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Rising of the Planes

Kenneth Hall

What Makes Airplanes Fly? History, Science, and Applications of Aerodynamics. 2nd ed. Peter P. Wegener. xii + 260 pp. Springer-Verlag, 1997. $34.95.

Aerodynamics, unlike many other important but highly technical scientific disciplines, attracts experts and neophytes alike. Whereas aerodynamics has benefited from the work of classically trained scientists and engineers, much of the early history of aviation was strongly influenced by more unlikely sources. The Wright brothers were bicycle mechanics, not trained engineers. Frank Lanchester, an English lawyer, postulated the importance of "circulation" in the generation of lift. The details were to be worked out later by now-famous aerodynamicists such as Wilhelm Kutta, Nikolai Joukowski and Ludwig Prandtl. Over the past 80 or so years, aerodynamics has evolved into a highly specialized field.

Nevertheless, aerodynamics and aviation continue to hold an almost mystical fascination for many lay people. For this audience, Peter P. Wegener has written a second edition of What Makes Airplanes Fly? The author explains in non-mathematical language the many disparate technical elements necessary for understanding modern aerodynamics, a task in many respects equal to that of Stephen W. Hawking's treatment of cosmology in A Brief History of Time. In Herculean fashion, Wegner describes the nature of fluids, properties of the atmosphere, conservation laws that govern fluid flow, boundary layer theory, the nature of lift and drag, propulsion systems and supersonic and hypersonic flight. Indeed, the breadth of material is as at least much as would typically be included in a two-course sequence in aerodynamics at most engineering schools.

To the author's credit, he does not oversimplify the essence of the material. A good example is his discussion of the importance of circulation on lift. Circulation is a measure of how much the flow in some sense circumnavigates the airfoil. The usual way lift is described in the popular literature is to say that the air passing over the airfoil must travel farther and faster over the upper surface than the lower surface, owing to the camber and thickness of the airfoil, before rejoining at the trailing edge of the airfoil. Thus invoking Bernoulli's equation, the story goes, the pressure is higher below the airfoil than above, producing lift.

Although partially true, such popular explanations are technically flawed (a fluid particle split by the leading edge does not meet up at the trailing edge). Moreover, such explanations do not represent the way in which aerodynamicists think about circulation and lift. Wegener, however, gives cogent and thoughtful explanations for the origins and effect of circulation that can be appreciated by the trained aerodynamicist and understood by the novice.

Although highly accurate, the book contains a few errors. In the section on propellers, Wegener writes, "Because the rotating blades run into portions of their own wakes, the lift distribution is uneven." Although helicopter blades often run into their own wakes, propeller blades usually do not. Such minor flaws can be forgiven considering the breadth of material adroitly covered. A more serious flaw in the second edition is a typographical error (repeated no fewer than three times!) in Bernoulli's equation, making the meaning of this most fundamental of fluid dynamic equations meaningless to the uninitiated.

Finally, Wegener's outlook for improvements in aerodynamics and aircraft over the next century is optimistic but measured. Arguing that environmental and funding constraints will limit the development of supersonic and hypersonic transports, he suggests that there are still fruitful areas for the aerodynamic improvement of conventional subsonic aircraft, including boundary-layer control to reduce drag, large high-speed fuel-efficient turboprops and aircraft with "intelligent wings," whose shape can be continuously varied for optimal performance.—Kenneth C. Hall, Mechanical Engineering and Materials Science, Duke University

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