, the action could result in destructive forces in the transmission system. This was the basic reason for the Wrights' great fear of "engine vibration," which confined them to the use of small cylinders and made a fairly heavy flywheel necessary on all their engines. When they were requested to install an Austro-Daimler engine in one of their airplanes, they designed a flexible coupling which was interposed between the engine and the propeller drive and this was considered so successful that it was applied to the flywheel of some engines of their last model, the 6–70, "which had been giving trouble in this regard."[11]

Although flat, angled, and vertical engines had all been operated successfully, the best and most modern automotive engines of the time were vertical, so their choice of a horizontal position was probably dictated either by considerations of drag or their desire to provide a sizable mounting base for the engine, or both. There is no record of their ever having investigated the matter of the drag of the engine, either alone or in combination with the wing. The merit of a vertical versus a horizontal position of the engine was not analogous to that of the pilot, which they had studied, and where the prone position undoubtedly reduced the resistance.

Having decided on the general makeup of their engine, the next major decision was that of just what form the principal parts should take, the most important of these being the cylinders and crankcase. Even at this fairly early date in the history of the internal combustion engine various successful arrangements and combinations were in existence. Individual cylinder construction was by far the most used, quite probably due to its case of manufacture and adaptability to change. Since 4-cylinder engines were just coming into general use (a few production engines of this type had been utilized as early as 1898), there were few examples of en-bloc or one-piece construction. The original German Daimler Company undoubtedly was at this time the leader in the development of high-output internal-combustion engines, and in 1902, as an example of what was possible, had placed in service one that possibly approximated 40 hp, which was an MEP of 70 psi. (Almost without exception, quoted power figures of this period were not demonstrated quantities but were based on a formula, of which the only two factors were displacement and rpm.) The cylinders of this Daimler engine were cast iron, the cylinder barrel, head, and water jacket being cast in one piece. The upper part of the barrel and the cylinder head were jacketed, but, surprisingly, the bottom 60 percent of the barrel had no cooling. The cylinders were cast in pairs and bolted to a two-piece aluminum case split at the line of the crankshaft. Ignition was make-and-break and the inlet valves were mechanically actuated. Displacement was 413 cu in. and the rpm was 1050.

Although a few examples of integral crankcase and water jacket combinations were in use, the Wrights were being somewhat radical when they decided to incorporate all four cylinders in the one-piece construction, particularly since they also proposed to include the entire crankcase and not just one part of it. It was undoubtedly the most important decision that they were required to make on all the various construction details, and probably the one given the most study and investigation. Many factors were involved, but fundamentally everything went back to their three basic requirements: suitability, time, and cost. There was no obvious reason why the construction would not work, and it eliminated a very large number of individual parts and the required time for procuring, machining, and joining them. Probably one very strong argument was the advanced state of the casting art, one of the oldest of the mechanical arts in existence and one the Wrights used in many places, even though other processes were available. What no doubt weighed heavily was that Dayton had some first-class foundries. The casting, though intricate and not machinable in their own shop, could be easily handled in one that was well outfitted. The pattern was fairly complex but apparently not enough to delay the project or cause excessive cost.