[15]: Considerable doubt surrounds Whitehead's actual flight accomplishments, but Pruckner's engines were certainly used, as several were sold to early pioneers, including Charles Wittemann. It is probable that the specific power output was not very great, for the air-cooled art of this time was not very advanced and Pruckner had a rather poor fin design. But the change to water cooling eliminated this trouble, and the engines were most simple, should have been relatively quite light, and with enough development could probably have been made into sufficiently satisfactory flying units for that period.
[16]: A drawing of the camshaft is held by The Franklin Institute.
[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).
[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.
[19]: The Papers of Wilbur and Orville Wright, volume 2, Appendix.
[20]: The Wrights apparently never applied for an engine patent of any kind. This no doubt grew out of their attitude of regarding the engine as an accessory and deprecating their work in this field. A reasonably complete patent search indicates that this particular cam device has never been patented, although a much more complex arrangement accomplishing the same purpose was patented in 1900, and a patent application on a cam-actuating mechanism substantially identical to that of the Wrights and intended for use in a golf practice apparatus is pending at the present time.
[a]: Concurrently with the Wrights' first engine work, Manly was developing the engine for the Langley Aerodrome, and a comparison of the Wrights' engine development with that of Manly is immediately suggested, but no meaningful comparison of the two efforts can be drawn. Beyond the objective of producing a power unit to accomplish human flight and the fact that all three individuals were superb mechanics, the two efforts had nothing in common. The Wrights' goal was an operable and reasonably lightweight unit to be obtained quickly and cheaply. Manly's task was to obtain what was for the time an inordinately light engine and, although the originally specified power was considerably greater than that of the Wrights, it was still reasonable even though Manly himself apparently increased it on the assumption that Langley would need more power than he thought. 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. A description of the Langley-Manly engine and the history of its development is contained in Smithsonian Annals of Flight number 6, "Langley's Aero Engine of 1903," by Robert B. Meyer (xi+193 pages, 44 figures; Smithsonian Institution Press, 1971)