Basic Aerodynamics

This was the last first in aviation, we had always said, a milestone, and that made it unique. Would we do it again? No one can do it again. And that is the best thing about it.

Jeana Yeager and Dick Rutan “Voyager” 1987

1.1 Introduction

Aerodynamics is the study of the flow of air around and within a moving object. Its main objective is understanding the creation of forces by the interaction of the gas motion with the surfaces of an object. Aerodynamics is closely related to hydrody­namics and gasdynamics, which represent the motion of liquid and compressible-gas flows, respectively.

Aerodynamics is the essence of flight and has been the focus of intensive research for about a century. Although this might seem to be a rather long period of development, it is really quite short considering the time span usually required for the formulation and full solution of basic scientific problems. In this relatively short time, mankind has advanced from the first gliding and primitive-powered airplane flights to interplanetary spaceflight.

Perhaps the most important motivation for this rapid development is the challenge to the human spirit represented by manned flight. However, practical needs also have strongly affected these endeavors, and we often find periods of rapid growth in aerodynamic knowledge associated with the solution of compel­ling problems in transportation, military applications, industry, and even sporting competitions. Figure 1.1 illustrates this growth in terms of the maximum speed attained by manned aircraft. Speed is a key measure of performance in almost every aspect of flight. It is of obvious vital importance in commercial flight as well as in military operations.

Even a casual study of the history of aviation yields considerable insight into the pressures that have motivated periods of almost explosive growth in the technology of flight. Much of the increase in speed during the 1940s and several subsequent decades was motivated strongly by military considerations. However, notice that two radical departures (shown as dashed lines in Fig. 1.1) from the curve for conven­tional airplanes occur in the 1920s and in the two decades between 1940 and 1960.

Figure 1.1. Evolution of aircraft speeds showing nearly exponential growth. Jet propelled aircraft speeds level off in the 1980’s as turbojet performance Mach number limits are exceeded.

The phenomenal growth in speed from the 100 miles per hour (mph) range to over 400 mph that occurred during the period 1920-1932 was spurred by international competition in the guise of the Schneider Trophy Seaplane Races. Rapid progress not only in high-speed aerodynamics but also in aeropropul – sion took place during that period. Similar growth occurred during the 1950s in supersonic flight. More recently, the international competitive spaceflight activities brought about rapid growth in propulsion, electronics, and materials, if not much in aerodynamics. There are signs that a new international competition is underway in the area of hypersonic aerodynamics and related technologies as policy decisions are made regarding the need for low-cost single-stage-to-orbit (SSTO) space vehicles.

Several of the key historical aircraft identified in Fig. 1.1 are illustrated in Figs. 1.2-1.12. Progressive improvements in aerodynamic configuration are apparent. In this textbook, we focus on the physical laws and related analytical and computa­tional methods used to arrive at the aerodynamic-problem solutions implied in the evolving vehicle shapes depicted in the aircraft illustrated in this chapter.

Figure 1.2. Beginnings: The 1903 Wright Flyer (Smithsonian Institute).

Figure 1.4. Outright winner of the Schneider Trophy: Supermarine S6B (1931).

Figure 1.5. Supermarine Spitfire Mk II – an outcome of the Schneider Trophy racing seaplane research.

Figure 1.6. Messerschmitt ME 262. The first operational jet-propelled aircraft, 1944.

Figure 1.8. X-15: First hypersonic airplane.

Figure 1.10. Boeing 787 Dreamliner transonic jet transport during first flight test.

Figure 1.12. NASA X-43 hypersonic test vehicle using Scramjet propulsion.

Although it is not required to gain an understanding of the text material, the student is encouraged to supplement the text coverage with a parallel study of the history of aeronautics. Those interested in engaging in creative work as their careers in aeronautics develop will benefit greatly from this extra effort. References at the end of each chapter and frequent historical notes (usually provided as footnotes) serve as a guide for such an in-depth study. Much useful material is now available on the Internet and other large-scale computer networks.

In solving the problems of aerodynamics, those involved have been required to create basic technology along with the associated mathematical and experimental methodology. It is vital that the student understand the framework of this technology in detail and learn not only the application of the tools but also the deeper physical meaning they represent. This textbook is designed to promote this type of critical study of the subject. A carefully paced discussion of the traditional tools, such as mathematical analysis and experiment along with modern computational methods, is used to provide the student a broad understanding of both the physical meaning and the modern implementation of a wide variety of techniques and problem solu­tions. It is significant that the book outline follows closely the historical outline in terms of the need for each successive new idea and problem solution.