Figure 4.—First flight engine, 1903, left side and rear views, with dimensions. (Drawing courtesy Howell Cheney Technical School.)
LEFT SIDE VIEW.
REAR VIEW
The selection of aluminum for the material was an integral part of the basic design decision. Despite the excellence and accuracy of the castings that could be obtained, there was nevertheless a minimum dimension beyond which wall thickness could not be reduced; and the use of either one of the two other proven materials, cast iron or bronze, would have made the body, as they called it, prohibitively heavy. The use of aluminum was not entirely novel at this time, as it had been utilized in many automobile engine parts, particularly crankcases; but its incorporation in this rather uncommon combination represented a bold step. There was no choice in the matter of the alloy to be used, the only proven one available was an 8 percent copper 92 percent aluminum combination.
By means of the proper webs, brackets and bosses, the crankcase would also carry the crankshaft, the rocker arms and bearings, and the intake manifold. The open section of the case at the top was covered with a screw-fastened thin sheet of cold-rolled steel. The main bearing bosses were split at a 45° angle for ease of assembly. The engine support and fastening were provided by four feet, or lugs, cast integral on the bottom corners of the case, and by accompanying bolts (Figure [2]). Although the crankcase continued to be pretty much the "body" of the internal combustion aircraft engine throughout its life, the Wrights managed to incorporate in this original part a major portion of the overall engine, and certainly far more than had ever previously been included.
The design of the cylinder barrel presented fairly simple problems involving not much more than those of keeping the sections as thin as possible and devising means of fastening it and of keeping the water jacket tight. They saved considerable weight by making the barrel quite short, so that in operation a large part of the piston extended below the bottom of it; but this could be accepted, as there were no rings below the piston pin (Figure [6]). The barrel material, a good grade of cast iron, was an almost automatic choice. In connection with these seemingly predetermined decisions, however, it should be remembered that their goal was an engine which would work without long-time development, and that, with no previous experience in lightweight construction to guide them they were nevertheless compelled to meet a weight limit, so that the thickness of every wall and flange and the length of every thread was important.
With the separate cylinder barrel they were now almost committed to a three-piece cylinder. It would have been possible to combine the barrel and head in a one-piece casting and then devise a method of attachment, but this would have been more complex and certainly heavier. For housing the valves, what was in effect a separate cylindrical, or tubular, box was decided upon. This would lie across the top of the cylinder proper at right angles to the cylinder axis, and the two valves would be carried in the two ends of this box. The cylinder barrel would be brought in at its head end to form a portion of the cylinder head and then extended along its axis in the form of a fairly large boss, a mating boss being provided on one side of the valve box. The cylinder barrel would then be threaded into the valve box and the whole tightened or fastened to the crankcase by means of two sets of threads, one at each end of the barrel proper. This meant that three joints had to be made tight with only two sets of threads. This was accomplished by accurate machining and possibly even hand fitting in combination with a rather thick gasket at the head end, one flat of which bore against two different surfaces. This can be seen in Figure [6], where the circular flange on the valve box contacts both the crankcase and the cylinder barrel. Altogether it was a simple, light, and ingenious solution to a rather complex problem.