Quest for Performance: The Evolution of Modern Aircraft
Chapter 1
[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.
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.
[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.
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.
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.
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.
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.
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.