Number 7 SMITHSONIAN ANNALS OF FLIGHT SMITHSONIAN AIR AND SPACE MUSEUM & SERIAL PUBLICATIONS OF THE SMITHSONIAN INSTITUTION The emphasis upon publications as a means of diffusing knowledge was expressed by the first Secretary of the Smithsonian Institution. In his formal plan for the Insti­ tution, Joseph Henry articulated a program that included the following statement: "It is proposed to publish a series of reports, giving an account of the new discoveries in science, and of the changes made from year to year in all branches of knowledge." This keynote of basic research has been adhered to over the years in the issuance of thousands of titles in serial publications under the Smithsonian imprint, com­ mencing with Smithsonian Contributions to Knowledge in 1848 and continuing with the following active series: Smithsonian Annals of Flight Smithsonian Contributions to Anthropology Smithsonian Contributions to Astrophysics Smithsonian Contributions to Botany Smithsonian Contributions to the Earth Sciences Smithsonian Contributions to Paleobiology Smithsonian Contributions to Zoology Smithsonian Studies in History and Technology In these series, the Institution publishes original articles and monographs dealing with the research and collections of its several museums and offices and of professional colleagues at other institutions of learning. These papers report newly acquired facts, synoptic interpretations of data, or original theory in specialized fields. These pub­ lications are distributed by mailing lists to libraries, laboratories, and other interested institutions and specialists throughout the world. Individual copies may be obtained from the Smithsonian Institution Press as long as stocks are available. S. DILLON RIPLEY Secretary Smithsonian Institution The Curtiss D-12 Aero Engine Curtiss D-12-E engine, 435 hp, 1930. (Smithsonian photo A-4593.) SMITHSONIAN ANNALS OF FLIGHT • NUMBER 7 The Curtiss D-12 Aero Engine by Hugo T. Byttebier SMITHSONIAN INSTITUTION PRESS City of Washington 1972 UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON : 1972 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 75 cents Stock Number 4705-000i Acknowledgments This history of the Curtiss D-12 engine is the result of research extend­ ing over several years. It has been made possible through assistance provided by many persons and organizations. From its inception the project received the careful attentions of Mr. Robert B. Meyer, Jr., curator, aero propulsion division, National Air and Space Museum, Smithsonian Institution. Other contributors include the following: Dr. Arthur Nutt, designer of the D-12 engine; Mr. Lee M. Pearson and Mr. Eric Collins of the Bureau of Naval Weapons; Mr. Royal Frey of the Air Force Museum at Wright-Patterson Air Force Base; General James Doolittle; Major G. E. A. Hallett; Mr. Roland Rohlfs; Mr. Erik Hildes-Heim; Mr. Harold Morehouse; Mr. George Page; Mr. Theodore Wright; Technical Sergeant Merle Olmsted; Mr. Steve Wittman; Mr. Alan Phillips; Mr. Paul Matt; Mr. Thomas Foxworth; the Curtiss- Wright and Pratt & Whitney companies; Lieutenant Colonel F. E. Rudston Fell, D.S.O., O.B.E., Wing Commander Norman Macmillan, and the Rolls-Royce and Fairey companies, of Great Britain; Mr. Anto­ nio M. Biedma of Argentina; and General Edmundo Vaca Medrano, former aeronautical attache at the Bolivian Embassy in Buenos Aires. Special thanks are due Mr. William Lewis, who interviewed Charles B. Kirkham, and Mr. Paul E. Garber, historian emeritus, National Air and Space Museum, who reviewed the original manuscript. Contents Page Acknowledgments v Introduction 1 Development of the Curtiss C-12 30 Development of the Curtiss CD-12 43 The Curtiss D-12 Engine 51 The Influence of the D-12 86 vn Introduction he date 28 September 1923 is an important one in the annals of Taviation. On that day, at Cowes in England, the sixth interna­ tional race for the Schneider Cup was held, and on that day the Schnei­ der race changed drastically from a contest to evaluate speed, reliability, and navigability of seaplanes to an all-out, high-speed competition between world powers. This change was brought about by two sleek, diminutive Curtiss racers that had been brought from America to challenge the European sea­ planes. The Curtiss planes ran away from their competitors so convinc­ ingly that even those who had foreseen the result were surprised at the extent of the victory. The apparent ease of the American win was forcefully significant, and the observant press was quick to praise the excellent combination of flying skill and advanced technology. The most important element of the victory soon was pinned down to the Curtiss D-12 engines. They proved to be powerful, light, and reliable, and they introduced a new dimension in power plants by having a frontal area for which one commentator found the correct adjective— "piccolissimo." The D-12 engine is an excellent example of a first-line design in the history of aero engine development. It was the result not only of brilliant inspiration but of patient application, perseverance, and hard work on the part of its developers. To understand the origins of the D-12 it is necessary to go back to the autumn of 1915. World War I was about to enter its second year, and to everyone concerned with aeronautics it had become obvious that the Germans had taken a lead in aero engine development. The quickly rising importance of the air arm caused the Allies to become increasingly aware of their shortage of adequate engines, especially of high-power units that would be decisive in the battle for aerial supremacy. Several American engineering firms tried to develop aero engines that would be acceptable to the Allied flying services, but only a few were able to secure orders. The inherent problems of aircraft engine devel- opment were made more acute by wartime difficulties. The fighting forces were continually exerting pressure on the technical limitations of an already highly sophisticated piece of machinery, thus imposing an unrelenting stress on designers and manufacturers. One of the greatest efforts by an American firm toward the achieve­ ment of a first-class, high-power aero engine was undertaken by the Wright Aeronautical Corporation, successor to the original Wright Company. Wright Aeronautical, whose directors had seen the great future that lay in engine manufacture, already had gained control of the Simplex Automobile Company; and it had acquired the services of that firm's chief engineer, Henry M. Crane, who had built up quite a reputation because of the excellence of his Crane-Simplex motor cars. In making the decision to embark upon the manufacture of high- power aero engines, the executives of Wright Aeronautical were aware that no quick results would be reached by starting on the design of yet another type of engine. Rather, they believed that the quickest and safest way to achieve big production potential was to build an engine based upon the most promising design being evolved in Europe. Thus, the firm sent Crane and another representative to France to evaluate the different types of aero engines being developed there and to deter­ mine the design best suited for production in the United States. The story of the D-12 engine starts in the fall of 1915 with the arrival of the Wright mission in France. At that time a novel type of aero engine was passing through its tests with fair results. It had been built according to the ideas of Marc Birkigt, a young Swiss engineer. Some ten years earlier Birkigt had linked his talent to capital from Spain to enable that country to compete in the field of automobile manu­ facture. In this he had succeeded brilliantly, with the trade name Hispano-Suiza having become well known in the automotive world. At the start of World War I, Birkigt had begun building an aviation engine along original ideas evolving from his company's successful auto­ mobile racing engines. When Wright's representatives arrived this aero engine was passing through a period of intense development prior to its being launched on large-scale production. The usual water-cooled engine at that time consisted mainly of a very stiff and relatively heavy crankcase upon which steel cylinders were bolted individually. Each cylinder was surrounded by its own cooling mantle, while the valve mechanism in the head was, in most cases, left totally exposed. Birkigt's Hispano-Suiza engine departed radically from that type of construction. Its cylinders were formed from an aluminum, single-block casting into which were screwed four forged steel barrels FIGURE 1 .—Cross-sectional arrangement, Hispano-Suiza, 200 hp, of World War I. (Smithsonian photo A-53098.) that were threaded on the outside for their entire length (Figures 1, 2). These barrels were closed at the top, forming a compartment that served as a combustion chamber. Because the barrels were screwed into the aluminum blocks, the passages for the cooling water were cored from the block; as a result, the heat imparted by the combustion process to the cylinder barrel had to pass through a part of the aluminum casting before it was affected by the cooling medium. This was one of the disadvantages of the design. A very light crankcase was attached to the cylinder blocks; thus, the Hispano could be described as having the crankcase hung on the cylin­ ders instead of having the cylinders on the crankcase. The valves, two per cylinder and mounted vertically in the cylinder head, were actuated 3 FIGURE 2.—Hispano-Suiza, Type I, 150 hp. (Smithsonian photo A-4593B.) by a single overhead camshaft which operated the valve stems directly, without the interposition of either pushrods or rockers. The entire valve mechanism was enclosed in an oiltight cover that fitted closely over the cylinder block. The overall design resulted in a compact, clean, light, and apparently simple unit which inevitably attracted the attention and admiration of the engineering world.