Part 6

Also for the first time one-piece forged valves were used, but just when these were incorporated is not certain and, surprisingly, they were applied to the inlet only, the exhaust valve being continued in the previous two-piece screwed and riveted construction. The reasoning behind this is not evident. If a satisfactory two-piece exhaust valve had finally been developed it would be logical to carry it over to the new design; but exhaust valves normally being much more troublesome, it would seem that a good exhaust valve would make an even better inlet valve and, in the quantities utilized, the two-piece design should have been much cheaper. In the original 6-cylinder engine the inlet valves operated automatically as in all previous models, but at the time of a later extensive redesign (1913) this was changed to mechanical actuation, and the succeeding engines incorporated this feature. All the valve-actuating mechanism was similar to that of the vertical 4, and the engine had the usual compression-release mechanism, the detail design being carried over directly from the 4-cylinder.

Design of the piston followed their previous practice, with wide rings above the pin and shallow grooves below the pin on the thrust face, and with the pin fastened in the piston by a set screw. The piston had four ribs underneath the head (see Figure 13b) radiating from the center and with the two over the pin bosses incorporating strengthening webs running down and joining the bosses. The piston length was reduced by 1 in., thus giving a much less clumsy appearance and, with other minor alterations, a weight saving of 40 percent (see Figures 13b and c). The rods were for the first time made of I-section forgings, a major departure, machined on the sides and hand finished at the ends, with a babbit lining in the big end, the piston pin bearing remaining steel on steel.

At least two different general carburetion and induction systems were utilized, possibly three. One, and most probably the original, consisted of a duplicate of the injection pump of the 4-cylinder fitted to a manifold which ran the length of the engine, with three takeoffs, each of which then split into two, one for each cylinder. Of this arrangement they tried at least two variations involving changes in the location and method of injecting the fuel into the manifold; and there seems to have been an intermediate manifold arrangement, using fuel-pump injection at the middle of the straight side, or gallery, manifold, which was fed additional air at both ends through short auxiliary inlet pipes. This would indicate that with the original arrangement, the end cylinders were receiving too rich a mixture, when the fuel in the manifold was not properly vaporized. Although the exhaust was on the same side of the engine as the inlet system, no attempt was made to heat the incoming charge at any point in its travel. An entirely different system adopted at the time of the complete redesign in 1913 consisted of two float-feed Zenith carburetors each feeding a conventional three-outlet manifold. This carburetor was one of the first of the plain-tube type, that is, with the airflow through a straight venturi without any spring-loaded or auxiliary air valves, and was the simplest that could be devised. When properly fitted to the engine, it gave a quite good approximation of the correct fuel and air mixture ratio over the speed-load running range, although it is considerably more than doubtful that this was maintained at altitude, as is stated in one of the best descriptions of the engine published at the time the carburetors were applied.

The compression ratio of this engine was lowered by almost 20 percent from that of the vertical 4. This, in combination with the low bore-to-stroke ratio, the unheated charge, and the later mechanically operated inlet valve, indicates that the Wrights were now attempting for the first time to secure from an engine something approaching the maximum output of which it was capable.

As the engine originally came out, it continued to utilize only one spark plug in each cylinder. The high-tension magneto had a wide range of spark advance adjustment, which again provided the only control of the engine when equipped with the original fuel pump injection.

The location of the valves and pushrods was similar to that in the 4, so that the cams were immediately adjacent to the camshaft bearings, which were carried in the crankcase ends and in the heavy webs. The camshaft was gear-driven and the cam shape was similar to that of the last 4s, with a quite rapid opening and closing and a long dwell, leaving the valve opening accelerations and seating velocities still quite high.

The crankshaft was a continuation of their basic design of rather light construction, particularly in the webs. The cheeks were even thinner (by 1/4 in.) than those of the 4 although the width was increased by 1/8 in. (see Figure 13e). For the first time they went to a forging, the rough contour type of the time, and utilized a chrome-nickel alloy steel.

Lubrication was by means of the usual gear pump, and the piston and rod bearings continued to be splash-fed. The rod big-end bearing carried a small sharp undrilled boss at the point where, on the other engines, had been located scuppers whose purpose was apparently still to throw lubricating oil on the cylinder wall carrying the more highly loaded side of the piston. The rod big-end bearing was lubricated by a hole on the top of the big-end boss catching some of the crankcase splash, which was then carried to the bearing by a groove.

When the 6-cylinder engine was completely redesigned in 1913 this led to the introduction in late fall of that year of a new model called the 6-60, the 60 designating the rating in horsepower. There is little in the Wright records to show why such a radical revision was thought necessary, but the general history of the period gives a rather clear indication. The competition had caught up to the Wrights in powerplants. Other engines were being installed in Wright airplanes, and Navy log books show these other engines being used interchangeably with those of the Wrights.

Most of the descriptions of the new model published at the time it was introduced concentrate on the addition of the two carburetors and the mechanical operation of the inlet valves, but these were only two of many major changes. The cylinder was completely revised, the intake being moved to the camshaft side of the engine from its position adjacent to the exhaust, so that the two ports were now on opposite sides of the cylinder. By proper positioning of the rocker-arm supports and choice of their length and angles, all valves were made operable from a single camshaft. The shrunk-on steel water jacket cylinder was retained, but the water connections were repositioned so that the water entered at the bottom and came out at the top of the cylinder. Over the life of the 6-cylinder engine several different valve types were used but the published specifications for the model 6-60 called for "cast iron heads"--the old two-piece construction. The piston pins were case hardened and ground and the crankshaft pins and journals were heat treated and ground.

The fuel and oil pumps were removed from the side of the crankcase and a different ignition system was applied, although still of the high-tension spark-plug type which by this time had become general practice on all so-called high-speed internal-combustion engines. A second threaded spark-plug hole was provided in the cylinder head and despite its more common use for other purposes, it is evident that the intention was to provide two-plug ignition. It is doubtful that at the specific output of this engine any power difference would be found between one-and two-plug operation, so that the objective was clearly to provide a reserve unit in case of plug failure. However, it was also used for the installation of a priming cock for starting and because of the prevalence of single-wire ignition systems on existing and illustrated engines, it seems to have been used mostly in this manner, even though dual-ignition systems later became an unvarying standard for aircraft engines.

Viewed externally, the only part of the engine that appears the same as the original 6 is the small lower portion of the crankcase; but what is more visually striking is the beauty of the new lines and extreme cleanness of the exterior design (see Figures 14 and 15). Many of their individual parts had shown the beauty of the sparse design of pure utility but it was now in evidence in the whole. Despite the proven practical value of their other models, this is the only one that can be called a good-looking engine, instantly appealing to the aesthetic sense, even though the vertical 4 is not an ugly engine. The appearance of their final effort, in a field they were originally reluctant to enter and concerning which they always deprecated the results of their own work, was a thing of which a technically trained professional engine designer could be proud.

The 6-60 was continued in production and development until it became the 6-70, and indications are that it eventually approached an output of 80 horsepower.

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.

[Footnote 17: Baker states that the first crankshaft was made from a slab of armor plate and if this is correct the alloy was a rather complex one of approximately .30-.35 carbon, .30-.80 manganese, .10 silicon, .04 phosphorus, .02 sulphur, 3.25-3.50 nickel, 0.00-1.90 chromium; however, all the rest of the evidence, including Orville Wright's statement to Dr. Gough, would seem to show that it was made of what was called tool steel (approximately 1.0 carbon).]

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...."

[Footnote 18: Their intended piston ring tension is not known. Measurements of samples from the 4-and 6-cylinder vertical engines vary greatly, ranging from less than 1/2 lb per sq in. to almost 1-1/4 lb. The validity of these data is very questionable as they apply to parts with unknown length of service and amount of wear. It seems quite certain, however, that even when new the unit tension figure with their wide rings was only a small fraction of that of the modern aircraft piston engine.]

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.

Appendix

Characteristics of the Wright Flight Engines

_1903 _1904-1905 _1908-1911 _1911-1915 First flight Experimental Demonstrations service_ engine[a]_ flights_ and service_ ------------------------------------------------------------------------- Cyl./Form 4/flat 4/flat 4/vertical 6/vertical Bore and stroke (in.) 4×4 4-1/8×4 4-3/8×4 4-3/8×4-1/2 Displacement (cu. in.) 201 214 240 406 Horsepower 8.25-16 15-21 28-42 50-75 RPM 670-1200 1070-1360 1325-1500 1400-1560 MEP 49-53 52-57 70-87 70-94 Weight (lb) 140-180 160-170 160-180 265-300 -------------------------------------------------------------------------