The Aerodynamic Design of Aircraft

When Dietrich Kuchemann’s The Aerodynamic Design of Aircraft was published posthumously in 1978, it was a unique and forward thinking text comprising the philosophy and life’s work of a unique and visionary intellect. Dietrich Kuchemann studied under Ludwig Prandtl at the Univer­sity of Gottingen from 1930 until receiving his doctorate in 1936. He was a research scientist at the Aerodynamische Versuchs Anstalt Gottingen for ten years before coming to work at the Royal Aircraft Establishment Farnborough. He would continue working at the RAE in a variety of scien­tific and leadership roles until his death in 1976. For the last four years of his life, he taught a course at Imperial College London upon which this book is based.

Dietrich Kuchemann was a preeminent aerodynamicist of his era. His early work with Dr. Johanna Weber, his lifelong scientific collaborator, helped usher in the jet age and resulted in the publication of Aerodynamics of Propulsion in 1953. His conception of and work on slender-wing super­sonic aircraft strongly influenced the development of the Concorde. In 1962 Kuchemann was awarded the Royal Aeronautical Society’s Silver Medal and in 1963 he was elected a Fellow of the Royal Society. He was appointed a Commander of the British Empire in 1964. In 1970 he received the Ludwig Prandtl Ring.

In addition to his many technical contributions, Kuchemann was a tireless advocate for the aerospace community. He was the founder and principal editor of the journal Progress in Aeronautical Sciences (now Pro­gress in Aerospace Sciences) from 1961 until his death. He served on the AGARD Fluid Dynamics Panel from 1965, including serving as its chair from 1973-1975.

The Aerodynamic Design of Aircraft is quite unlike most engineering texts. Engineering texts are typically focused on a set of tools—the rigorous derivation and presentation of analysis techniques sufficient to span a discipline. Kuchemann’s approach is to focus on the problem and its solution—what kind of flow is best for a given class of aircraft and how to achieve it. In this approach, Kuchemann fully embraces the true inverse nature of design; rather than answer “what flow given the shape,” he strives to answer “what flow given the purpose” and then “what shape given the flow.”

Kiichemann establishes three classes of aircraft based on the character of flow involved. Each class is suitable for a distinct cruise speed regime: classical and swept aircraft for subsonic and transonic cruise, slender-wing aircraft for supersonic cruise, and wave-rider aircraft for hypersonic cruise. The desired flow for each kind of aircraft has distinct structure: classical and swept aircraft with streamlined and attached flow separating from the trailing edge of airfoils, slender-wing aircraft with vor­tices attached to sharp leading edges, and wave-rider aircraft with a strong shock attached to the leading edges of the underside of the wing.

In addition to the structure of each flow, Kiichemann establishes the need for each flow to be “healthy.” A healthy flow exhibits the desired struc­ture and is stable and controllable not only at the design point, but also at off-design operating conditions. The forces and moments on the aircraft must change gradually and continuously throughout the flight envelope.

My own experience with this book is one that almost didn’t happen. For years, I knew this book, then long out-of-print, only by reputation; although highly regarded and oft cited, I had never managed to find a copy of my own. My desire for this text had led me to request an automated notification from an online auction site. That request went unfulfilled for more than a year. I remarked on this situation before a technical meeting of aircraft design practitioners, researchers, and educators in the spring of 2009 only to find out that another member of the group had requested the same noti­fication from the same site. We were all quite amused that if a copy were to become available, two friends would unknowingly become embroiled in a fierce bidding war. As luck would have it, just a few days after that meeting, I was notified of an available copy and was able to secure purchase before my friend.

Upon its arrival, I found that this book exceeded its reputation as a unique treatment of aerodynamic design. I became determined that The Aerodynamic Design of Aircraft must be reprinted and made available again to the aerospace community. Later that year, I received enthusiastic support for this project from the membership and leadership of the AIAA Aircraft Design Technical Committee. With their backing, I approached the publishing staff of the AIAA about pursuing this project. Although this project has been long in coming, its completion stands as a tribute to the dedication of Dietrich Kiichemann’s family and the editors of the AIAA Education Series.

The Aerodynamic Design of Aircraft is as relevant and as forward looking today as it was in 1978. The swept wing aircraft, in becoming the mainstay of the aerospace industry, has achieved a high degree of sophistication, but much of the nuance contained in this text, required to continue advancing these designs, is lost outside a few experienced practitioners in industry. Since the Concorde, there has been no second generation of supersonic

Foreword xiii

transport aircraft. Acceptable sonic boom is the primary challenge facing developers of supersonic business jets, but these aircraft will also have to balance cruise efficiency and must still achieve healthy flow. The past few years have seen the successful development and test of several scramjet engine and hypersonic vehicle technology programs. These programs build the case for the development of a generation of hypersonic wave – rider vehicles as foreseen by this text.

Kiichemann understands, explains, and advocates the integration of aerodynamic and propulsive roles of air vehicles in a way now considered essential for the success of the next generation of vehicles of all classes. Recent years have seen dramatic progress in the use of computation for aerodynamics. Computational fluid dynamics provides an incredibly powerful tool for sophisticated aerodynamic analysis. Aerodynamic shape optimization techniques are poised to provide an equally powerful tool for sophisticated aerodynamic design. Despite the rigor and power of these techniques, they fail to provide the guidance of this text as to what flow or shape is desired. Kiichemann’s call for healthy flow having the same characteristics off – and on-design rings true today. The physical insight and intuition conveyed by this text are timeless.

With the republication of this text, Dietrich Kiichemann’s influence will extend to the next generation of the aerospace industry and the vehicles it will produce. I know he would be proud that so many, including my friend, will finally have access to this work.

Rob McDonald

San Luis Obispo, CA June 2012

Dietrich Kuchemann died on February 23, 1976. Fortunately for the world of aerodynamics, he had by then assembled the material of this book and was engaged in a final editing. The devoted cooperation of several col­leagues in the Royal Aircraft Establishment and the generosity of RAE man­agement in providing typing and other services have enabled the task of preparation to be completed. The book appears now as one of the lasting records of the work of a great aerodynamicist, perhaps the greatest of his generation.

The author’s working lifetime was spent in research, first at the AVA Gottingen and then at the RAE Farnborough where he became Head of the Aerodynamics Department. During the last four years of his life, in addition to continuing his research, he gave a course of lectures to students of the Aeronautical Engineering Department of Imperial College, London. He took this opportunity to set out clearly his convictions regarding the pre-eminent position of fluid dynamics in the complex process of aircraft design. The book follows the general line of these lectures, but with a fuller development of ideas and material it emerges as much more than a textbook for students. Overall it provides a coherent explanation of why air­craft are the shapes they are for the tasks they have to perform, an introduc­tion to the methods used in their detailed aerodynamic design and a unified vision of science applied in an orderly way to human progress. It would be presumptuous to place limits on its readership.

The choice of title indicates the author’s personal approach. Aerody­namics is for him an applied science that is meaningful only when it has the practical design of aircraft as its aim. The best methods are those that work in this context; they contribute to a conceptual framework that the scientist is aiming to put into the hands of the aircraft designer. Simplifying assumptions are preferred to so-called “exact” methods, so long as the assumptions help to understand and elucidate the fluid mechanical pro­cesses and can be exposed to critical examination, usually by way of exper­iment. At the same time, the conceptual framework that emerges from this approach is recognised to constitute only the first step—an introduction— to the detailed process of aircraft design. The designer, knowing what his aircraft is required to do, needs to select the type of flow pattern appropriate to the task—this leads him automatically to the correct framework within which his design can then be worked out in detail, using the aerodynamic concepts and theoretical methods outlined in the book.

On this basis, the arrangement of the book is straightforward, as the list of chapters indicates. Professor Kuchemann introduces his subject (Chapter 1) by developing a personal philosophy in which aviation takes its place as an essential element in the development of human society and in which aerodynamics is an essential element, indeed the dominant element, in the development of aviation. From the idea that aviation can eventually bring the whole world within a few hours’ traveling time, there emerges the need for aircraft with cruising speeds which are greater in pro­portion to their operational ranges. From the need to operate in different regimes of Mach number emerge three essentially distinct types of air­craft—the classical aircraft with moderate to high aspect ratio, swept or unswept; the slender aircraft marked by its low aspect ratio wing of delta­like planform; and the waverider, a sharp-edged lifting body riding on a strong shock wave.

When once identified, these types of aircraft may be examined more widely as regards their potential for other Mach number ranges and other operational scenarios. In all cases, however, the different types should have in common certain vital features stemming from Ludwig Prandtl’s idea of a “healthy” flow, namely that the flow is an efficient means of generating aero­dynamic lift and is capable of persisting in steady and stable form over ranges of Mach number, Reynolds number, angles of incidence and angles of sideslip that embrace the flight envelope of the aircraft.

The groundwork for this approach is laid carefully. A section on the fun­damental processes of fluid mechanics (Chapter 2) is a unique and masterly exposition of principles and ideas concerning the relations between flow patterns and aircraft shapes that is basic to the author’s approach to design. Chapter 3 treats broadly the means for generating lift and propulsive force and introduces the mathematical techniques that are needed to give quantitative expression to the fluid mechanical concepts.

Chapters 4 to 8 contain the detailed treatment of the types of flow that relate to the different types of aircraft. A short final chapter reiterates the author’s conviction of the fundamental place of aerodynamics as the key to aircraft design and looks forward to much development of the subject still to come. Collected at the end are over 1900 references to reports and papers that have been referred to in the text.

Many friends and colleagues of the author have contributed in various ways to the production of the book. Particular acknowledgement is made to J. A. Bagley, J. H. B. Smith, E. G. Broadbent and P. L. Roe, members of the Aerodynamics Department of the RAE, who completed the editing, also to Dr. Johanna Weber who checked the accuracy of drafts and

PREFACE xv/i

references and whose contribution extends over a lifetime of collaboration with Professor Kuchemann. For the work of typing, proof reading and similar assistance, thanks are due to Mrs. Elma Turner, the author’s former secretary, and to Miss Susan Damms and Mrs. Irene Joth. At Imperial College, Dr. P. J. Finley assisted in many ways and the advice and encour­agement of Professor P. R. Owen was much valued.

J. Seddon

Farnham, Surrey 1978