Preface
This book is intended to help students and practicing engineers to gain a greater physical understanding of aerodynamics. It is not a handbook on how to do aerodynamics, but is motivated instead by the assumption that engineering practice is enhanced in the long run by a robust understanding of the basics.
A real understanding of aerodynamics must go beyond mastering the mathematical formalism of the theories and come to grips with the physical cause-and-effect relationships that the theories represent. In addition to the math, which applies most directly at the local level, intuitive physical interpretations and explanations are required if we are to understand what happens at the flowfield level. Developing this physical side of our understanding is surprisingly difficult, however. It requires navigating a conceptual landscape littered with potential pitfalls, and an acceptable path is to be found only through recognition and rejection of multiple faulty paths. It is really a process of argumentation, thus the “arguing” in the title. This kind of argumentation is underemphasized in other books, in which the path is often made to appear straighter and simpler than it really is. This book explores a broader swath of the conceptual landscape, including some of the false paths that have led to errors in the past, with the hope that it will leave the reader less likely to fall victim to misconceptions.
We'll encounter several instances of serious misinterpretations of mathematical theory that are in wide circulation and of erroneous physical explanations that have found their way into our folklore. In any case where a misconception has been widely enough propagated, the “right” explanation would not be complete without the debunking of the “wrong” one. I have tried to do this kind of debunking wherever it seemed appropriate and have not hesitated to say so when I think something is wrong. This is part of what makes aerodynamics so much fun. It's one of those little perversities of human nature that coming up with a good explanation is much more satisfying when you know there are people out there who have got it wrong. But debunking bad explanations serves a pedagogical purpose as well, because the contrast provided by the wrong explanation can strengthen understanding of the right one.
This effort devoted to basic physical rigor and avoiding errors comes at a cost. We'll spend more time on some topics than some will likely think necessary. I realize some parts of the discussion are long and are not easy, but I hope most readers will find it worth the effort.
We are now well into what I would call the computational era in aerodynamics, made possible by the ever-advancing capabilities of computers. In the 1960s, we began to calculate practical numerical solutions to linear equations for inviscid flows in 3D. In the 1970s, it became economical to compute solutions to nonlinear equations for inviscid transonic flows in 3D and to include viscous effects through boundary-layer theory and viscous/inviscid coupling. By the 1990s, we were routinely calculating solutions to the Reynolds-averaged Navier-Stokes (RANS) equations for full airplane configurations. These computational fluid dynamics (CFD) capabilities have revolutionized aerodynamics analysis and design and have made possible dramatic improvements in design technology. CFD is now such a vital part of our discipline that this book would not be complete if it did not address it in some way. While this is not a book about CFD methods or about how to use CFD, there are conceptual aspects of CFD that are relevant to our focus, and these are considered in chapter 10.
I believe that although we now rely on CFD for much of our quantitative work, it is vitally important for a practicing engineer to have a sound understanding of the underlying physics and to be familiar with the old simplified theories that our predecessors so ingeniously developed. These things not only provide us with valuable ways of thinking about our problems, they also can help us to be more effective users of CFD.
The unusual scope of the book is deliberate. The book is not intended to be a handbook. Nor is it intended as a substitute for the standard textbooks and other sources on aerodynamic theory, as I have omitted the mathematical details whenever the physical understanding I seek to promote can be conveyed without them. This applies especially to the discussion of the basic physics in the early chapters. Those looking for rigorous derivations of the mathematical details will have to look elsewhere. Also, exhaustive scope is not a practical goal. So, for the details on many of the topics treated here, and for any treatment at all of the many topics neglected here, the reader will have to consult other sources. This book is also not intended as an introduction to the subject. Though it would not be impossible for someone with no prior exposure to follow the development given here, some experience with the subject will make it much easier. And while I assume no prior knowledge of the subject, I do assume a higher level of technical sophistication than is often assumed in undergraduate-level texts.
An understanding of the physical basics is more secure if it includes an appreciation of the “big picture,” the logical structure of the body of knowledge and the collection of concepts we call aerodynamics. I have tried to at least touch on all of the topics that are so basic that the overall framework could not stand without them. I also devote more attention than most aerodynamics textbooks to the relationships between the parts, to how it all “fits together.” Beyond that, several considerations have guided my choice of topics and the kinds of treatment I've given them. One is my own familiarity and experience. Another is my observation of some common knowledge gaps, things that don't seem to be covered well in the usual aero engineering education. But we'll also spend a good part of our time on some of the very familiar things that we tend to take for granted. Our understanding of these things is never so good that it can't benefit from taking a fresh look. We'll put a different spin on some familiar topics, for example, what the Biot-Savart law really means and why it causes so much confusion, what “Reynolds number” and “incompressible flow” really mean, and a real physical explanation for how an airfoil produces lift.
As we'll see in chapter 1, the subject matter of aerodynamics consists of physical principles, conceptual models, mathematical theories, and descriptions and physical explanations of flow phenomena. Some of this subject matter has direct practical applications, and some doesn't. We'll spend considerable time on some topics that have no apparent practical import, for example, physical explanations of things for which we have perfectly good quantitative theories and esoterica such as how lift is felt in the atmosphere at large. We'll do these things because they provide general fluid-mechanics insight and because they serve to expand our appreciation of the cognitive dimension of the subject, the processes by which we think about aerodynamic phenomena and the practical problems that arise from them. They also help us to see how mistaken thinking can arise and how to avoid it. The medical profession in recent years has begun to pay more attention to the cognitive dimension of their discipline, studying how doctors think, in an effort to improve the accuracy of their diagnoses and to avoid mistakes. Doing some of the same would be good for us as well.
Aerodynamics as a subject encompasses a wide variety of flow situations that in turn involve a multitude of detailed flow phenomena. The subject is correspondingly multifaceted, with a rich web of interconnections among the phenomena themselves and the conceptual models that have been developed to represent them. Such a subject has a logical structure of course, but it is not well suited to exposition in a single linear narrative, and there is therefore no ideal solution to the problem of organizing it so that it flows completely naturally as a single string of words. The organization I have chosen is based not on the historical development or on a progression from “easy” concepts to “advanced,” but on a general conceptual progression, from the basic physics, to the flow phenomena, and finally to the conceptual models. I have tried to organize the material so that it can be read straight through and understood without the need to skip forward. I have also tried to provide direct references whenever I think referring back to previous chapters would be helpful and to alert the reader when further discussion of a topic is being deferred until later.
The general flow of the book is as follows. First, we take an overview of the conceptual landscape in chapter 1. Then we consider the basic physics as embodied in the NS equations in chapters 2 and 3. We turn to the phenomenological aspects of general flows in boundary layers and around bodies in chapters 4 and 5. We then enter the more specific realm of aerodynamic forces and their manifestations in flowfields to deal with drag in chapter 6 and lift generation, airfoils, and wings in chapters 7 and 8. All of this sets the stage for a bit of a regression into theory, with discussions of theoretical approximations and CFD in chapters 9 and 10.
When I started writing I had something less ambitious in mind, something more on the scale of a booklet with a collection of helpful ways of looking at aerodynamic phenomena and a catalog of common misconceptions and how to avoid them. As the project progressed, it became clear that effective explanations required more background than I had anticipated, and the book gradually grew more comprehensive. The first draft in something close to the final form was completed in late 2008 and was reviewed by several Boeing colleagues (acknowledged below). Their feedback was incorporated into a second draft that was used in a 20-week after-hours class for Boeing engineers in 2009. Feedback from class participants and others led to significant revisions for the final draft. As it turned out, the general argumentative approach I've taken to the subject extended to the writing process itself. Many sections saw multiple and substantial rewrites as my thinking evolved.
I gratefully acknowledge the help of many people in getting me through this long process. First, my wife, Theresa, who put up with the many, many weekends that I spent in front of our home computer. Then The Boeing Company, which allowed me to spend considerable company time on the project, Boeing editors, Andrea Jarvela, Lisa Fusch Krause, and Charlene Scammon, who turned my raw Word files and graphics into a presentable draft and helped me take that draft through several revisions, and Boeing graphics artist John Jolley, who redrew nearly half the graphics. Finally, the friends and colleagues without whose help the book would have been much poorer. Mark Drela (MIT), Lian Ng, Ben Rider, Philippe Spalart, and Venkat Venkatakrishnan provided very detailed feedback and suggestions for improvement. Steve Allmaras and Mitch Murray made special CFD calculations just for the book. My former Boeing colleague Guenter Brune wrote the excellent 1983 Boeing report on flow topology that introduced me to the topic and served as the basis of much of Section 5.2.3. Another former Boeing colleague, Pete Sullivan, did the CFD calculations plotted in Section 6.1.5. And many others contributed feedback on various drafts of the manuscript: Anders Andersson, John D. Anderson (University of Maryland), Byram Bays-Muchmore, Bob Breidenthal (University of Washington), Julie Brightwell, Tad Calkins, Dave Caughey (Cornell University), Tony Craig, Jeffrey Crouch, Peg Curtin, Bruce Detert, Scott Eberhardt, Winfried Feifel, David Fritz, Arvel Gentry, Mark Goldhammer, Elisabeth Gren, Rob Hoffenberg, Paul Johnson, Wen-Huei Jou, T. J. Kao, Edward Kim, Alex Krynytsky, Brenda Kulfan, Louie LeGrand, Adam Malachowski, Adam Malone, Tom Matoi, Mark Maughmer (Penn State University), the late John McMasters, Kevin Mejia, Robin Melvin, Greg Miller, Deepak Om, Ben Paul, Tim Purcell, Steve Ray, Matt Smith, John Sullivan (Purdue University), Mary Sutanto, Ed Tinoco, David Van Cleve, Paul Vijgen, Dave Witkowski, Conrad Youngren (New York Maritime College), and Jong Yu.
Thanks also to the copyright owners who kindly gave permission to use the many graphics I borrowed from elsewhere. They are acknowledged individually in the figure titles.
Doug McLean,
April 2012.