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Quest for Performance: The Evolution
of Modern Aircraft
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- Part I: THE AGE OF
PROPELLERS
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- Chapter 2: Design Exploration,
1914-18
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- Background
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- [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.
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- 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.
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- 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.
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- 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.
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- 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.
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