Minor Design Details and Performance of the Wright Engines
In the Wright brothers’ various models were many minor design items which altogether required a great deal of consideration, but which did not materially affect overall engine performance. The results generally could all be classed as good practice; however, one of these utilized in the 4-cylinder vertical engine was rather unorthodox and consisted of offsetting the cylinders with relation to the crankshaft. This arrangement, which can be seen in the drawing (Figure 11) was apparently an attempt to reduce the maximum side load on the piston during the power stroke, but since the peak gas loading usually occurs at about 10 to 15 percent of the power stroke, this probably did not have much effect, and it was not carried over to the 6-cylinder design.
All engine bearings were of the plain sleeve type and, except for the bronze and steel bearings in the connecting rod, were of babbit. The advantages of babbit for bearings were discovered very early in the development of the mechanical arts, and apparently the Wrights never encountered a bearing loading sufficiently high to cause a structural breakdown in this relatively weak material.
Valve openings show no variation through the successive production engines, although the Wrights most probably experimented with different amounts. The 1903 engine and the vertical 4-and 6-cylinder all had lifts of 5/16 in., but the valve-seat angles varied somewhat; the records show included angles of 110° to 90°—not a large difference.
The valve-operating mechanism was the same from the first vertical 4 onward. The high side thrust caused by the cam shape required for the very rapid valve opening they chose was, no doubt, the reason for the use of the hinged cam follower, and since the same general cam design was used in their last engine, the 6-cylinder, the same method of operation which had apparently proved very serviceable was continued. How satisfactory was the considerably simpler substitute used in the Bariquand et Marré version of the 4-cylinder engine is not known. Possibly it was one of the alterations in the Wrights’ design that Wilbur Wright objected to, although in principle it more closely conforms to the later fairly standard combination valve tappet and roller construction: The available drawings do indicate, however, that the cam of the Bariquand et Marré engine was also altered to give a considerably less abrupt valve opening than the Wright design, so that there was less side thrust. For the Wright 6-cylinder engine their 4-cylinder cam was slightly altered to provide a rounding off near the top of the lobe, thus providing some reduction in the velocity before maximum opening was reached. All their cam designs indicate a somewhat greater fear of the effect of seating velocities than of opening accelerations.
Since the range of cylinder diameters utilized did not vary greatly, the valve sizes were correspondingly fairly uniform. The diameter of the valves for the original 4-in.-bore cylinder was 2 in., while that for the 4-3/8-in. bore used in the 6-cylinder engine was actually slightly smaller, 1-7/8 in. Possibly the Wrights clung too long to the automatic inlet valve, although it did serve them well; but possibly, as has been previously noted, there were valid reasons for continuing its use despite the inherently low volumetric efficiency this entailed.
The inherent weakness in the joints of the three-piece connecting rod has been pointed out, but aside from this, the design was excellent, for all the materials and manufacturing methods required were readily available, and structurally it was very sound. Tubular rods were still in use in aircraft engines in the 1920s.
The Wrights had a surprisingly thorough grasp of the metallurgy of the time, and their choice of materials could hardly have been improved upon. Generally they relied upon the more simple and commonly used metals even though more sophisticated and technically better alloys and combinations were available.17 Case hardening was in widespread use in this period but their only utilization of it was in some parts of the drive chains purchased completely assembled and in the piston pins of their last engine. The treatment of the crankshafts of all their engines except the final 6-cylinder was typical of their uncomplicated procedure: the particular material was chosen on the basis of many years of experience with it, hardening was a very simple process, and the expedient of carrying this to a point just below the non-machinable range gave them bearing surfaces that were sufficiently hard, yet at the same time it eliminated the possibility—present in a heat-treating operation—of warping the finished piece.
In the entire 1903 engine only five basic materials—excepting those in the purchased “magneto” and the platinum facing on the ignition-system firing points—were used: steel, cast iron, aluminum, phosphor bronze, and babbit. The steels were all plain carbon types with the exception of the sheet manifold, which contained manganese, and no doubt this was used because the sheet available came in a standard alloy of the time.
Overall, the Wright engines performed well, and in every case met or exceeded the existing requirements. Even though aircraft engines then were simpler than they became later and the design-development time much shorter, their performance stands as remarkable. As a result, the Wrights never lacked for a suitable powerplant despite the rapid growth in airplane size and performance, and the continual demand for increased power and endurance.
Few service records dating from before 1911, when the military services started keeping log books, have been found. Some of those for the period toward the end of their active era have been preserved, but for that momentous period spanning the first few years when the Wrights had the only engines in actual continuous flight operation, there seems to be essentially nothing—perhaps because there were no standard development methods or routines to follow, no requirements to be met with respect to pre-flight demonstrations or the keeping of service records. Beginning in 1904, however, and continuing as long as they were actively in business, they apparently had in progress work on one or more developmental or experimental engines. This policy, in combination with the basic simplicity of design of these engines, accounted in large measure for their ability to conduct both demonstrations and routine flying essentially whenever they chose.
Time between engine overhauls obviously varied. In mid 1906 an engine was “rebuilt after running about 12 hours.” This is comparatively quite a good performance, particularly when it is remembered that essentially all the “running” was at full power output. It was considerably after 1920 before the Liberty engine was redesigned and developed to the stage where it was capable of operating 100 hours between overhauls, even though it was being used at cruising, or less than full, power for most of this time.
The Wrights of course met with troubles and failures, but it is difficult, from the limited information available, to evaluate these and judge their relative severity. Lubrication seems to have been a rather constant problem, particularly in the early years. Although some bearing lubrication troubles were encountered from time to time, this was not of major proportions, and they never had to resort to force-feed lubrication of the main or rod big-end bearings. The piston and cylinder-barrel bearing surfaces seem to have given them the most trouble by far, and examination of almost any used early Wright engine will usually show one or more pistons with evidence of scuffing in varying degrees, and this is also apparent in the photographs in the record. This is a little difficult to understand inasmuch as most of the time they had the very favorable operating condition of cast iron on cast iron. Many references to piston seizure or incipient seizure, indicated by a loss of power, occur, and this trouble may have been aggravated by the very small piston clearances utilized. Why these small clearances were continued is also not readily explainable, except that with no combination of true oil-scraper rings, which was the basic reason why the final form of aviation piston engine was able to reach its unbelievably low oil consumptions, their large and rather weak compression rings were probably not doing an adequate job of oil control, and they were attempting to overcome this with a quite tight piston fit.18 In any event, they did encounter scuffing or seizing pistons and cylinder over-oiling at the same time. As late as 4 May 1908 in the Wright Papers there appears the notation: “The only important change has been in the oiling. The engine now feeds entirely by splash....”
Their troubles tended to concentrate in the cylinder-piston combination, as has been true of almost all piston engines. References to broken cylinders are frequent. These were quite obviously cylinder barrels, as replacement was common, and this again is not readily explainable. The material itself, according to Orville Wright, had a very high tensile strength, and in the 1903 engine more than ample material was provided, as the barrel all the way down to well below the attachment to the case was 7/32 in. thick. The exact location of the point of failure was never recorded, but in its design are many square corners serving as points of stress concentration. Also, of course, no method was then available for determining a faulty casting, except by visual observation of imperfections on the surface, and this was probably the more common cause. It is interesting, however, that the engine finally assembled in 1928 for installation in the 1903 airplane sent to England has a cracked cylinder barrel, the crack originating at a sharp corner in the slot provided at the bottom of the barrel for screwing it in place.
Valve failures were also a continuing problem, and Chenoweth reports that a large proportion of the operating time of the 1904-1906 development engine was concentrated on attempts to remedy this trouble. None of their cams, including those of the 6-cylinder engine, evidence any attempt to effect a major reduction in seating velocities. United States Navy log books of 1912 and 1913 record many instances of inlet valves “broken at the weld,” indicating that some of the earlier 6-cylinder engines were fitted with valves of welded construction.
For the engineer particularly, the fascination of the Wrights’ engine story lies in its delineation of the essentially perfect engineering achievement by the classic definition of engineering—to utilize the available art and science to accomplish the desired end with a minimum expenditure of time, energy, and material. Light weight and operability were the guiding considerations; these could be obtained only through constant striving for the utmost simplicity. Always modest, the Wrights seem to have been even more so in connection with their engine accomplishments. Although the analogy is somewhat inexact, the situation is reminiscent of the truism often heard in the aircraft propulsion business—few people know the name of Paul Revere’s horse. Yet, as McFarland has pointed out, “The engine was in fact far from their meanest achievement.” With hardly any experience in this field and only a meagerly equipped machine shop, they designed and assembled an internal combustion engine that exceeded the specifications they had laid down as necessary for flight and had it operating in a period of about two months elapsed time. The basic form they evolved during this unequalled performance carried them through two years of such successful evolutionary flight development that their flying progressed from a hop to mastery of the art. And the overall record of their powerplants shows them to have been remarkably reliable in view of the state of the internal combustion engine at that time.