Ludwig Prandtl (4 February 1875 - 15 August 1953) was a German physicist. He was a pioneer of aerodynamics, and developed the mathematical basis for the fundamental principles of subsonic aerodynamics in the 1920s. His studies identified the boundary layer, thin-airfoils, and lifting-line theories. He was also the eponym of the Prandtl number.

Prandtl was born in Freising, near Munich in Bavaria, in 1875. His mother suffered from a lengthy illness and, as a result, Ludwig spent more time with his father, a professor of engineering. His father also encouraged him to observe nature and think about his observations.

He entered the University of Munich in 1894 and graduated with a Ph.D. in six years. His work at Munich had been in solid mechanics, and his first job was as an engineer designing factory equipment. There, he entered the field of fluid mechanics when he had to design a suction device. After carrying out some experiments, he came up with a new device that worked well and used less power than the device he replaced.

In 1901 Prandtl became a professor of mechanics at a technical school in Hannover. It was here that he developed many of his most important theories. In 1904 he delivered a groundbreaking paper, Fluid Flow in Very Little Friction, in which he described the boundary layer and its importance for drag and streamlining. The paper also described flow separation as a result of the boundary layer, clearly explaining the concept of stall for the first time. Several of his students made attempts at closed-form solutions, but failed, and in the end the approximation contained in his original paper remains in widespread use.

The effect of the paper was so great that Prandtl became director of the Institute for Technical Physics at the University of Göttingen later in the year. Over the next decades he developed it into a powerhouse of aerodynamics, leading the world until the end of World War II. In 1925 the university spun off his research arm to create the Kaiser Wilhelm Institute for Fluid Mechanics.

Following earlier leads by Frederick Lanchester from 1902-1907, Prandtl worked with Albert Betz and Max Munk on the problem of a useful mathematical tool for examining lift from "real world" wings. The results were published in 1918-1919, known as the Lanchester-Prandtl wing theory or lifting-line theory . He also made specific additions to study cambered airfoils, like those on World War I aircraft, and published a simplified thin-airfoil theory for these designs. This work led to the realization that on a wing of finite length, all of them, wing-tip effects became very important to the overall performance and characterization of the wing. Considerable work was included on the nature of induced drag and wingtip vortexes, which had previously been ignored. With these tools, early aircraft designers were first able to make real theoretical studies of their aircraft even before they were built.

Prandtl and his student Theodor Meyer developed the first theories of supersonic shock waves and flow in 1908. The Prandtl-Meyer expansion fans allowed for the construction of supersonic wind tunnels. He had little time to work on the problem further until the 1920s, when he worked with Adolf Busemann and created a method for designing a supersonic nozzle in 1929. Today, all supersonic wind tunnels and rocket nozzles are designed using the same method. A full development of supersonics would have to wait for Theodore von Kármán's work, a student of Prandtl at Göttingen.

Other work examined the problem of compressibility at high subsonic speeds, known as the Prandtl-Glauert correction . This became very useful during World War II as aircraft began approaching supersonic speeds for the first time. He also worked on meteorology, plasticity and structural mechanics .

Prandtl worked at Göttingen until he died on August 15, 1953. His work in fluid dynamics are generally more detailed and easier to understand than others, and are still used today in many areas of aerodynamics. He is often referred to as the father of modern aerodynamics.