The Wright Brothers' Engines and Their Design

by Leonard S. Hobbs

Figure 2.—First flight engine, 1903, underside and flywheel end. (Photo courtesy Science Museum, London.)

In late 1947 work on the Educational and Musical Arts drawings was initiated under the direction of Louis P. Christman and carried through to completion by him. Christman has stated that Orville Wright was critical of the Science Museum drawings but just what he thought incorrect is not known. Whatever his reasons, he did encourage Christman to undertake the major task of duplication. Christman worked directly with Orville Wright for a period of six weeks and had access to all the records and parts the Wrights had preserved. The resultant drawings are also very complete and, regardless of the differences between these two primary sets, both give a sufficiently accurate picture of the first engine for all purposes except that of exact reproduction in every detail.

There exists a still unsolved puzzle in connection with what seems to be yet another set of drawings of the first engine. In December 1943, in writing to the Science Museum telling of his decision to have the airplane and engine brought back to the United States, Orville Wright stated, "I have complete and accurate drawings of the engine. I shall be glad to furnish them if you decide to make a replica."[7] No trace of these particular drawings can be found in any of the museums, institutions, or other repositories that normally should have acquired them and the executors of Orville Wright's estate have no record or knowledge of them. The date of his letter is four years before the Dayton drawings were commenced; and when Christman was working on these with Orville Wright they had copies of the Science Museum drawings, with complete knowledge of their origin, yet Christman has no knowledge of the drawings referred to in Orville's letter to the Museum. Finally, the evidence is quite conclusive that there were no reproducible or permanent drawings made at the time the first engine was constructed, and, of course, the reconstructed engine itself was sent to England in 1928 and not returned to this country until 1948.[8]

The Engine of the First Flight, 1903

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.