Leonard S. Hobbs

“ They had obtained a power increase of about 40 percent, with a weight decrease of 10 percent, and now had an engine whose design was almost standard form for good internal combustion engines for years to come. In fact, had they split the crankcase at the crankshaft center line and operated the inlet valves mechanically, they would have had what could be termed a truly modern design. They needed more cylinder cooling, both barrel and head, particularly the latter, and an opened-up induction system for maximum power output, but this was not what they were yet striving for. ”

“ A fundamental that every professional designer knows is that it is with this particular sort of part that weight gets out of control; even an additional 1/16 in., if added in a few places, can balloon the weight. With their usual foresight and planning, the Wrights carefully checked and recorded the weight of each part as it was finished, but even this does not quite explain how these two individuals, inexperienced in multicylinder engines—much less in extra-light construction—could, in two months, bring through an engine which was both operable and somewhat lighter than their specification. ”

“ The rate of fuel supply to the engine would decrease as the level in the fuel tank dropped, but as the head being utilized was a matter of several feet and the height of the supply tank a matter of inches, the fuel-air ratio was still maintained well within the range that would ignite and burn properly in the contemplated one-power condition of their flight operation. ”

“ Despite the fact that the Wright shop engine was not a flight unit, it is interesting both because it was a well designed stationary powerplant with several exceedingly ingenious features, and because its complete success was doubtless a major factor in the Wrights' decision to design and build their own first flight engine. ”

“ 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 (p. 13) model, the 6-70, "which had been giving trouble in this regard." ”

“ Since a simple ignition switch was all that was required to stop the power output, regardless of whether a fuel-control valve or a spark-advance control was used, it must be concluded that the primary function of the compression release was to facilitate starting, and any other useful result was something obtained at no cost. The compression release was later generally abandoned, and until the advent of the mechanical starter during the (p. 31) 1920s, starting an engine by "pulling the propeller through" could be a difficult task. ”

“ 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. ”

“ Indeed, working motors of one-half this weight per horsepower [9 pounds per horsepower] have been constructed by several different builders." It is quite obvious that with their general knowledge and the experience they had acquired in designing and building a successful shop engine for their own use, they had no cause to doubt their ability to supply a suitable powerplant when the need arose. After the characteristics of the airframe had been settled, and the engine requirements delineated in rather detailed form, they had reached the point of decision on what they termed the motor problem. ”

“ The piston pin was locked in the piston by the usual set screw, but here no provision was made for the alternate practice of clamping the rod on the pin. This piston-pin setscrew construction had become a standard arrangement in automobile practice. The piston rings were the normal wide design of that time, with what would now be considered a low unit pressure.
Quite early in the life of this engine model the practice was initiated of incorporating shallow grooves in the surface of the more highly loaded thrust face of the piston below the piston pin to provide additional lubrication. ”

“ His first engines were designed by a clever engineer, Anton Pruckner, who left at the end of 1901, after which Whitehead himself became solely responsible for them. It was stated that the Wrights visited the Whitehead plant in Bridgeport, Connecticut, and that Wilbur remained for several days, spending his time in their machine shop. This was not only categorically denied by Orville Wright when he heard of it but it is quite obvious that the 1903 or any other of the Wright engine designs bears little resemblance to Pruckner's work. ”

“ The advantages of simplicity and a reduced number of parts still existed, but there also was a sizable power increase to be had which possibly would have more than balanced off the increased cost and weight. They did not utilize mechanical operation until after a major redesign of their last engine model. Very possibly the answer lies in the phenomenon of fuel detonation. This was only beginning to be understood in the late 1920s, and it is quite evident from their writings that they had little knowledge of what made a good fuel in this respect. ”

“ Much later, when this type of vibration became understood, it was found virtually impossible, with power outputs of any magnitude, to design an undampened shaft, within the space and weight limitations existing in an ordinary engine, strong enough to withstand the stress generated when the frequency of the imposed vibration approximated the natural frequency of the shaft. ”

“ 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. ”

“ A rather thorough stress analysis of the rod shows it to compare very favorably with modern practice. In the absence of an indicator card for the 1903 engine, if a maximum gas pressure of five times the MEP is assumed, the yield-tension factor of safety is measurably higher than that of two designs of piston engines still in wide service, and the column factor of safety only slightly less. The shear stresses in the brazed and threaded joints are so low as to be negligible. ”

“ This maximum-capacity theory of valve operation, with its neglect of acceleration forces and seating velocities, may well have been at least partially if not largely the cause of their exhaust-valve troubles and the seemingly disproportionate amount of development they devoted to this part, as reported by Chenoweth, although it is also true that the exhaust valve continued to present a problem in the aircraft piston engine for a great many years after, even with the most scientific of cam designs. ”

“ This is most probably because in the early flight years, when the Wrights were so meticulously measuring and recording technical information on the important factors affecting their work, the flights were of such short duration that fuel economy was of very minor importance. After success had been achieved, they ceased to keep detailed records on very much except their first interest—the flying (p. 64) machine itself—and when the time of longer flights arrived, the fuel consumption that resulted from their best engine design efforts was simply accepted. ”

“ Each cylinder was fastened to the crankcase by four nuts on studs driven into the aluminum case. Valves and rocker arms were similar to those of the early engines, the automatic inlet valve being retained. The continued use of the two-piece valve is not notable, even though one-piece forgings were available and in use at this time; the automobile continued for many years to use this construction. The camshaft was placed at the bottom of the engine, inside the crankcase, and the rocker arms were actuated by pushrods which were operated by hinged cam followers. ”

“ The cylinder, of cast iron quite finely and completely finned for its (p. 65) day, was air-cooled, or rather, air-radiated, as there was no forced circulation of air over it, the atmosphere surrounding the engine simply soaking up the dissipated heat. Although this was possibly a desirable adjunct in winter, inside the small shop in Dayton, the temperature there in summer must have been quite high at times. The operating fuel was city illuminating gas, which was also utilized to heat, by means of a burner, the ignition tube. ”

“ The make-and-break system, on the other hand, required for each cylinder what was physically the equivalent of a spark plug, that is, a moving arm and contact point inside the cylinder, a spring-loaded snap mechanism to break the contact outside the cylinder, and a camshaft and cams to actuate the breaker mechanism at the proper time. Furthermore, as the Wrights applied it, the system required dry cells and a coil for starting, although these did not accompany the engine in flight. ”

“ Over the years, the Wrights seem to have made many changes in the engine: the bore was varied at different times, rod assembly methods were altered, and rod ends were changed from bronze to steel. Chenoweth states that on later engines an oil-control ring was added on the bottom of the piston, necessitating a considerable increase in the length of the cylinder barrel. This arrangement could not have been considered successful, as it apparently was applied to only a limited number of units and was not carried over to the later 6-cylinder engine model. ”

“ Consequently, the acquisition of a powerplant intended for actual flight was considerably deferred.
Nevertheless, they were continuously considering the power requirement and its problems. In his September 1901 lecture to the Western Society of Engineers, Wilbur Wright made two statements: "Men also know how to build engines and screws of sufficient lightness and power to drive these planes at sustaining speed"; and in conjunction with some figures he quoted of the required power and weight: "Such an engine is entirely practicable. ”

“ The fuel pump was not necessarily always used, as the engine in some of the illustrations did not incorporate one, the fuel apparently being fed by gravity, as on the original engine. Chenoweth describes an arrangement in which exhaust heat was applied to the inlet manifold to assist the fuel vaporization process, but it is believed that this was one of the many changes made in the engine during its lifetime and not necessarily a standard feature. ”

“ 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. ”

“ The rocker arm (102, Figure 6) is probably the best example of a small part which met all of their many specific requirements with an extreme of simplicity. It consisted of two identical side pieces, or walls, of sheet steel shaped to the desired side contour of the assembly, in which were drilled three holes, one in each end, to carry the roller axles, and the third in the approximate middle for the rocker axle shaft proper. This consisted of a piece of solid rod positioned by cotter pins in each end outside the side walls (see Figure 5) . ”

“ The Wrights were meticulous about checking the actual performance of their engines but at that time ratings generally were seldom authenticated and even when different engines were tried in the same airplane the results usually were not measured with any accuracy or recorded with any permanency. There is evidence that the competition became effective enough to compel the complete redesign of their engine so that it was essentially a new model. ”

“ 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. ”

“ 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 ”

“ 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. ”

“ Actually, of course, they never went larger than a 4-3/8-in. cylinder bore, and later aircraft engine experience would seem generally to confirm their judgment, for with the piston engine it has always been much more difficult to make the larger bores operate satisfactorily at any given specific output.
While the 90°V, 8-cylinder arrangement would have enabled them to utilize a great number of the 4-cylinder-engine parts, it would have given them a somewhat larger engine than was their apparent desire, unless they reduced the cylinder size. ”

“ The cams were machined as separate parts and then sweated onto the shaft. Their shape shows the principal concern in the design to have been obtaining maximum valve capacity—that is, a quite rapid opening with a long dwell. This apparent desire to get rid of the exhaust gas quickly is manifested again in the alacrity with which they adopted a piston-controlled exhaust port immediately they had really mastered flight and were contemplating more powerful and more durable engines. ”

“ The cost and time required were very much greater than the Wrights expended. He ended up with an engine of extraordinary performance for its time, containing many features utilized in much later important service engines. His weight per horsepower was not improved upon for many years. The Wrights' engine proved its practicability in actual service. The Manly engine never had this opportunity but its successful ground tests indicated an equal potential in this respect. ”