Ludwig
Prandtl
Born: 4 February
1875 in Freising, Germany Died: on 15 August 1953.
in Gottinger, Germany
In the early
decades of the 20^{th} century, Ludwig Prandtl formulated several
important aerodynamic theories. The most notable of these were his boundary
layer, thinairfoil, and liftingline theories. He was also a teacher of many
prominent aerodynamicists.
Ludwig Prandtl
was born in Freising, Bavaria, in 1874. His father was a professor of
engineering. His mother suffered from a lengthy illness and, as a result, the
young Ludwig spent more time with his father, becoming interested in his
father's physics and machinery books. His father also encouraged him to observe
nature and think about his observations. This upbringing fostered the young
Prandtl's interest in science and experimentation.
Prandtl began
his formal scientific studies at the age of 20 in Munich, Germany, and graduated
with a Ph.D. from the University of Munich within 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 original device that worked well and used less power than the device that had
been used.
Prandtl became a
professor of mechanics at a technical school in Hannover, Germany, I 1901. There
he developed his boundary
layer theory and studied supersonic fluid flows
through nozzles.
In 1904, he delivered
a revolutionary paper to the Third International Mathematical Congress at
Heidelberg, Germany. Titled 밬eber Flussigkeitsbewegung Bei Sehr Kleiner
Reibung?(Fluid Flow in Very Little Friction), the paper described his boundary
layer theory.
Prandtl's
boundary layer theory contributed to an understanding of skin friction drag and
how streamlining reduces
the drag experienced by
airplane wings and other moving bodies. Prandtl examined the drag
that resulted from the friction that was created when a fluid such as air passed
over an object's surface.
Prandtl
determined that there was an extremely thin layer of fluid around a wing or
airfoil that stuck to it because of friction. The friction caused this thin
layer of fluid, called the boundary layer, to move, or flow, around the
wing very slowly as if it were being dragged or pulled over the surface. The
farther away from the wing's surface the layer of air was, the less it was
affected by friction and the faster it moved until it reached the outer edges of
the boundary layer, where the airflow was normal and the fluid moved at normal
speed.
Prandtl also
observed that flow separation was another possible result of friction. When a
certain type of flow occurred, the boundary layer separated from the surface of
the wing. This resulted in a region of slowmoving air behind the wing. This
slowmoving air had lower pressure than the air flowing over the front of the
wing. This change in pressure distribution around the wing resulted in a
pressure drag toward the rear of the aircraft that much exceeded friction
drag.
His 1904 paper
raised Prandtl's prestige as an aerodynamicist. He became director of the
Institute for Technical Physics at the University of G?tingen later in the year,
where he worked with many outstanding students, creating the greatest
aerodynamics research center of his time.
In the years
that followed, Prandtl began work on calculating the effect of induced drag on
lift. Induced drag is the drag created by the vortices that trail an aircraft
from the tips of its wings. These vortices, or whirling motions of fluid, affect
the pressure distribution over the wings and result in a force in the direction
of drag. Hence, induced drag is a kind of pressure drag. He worked with Albert
Betz and Max Munk for almost ten years to solve this problem. The result was his
lifting line theory, which was published in 19181919. It enabled accurate
calculations of induced drag and its effect on lift.
In England,
Prandtl's lifting line theory is referred to as the LanchesterPrandtl theory.
This is because the English scientist Frederick Lanchester published the
foundation for Prandtl's theory years earlier. In his 1907 book
Aerodynamics, Lanchester had described his model for the vortices that
occur behind wings during flight. Prandtl's model for his theory was similar to
Lanchester's, although Prandtl claimed that he had not considered Lanchester's
model when he had begun his work in 1911.
During World War
I, Prandtl created his thinairfoil theory that enabled the calculation of lift
for thin, cambered airfoils. It
is still used today. He later contributed to the PrandtlGlauert rule for
subsonic airflow that describes the compressibility effects of
air at high speeds. Prandtl also made important advances in developing theories
of supersonic flow and turbulence.
Prandtl worked
with his student, Theodor Meyer, to develop the first theory for calculating the
properties of shock and expansion waves in supersonic flow in 1908. In 1929, he
worked with Adolf Busemann and created a method for designing a supersonic
nozzle. Today, all supersonic windtunnel nozzles and
rocketengine nozzles are designed using the same method. Prandtl also developed
a rule for correcting lowspeed airfoil lift calculations that accounted for the
way air compressed at high speeds. This became very useful during World War II
as aircraft began approaching supersonic speeds.
In 1925 Prandtl became
director of the Kaiser Wilhelm Institute for Flow Investigation at G?tingen. By
the 1930s, he was known worldwide as the leader in the science of fluid
dynamics, the study of the effect of fluid motion on objects. He continued his
research in many areas, such as meteorology and structural mechanics. Prandtl
also taught famous aerodynamicists such as Theodor von Kamman and Klaus
Oswatitsch.
Ludwig Prandtl was a
likeable man and an accomplished pianist. He worked at G?tingen until his death
on August 15, 1953. His work and achievements in fluid dynamics resulted in
equations that were easier to understand than others and are still used today in
many areas of aerodynamics. It is for this reason that he is referred to as the
father of modern aerodynamics.
Article by: J J O'Connor and E F Robertson
September 2000
