The Airplane - Jay Spenser [57]
The culprit was compressibility (also called Mach phenomena), which cropped up as human beings began flying so fast that the air could no longer get out of the way in time. Instead of maintaining a constant density around the airplane, air molecules bunched up until shock waves formed, setting loose these compressibility gremlins.
There was obviously more to learn.
Aviation’s early pioneers never knew how a wing achieves flight. They also didn’t care. The simple fact that wings worked meant people could fly by following nature’s lead.
In this sense, engineering, not science, gave us the gift of wings. Pascal, Bernoulli, Euler, and others laid down a body of theory to help explain the behavior of fluids, air included, at rest or in motion. However, that knowledge did not play a role in solving flight’s challenges. Instead, science’s role was initially to explain the underlying physics of flight after the fact.
It took a while to understand how wings actually work in terms of the circulation of air. Early in the twentieth century, the towering intellect in this cerebral quest was Dr. Ludwig Prandtl, a physicist and applied mathematician hailed as the “father of modern aerodynamics.” Born in Bavaria in 1875, Prandtl mathematically described fundamental fluid-flow principles, identified key aerodynamic concepts (including the boundary layer) that underpin our understanding of flight, and gave airplane designers the ability to begin predicting the performance of wings that had not yet been built.
Prandtl spent most of his career at Germany’s famous Göttingen University, which for decades he and his graduate students kept at the forefront of global aerodynamics research. Many of these disciples went elsewhere, carrying with them the light of theory to advance humanity’s conceptual understanding of flight.
Max Munk was one of them. Immigrating to the United States after World War I, Munk joined the National Advisory Committee for Aeronautics (NACA), predecessor to today’s National Aeronautics and Space Administration (NASA). At the start of the 1920s, Dr. Munk came up with the variable-density wind tunnel, which by employing higher air pressures allows accurate aerodynamic data to be obtained from the testing of subscale models. No longer would full-size airplanes have to be tested.
Hungarian aerodynamicist Theodore von Kármán was another noted Prandtl disciple. Immigrating to the United States in 1930, Dr. von Kármán directed the California Institute of Technology’s Guggenheim Aeronautical Laboratory, cofounded the Jet Propulsion Laboratory, and would for many decades serve as a leading aerospace advisor to the U.S. government.
Long before there were high-speed airplanes, Ludwig Prandtl’s mathematical musings gave the world a theoretical basis for grappling with compressibility and shock waves. Other scientists also contributed to the emerging body of supersonic theory. In the 1920s, Swiss scientist Jakob Ackeret—another Prandtl alumnus—reduced these individuals’ complex theories into a simple method for calculating the lift and drag of supersonic airfoils.
In a landmark paper presented in Rome in 1935, Adolf Busemann, also a Prandtl student, became the first person to propose sweeping back an airplane’s wings as a way to reduce shock wave formation in high-speed flight. Since it takes energy to generate shock waves, which are like a boat’s wake but in three dimensions, sweeping the wings back promised to reduce the amount of power required to fly at high speeds.
Wing sweep