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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 1
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- Introduction
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- [3] The first flight
of a powered, heavier-than-air aircraft was, of course, made by
Orville Wright on December 17, 1903. In the decade following this
historic event, aircraft development was characterized by a
proliferation of types, conceived primarily by inventors of
varying degrees of competence. A few of these aircraft flew
moderately well, some poorly, and some not at all. There was
little scientific and engineering foundation for aircraft design,
and many aircraft built during this period were constructed by
nontechnical people as amateur, backyard-type projects. Most of
these aircraft were designed for no other mission than to fly, and
most were employed for exhibition purposes, races, or other
spectacular types of events. No definitive aircraft configuration
types had emerged by 1914, the beginning of World War I, and
flying was regarded by most intelligent people-if at all-as a sort
of curiosity not unlike tightrope walking at the circus. These
viewpoints were utterly changed by the tactical and strategic uses
of aircraft in the First World War. The demands of combat
aviation, together with the opposing powers constantly vying for
air superiority, resulted in the development of the airplane from
a curiosity in 1914 to a highly useful and versatile vehicle,
designed to fulfill specific roles, by the end of the war in
November 1918.
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- The evolution of propeller-driven
airplanes from 1914 to the present falls into five distinct,
identifiable time periods that provide the framework for chapters
2 through 6. Significant design trends, as evidenced by changes in
aircraft physical and performance characteristics, are discussed
in chapter 7. Chapters 2 to 7 are restricted to a discussion of
aircraft designed to operate from land-based fields and airports.
Consequently, the flying boat, once an important class of aircraft
but now almost extinct, is not included in these chapters;
however, a brief description of the evolution of this unique and
picturesque type of aircraft is contained in chapter 8.
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- [4] As indicated in
the preface, the discussion is restricted primarily to aircraft
types developed in the United States. Chapter 2 on World War I
aircraft is an exception; European aircraft form the basis for the
material presented in this chapter since the United States
developed no significant combat aircraft during the war years
1914-18.
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- The aircraft discussed in the following
chapters, together with some of their physical and performance
characteristics, are listed in tables I to IV in appendix A. The quantities tabulated are defined in the list
of symbols contained in appendix B, and generally require no further elaboration.
However, three of the aircraft aerodynamic characteristics
presented deserve some further discussion. These are the zerolift
drag coefficient CD,0, the drag area
f, and the value of the maximum lift-drag ratio
(L/D)max.
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- The zero-lift drag coefficient
CD,O is a nondimensional number that relates the
zero-lift drag of the aircraft, in pounds, to its size and the
speed and altitude at which it is flying. Generally speaking, the
smaller the value of this number, the more aerodynamically clean
the aircraft. For example, the value Of CD,O for the North
American P-51 "Mustang" fighter of World War II fame is about
0.0161 (table
III) as compared with about 0.0771
for the Fokker E-III fighter of World War I (table I). Accordingly, the P-51 is a much cleaner aircraft
than the Fokker E-III.
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- The drag area f is the product of the
zero-lift drag coefficient and the wing area. The resulting number
is of interest because it represents, approximately, the area of a
square flat plate, or disc, held normal to the direction of
flight, which has the same drag in pounds as the aircraft at a
given speed and altitude. (The relationship is exact for a
flat-plate drag coefficient of 1.0. According to reference
72, the actual drag coefficient of such a plate is
1.171.) For example, the drag area of the P-51 fighter is 3.57
square feet as compared, with, 12.61 square feet for the much
smaller Fokker E-III of World War I. The improvement in
aerodynamic efficiency over the 25-year period separating the two
aircraft is obvious. Comparisons of the drag area of aircraft of
different periods designed for the same missions can thus provide
some indication of comparative aerodynamic cleanness or
streamlining. Furthermore, the maximum speed is approximately
proportional to the cube root of the ratio of the power to the
drag area (ref. 90). The larger this ratio, the higher the top
speed.
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- The value of the maximum lift-drag ratio
(L/D)max is a measure of the aerodynamic cruising efficiency
of the aircraft. In essence, it is inversely related to the amount
of thrust required to sustain a given [5] weight in the air
and is proportional to the miles of flight per pound of fuel for a
given propulsion system efficiency and aircraft weight. The higher
the value of (L/D)max, the higher the
cruising efficiency of the aircraft. The value of the maximum
lift-drag ratio is a function of the zero-lift drag coefficient
and the drag associated with the generation of lift. The
drag-due-to-lift is, in turn, related to the wing aspect ratio
(basically, the ratio of span to average chord) and becomes
smaller as the aspect ratio is increased. The value of the aspect
ratio A is given for each of the aircraft listed in the tables.
Values of (L/D)max, for
propeller-driven aircraft vary from about 6.4 for early World War
I fighters to about 16 for transports such as the Lockheed 1049G
of the 1950's. The values Of CD,O
and (L/D)max given in the
tables were estimated from published aircraft performance data
according to the methods described in appendix C.
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- The references used in obtaining the
characteristics of the aircraft are listed in tables I-IV or are specifically cited in the text.
Jane's All the World's
Aircraft (refs. 1-16) has been used extensively in compiling the
characteristics of the aircraft presented in the tables. This
definitive series of books has been published each year since 1909
and forms an invaluable source for anyone interested in aircraft
development. A few references that provide useful background
material, but which are not specifically cited in the text, are
offered for additional reading on the subject of aircraft
development. For convenience, references 17 to
124 are listed
alphabetically.
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