Ludwig Prandtl

German physicist

• Born: February 4, 1875
• Birthplace: Freising, Germany
• Died: August 15, 1953
• Place of death: Göttingen, West Germany (now in Germany)

Prandtl, one of the fathers of theoretical aerodynamics, is credited with discovering many of the pivotal concepts on which modern aviation is based. He was also the founder of the highly acclaimed school of aerodynamics and hydrodynamics at the University of Göttingen and the first director of what would become the Max Planck Institute for Fluid Mechanics.

Early Life

Ludwig Prandtl (PRAHN-tahl) took an early interest in the forces and characteristics of nature as a result of the strong influence of his father, Alexander Prandtl, who was a professor of engineering at a college in Weihenstephan, Germany. Magdalene, Ludwig’s mother, played a lesser role in her son’s life because of her extended periods of chronic illness. Ludwig took an early interest in science and engineering, deciding in 1894 to study the latter as his major at the Munich Technische Hochschule, a facility for higher education at the time, comparable to a modern college or university. He successfully completed the course of study and, in 1898, went on to earn his doctorate in physics.

Prandtl’s doctoral thesis, an experimental study in the distribution of tension and torque along a beam arranged at right angles from its source, became an important work in the field of the mechanics of solids. It was an indication of his exceptional abilities as a scientist that his first major paper and one in an area that would be of peripheral interest in his later work would generate significant interest from the contemporary scientific community.

His doctoral thesis and the help of his mentor, a noted German physicist, August Foppl, would earn for him a job on graduation as an engineer in the Augsburg-Nürnberg Machine Factory. It was at the factory that Prandtl was introduced to the study of the characteristics of the flow of air over objects, otherwise known as fluid mechanics. This emerging science was in its infancy at the turn of the century, with the first practical wind tunnels and heavier-than-air craft only slowly coming into use. (Prandtl himself would later in his life be responsible for the construction of the first functional wind tunnel in Germany in 1909.) Prandtl’s own introduction to the field came as a result of a project to refit a vacuum device used in the factory. Prandtl’s work at the factory would be the last he would undertake outside the world of academia, but it led to his most important discoveries and to a brilliant career as a founder of an emerging modern science.

Life’s Work

In 1901, to allow him to continue his studies into fluid mechanics, Prandtl accepted a position as a professor at the Technische Schule in Hannover. His first observations at Hannover involved the flow of thin liquids through a pipe. He noticed that the shape of the liquid flow did not fully conform to the shape of the pipe through which it flowed. A minute layer of liquid, no matter how low in viscosity the substance, would always form between the interior surface of the pipe and the main body of the liquid. This layer between the wall and the fluid actually controlled the pressure of the liquid flow and, consequently, the rate of flow itself. In practical terms, this boundary of stationary fluid is much the same as the layer of air that forms on a wing or airfoil in flight. This boundary actually helps to provide the lift and drag of an air wing necessary to control its movement through the air. Prandtl’s discovery of the boundary theory, as he named it, would be his single most meaningful discovery and would revolutionize powered aviation, leading to major innovations in the streamlining of aircraft wing and fuselage designs.

In 1904, shortly before Prandtl’s publication of his paper on the boundary theory, he was invited to head the new Institute for Technical Physics at the University of Göttingen. The institute would serve as Prandtl’s primary base of operations for the remainder of his life and would become one of the world’s leading centers for theoretical research into fluid mechanics. At Göttingen, Prandtl addressed many of the theoretical questions about piloted flight that were arising as a result of the breakthroughs in aviation technology. He conducted research into, among many subjects, the characteristics of airflow around a body traveling at either subsonic or supersonic speeds. He directed research projects by the institute’s graduate students into wing drag, the mechanics of solids, and other areas. Part of his work during the years before World War I also involved developing testing procedures for electrical fans for the German government and industry. In 1909, Prandtl married Gertrude Foppl, the daughter of his former mentor August Foppl. The couple eventually had two daughters.

Prandtl’s mathematical theories also played a significant part in the advent and popular acceptance of the single-winged airplane. He was himself an advocate of the controversial design concept, in direct conflict with the prevailing opinion held by the aircraft design community in favor of bi- and tri-winged aircraft. He also contributed through his theoretical discoveries to improvements in the design of lighter-than-air craft known as dirigibles, which were commonly in use in the early years of the century as both civilian and military air carriers.

After World War I, in 1918-1919, Prandtl published a breakthrough paper on the way in which air flows around airplane wings of a finite span. The paper, which duplicated and expanded on work done simultaneously by a British physicist named Frederick William Lanchester, became known as the Lanchester-Prandtl wing theory, one of many theoretical innovations that would bear Prandtl’s name over the next several years.

In the mid-1920’s, Prandtl and other researchers in Göttingen and elsewhere undertook the study of air turbulence created by a body moving through the air. In conjunction with another scientist, Theodor von Kármán, a former student of Prandtl, he developed a device for analyzing the distance turbulent air travels before its motion is dissipated. Prandtl’s paper on the subject, presented in 1933, led to radical changes in accepted theories about air turbulence and to concepts used by pilots around the world. Prandtl also conducted extensive studies into the question of how objects traveling at high subsonic speeds are compressed by the air flow over their surfaces. This theory, known as the Prandtl-Glaubert rule, along with his other research into supersonic airflow, played a vital role in developing successful designs for supersonic aircraft.

During the same period, Prandtl, already a world-renowned pioneer in his field, was named to head a technical facility in Germany that would later be known as the Max Planck Institute for Fluid Mechanics. Part of this facility, following Prandtl’s leadership and inspiration, would later become a major engineering design center and, in the 1970’s and 1980’s, an important contributor of spaceflight hardware and support services to the National Aeronautics and Space Administration (NASA) of the United States and to the European Space Agency.

Although it was not his area of primary investigation, Prandtl was also interested in questions concerning the elasticity and plasticity of a variety of solids, and the reaction of solid structures to torsion forces. In the latter, he developed a soap-film analogy that was found to be exceptionally useful in analyzing the effects of torsion forces on structures with noncircular cross sections. Prandtl’s work often centered on the equipment and mathematical models for use in testing natural reactions and design concepts. Such devices include the tubes that bear his name used in the measurement of the static and complete pressure of a liquid flow at any point. He was also instrumental in advancing the development of air-tunnel technology and other equipment used in aerodynamic testing and design.

Unlike many of the scientists in Germany during the years between World War I and the end of World War II, Prandtl managed to avoid responding to political pressure from the ruling Nazi Party and to maintain civilian control over his work. He also continued to publish technical papers on his work regularly and to receive widespread attention from the international scientific community. In later years, after World War II, Prandtl expanded his efforts to include meteorology, a subject on which he published a paper in 1950. On August 15, 1953, Prandtl died in Göttingen, West Germany.

Significance

Prandtl was one of a generation of scientific pioneers whose practical innovations during and after the Industrial Revolution made possible many of the conveniences known to modern humankind. As the founder of several pivotal theories of fluid mechanics used in the production of aircraft, he helped create the age of rapid, safe air transportation. Prandtl is known as the founder of aerodynamics, because he developed some of the fundamental concepts on which modern air travel is based. There is, however, much more to his story than the list of his singular accomplishments. Prandtl both developed numerous theories and helped create much of the basic methodology used by both his own students and others in the generation of physicists who followed him. Men such as Kármán, although only slightly younger than Prandtl, owed much of their understanding of how to approach theoretical problems to the training and example of Prandtl. Prandtl’s contributions extend into education as well as science. He helped build two institutions, the Institute for Technical Physics at the University of Göttingen and the Max Planck Institute for Fluid Mechanics, both of which have made significant contributions to science during and after Prandtl’s time.

Beyond his accomplishments, Prandtl was one of a class of scientists in the late nineteenth and early to mid-twentieth centuries who possessed an extraordinarily single-minded drive to advance their areas of investigation. Konstantin Tsiolkovsky in Russia, Pierre and Marie Curie in France, and Robert H. Goddard in the United States were chronologic and spiritual contemporaries of Prandtl, who worked not only to solve individual scientific questions but also to create a new field of study and to advance the broader body of human knowledge. It was the collegial perspective that also helped build an international scientific community that could coordinate and fully exploit limited financial, human, and natural resources to the best, most productive end.

As part of this community, Prandtl focused his efforts on practical, technological questions, the solutions for which could be put to direct use in aircraft design and other areas. Prandtl and his contemporaries were part of the modern class of scientists who used their studies to solve problems that derived from practical, secular needs. In this regard, Prandtl’s discoveries are every bit as significant as those of more visible inventors such as Orville and Wilbur Wright.

Bibliography

Eckert, Michael. The Dawn of Fluid Dynamics: A Discipline Between Science and Technology. Weinheim, Germany: Wiley-VCH, 2006. Describes the evolution of fluid dynamics in the twentieth century by focusing on Prandtl’s work. Intended for scientists, engineers, and historians of science and technology.

Lienhard, John H. “Ludwig Prandtl.” In Vol. 11 of Dictionary of Scientific Biography, edited by Charles Coulston Gillispie. New York: Charles Scribner’s Sons, 1975. This reference series includes concise, well-written biographies of many scientific figures. Listings are generally confined to basic facts about the individual’s life and works, with minimal coverage of the motives or reasons behind the person’s work.

Liepmann, H. W., and A. Roshko. Elements of Gasdynamics. New York: John Wiley & Sons, 1957. This book illustrates the role that Prandtl played in the theory of compressible flow.

Prandtl, Ludwig. Applied Hydro- and Aeromechanics: Based on Lectures of Ludwig Prandtl. Translated by J. P. Den-Hartog. New York: McGraw-Hill, 1934. This collection of technical lectures given by Prandtl is one of the earliest English-language versions of his works still readily available.

‗‗‗‗‗‗‗. Essentials of Fluid Dynamics, with Applications to Hydraulics, Aeronautics, Meteorology, and Other Subjects. New York: Hafnere, 1952. This technical volume provides an in-depth look at Prandtl’s work in fluid dynamics and other areas of interest to him throughout his long career. One of the few more readily available English-language works of Prandtl.

Schlichting, Hermann. Boundary Layer Theory. Translated by J. Kestin. 4th ed. New York: McGraw-Hill, 1960. This work includes many references to Prandtl and to his students. Contains a good discussion of his involvement in the viscous flow theory.