Quest for Performance: The Evolution of Modern Aircraft
Chapter 2: Design Exploration, 1914-18
[7] A multitude of aircraft types were tested in combat in the war period 1914-18, and literally hundreds of prototypes were built and flown. These numbers become believable when one considers that the prototype of a fighter aircraft could be designed, constructed, and test flown within a period of a few weeks. In contrast to the essentially job-shop approach to aircraft construction that prevailed prior to 1914, an aircraft industry was developing, nurtured by large expenditures of money by the belligerent governments. The engineering principles of aircraft design were also beginning to take shape. Government laboratories, such as the Royal Aeronautical Establishment in England, contributed greatly to the foundations of aeronautical engineering. Scientific and engineering laboratories also existed in France, Italy, and Germany; and the National Advisory Committee for Aeronautics (NACA) was established in the United States by act of Congress in 1915. The results of NACA research, however, did not begin to have a significant impact on aircraft design until the mid-to late 1920's. In contrast to the European powers, the United States had essentially no air force and no real aircraft industry when war was declared on Germany in April 1917. Accordingly, the United States relied almost entirely on tried and proven European aircraft designs. Many of these aircraft were produced by European companies for use by the American Expeditionary Force, while others were manufactured under license in the United States.
Aircraft types of amazing variety were built in the continual quest for better fighting machines. Monoplanes, biplanes, and triplanes were employed in military operations at various stages of the war, and several quadruplanes were tested in prototype form. The wings of most of these aircraft were supported externally by a combination of wires and struts, although several designers developed aircraft with internally [8] braced cantilever wings. Perhaps the most notable was the Dutch designer Anthony H. G. Fokker, who supplied many cantilever-wing fighter aircraft to the German air force. Both pusher- and tractor-type engine installations were employed, and multiengine bombers frequently utilized a combination of pusher and tractor powerplant installations. The pusher-type configuration was used extensively as a fighter, particularly by the British, in the early stages of the war. The internal structure of most of the aircraft consisted of a wooden framework braced with wire and covered externally with cloth. Some aircraft employed a mixture of metal and wood in their construction, and experiments were conducted with all-metal aircraft whose wings were internally braced. Dormer and Junkers in Germany were among the pioneers in all-metal aircraft construction. The types of alloys available at the time, however, did not lend themselves to the light weight required in aircraft design, and the concepts of light, stressed-skin metal construction lay in the future. All-metal aircraft did not play an important role in World War 1. The use of plywood as an external covering, together with a minimum of internal structure, particularly in fuselage design, was also employed by several manufacturers. This type of construction, called monocoque, is described in more detail later.
Two vastly different engine types were employed in World War I aircraft: the stationary engine, usually water cooled, and the rotary engine. Water-cooled engines of 4, 6, 8, and 12 cylinders were extensively utilized. In concept, these engines were not unlike the present-day automobile engine; a few of the in-line engines were air cooled. The rotary engine had cylinders arranged radially around a crankshaft; but unlike the modern radial engine, the crankshaft was fixed to the aircraft, and -the cylinders and crankcase, with propeller attached, rotated around it. This engine type was relatively light and was cooled easily by engine rotation, advantages that accounted for its extensive use. The rotary engine, perfected in France, had a primitive control system and introduced undesirable gyroscopic moments in the aircraft that adversely affected flying characteristics. The rotary engine is a curiosity that rapidly vanished from the scene following the close of World War I.
The design of a successful aircraft, even today, is not an exact science. It involves a combination of proven scientific principles, engineering intuition, detailed market or mission requirements, and perhaps a bit of inventiveness and daring. Aircraft design during World War I was more inventive, intuitive, and daring than anything else. [9] Prototypes were frequently constructed from full-size chalk drawings laid out on the factory floor. The principles of aerodynamics that form so important a part of aircraft design today were relatively little understood by aircraft designers during the war. An indication of the state of the art in this area is given in the textbooks by Barnwell and Sayers published in 1917 (ref. 27) and by Klemin in 1918 (ref. 79). Structural design was haphazard, and stress analysis did not become an accepted part of the design process in many companies until midway through the World War. In an area of engineering in which structural strength, light weight, and aerodynamic efficiency are so important, it is indeed surprising that a number of relatively good aircraft were produced.
The evolution of the airplane during the turbulent years of World War I is described briefly in the following sections of this chapter. Fighter aircraft, which usually reflected the latest in design refinements, are considered first, after which consideration is given to heavy bombers and army cooperation aircraft.