AERODYNAMICS

branch of physics that deals with the motion of air and other gaseous fluids and with the forces acting on bodies passing through such a fluid. Aerodynamics seeks, in particular, to explain the principles governing the flight of aircraft, rockets, and missiles. It is also concerned with the design of automobiles, high-speed trains, and ships, as well as with the construction of such structures as bridges and tall buildings to determine their resistance to high winds. Observations of the flight of birds and projectiles stirred speculation among the ancients as to the forces involved and the manner of their interaction. They, however, had no real knowledge of the physical properties of air, nor did they attempt a systematic study of those properties. Most of their ideas reflected a belief that the air provided a sustaining or impelling force. These notions were based to a large degree on the principles of hydrostatics (the study of the pressures of liquids) as they were then understood. Thus, in early times, it was thought that the impelling force of a projectile was associated with forces exerted on the base by the closure of the flow of air around the body. This conception of air as an assisting medium rather than a resisting force persisted for centuries, even though in the 16th century it was recognized that the energy of motion of a projectile was imparted to it by the catapulting device. Near the end of the 15th century, Leonardo da Vinci observed that air offered resistance to the movement of a solid object and attributed this resistance to compressibility effects. Galileo later established the fact of air resistance experimentally and arrived at the conclusion that the resistance was proportional to the velocity of the object passing through it. In the late 17th century, Christiaan Huygens and Sir Isaac Newton determined that air resistance to the motion of a body was proportional to the square of the velocity. Newton's work in setting forth the laws of mechanics marked the beginning of the classical theories of aerodynamics. He considered the pressure acting on an inclined plate as arising from the impingement of particles on the side of the plate that faces the airstream. His formulation yielded the result that the pressure acting on the plate was proportional to the product of the density of the air, the area of the plate, the square of the velocity, and the square of the sine of the angle of inclination. This failed to account for the effects of the flow on the upper surface of the plate where low pressures exist and from which a major portion of the lift of a wing is produced. The idea of air as a continuum with a pressure field extending over great distances from the plate was to come much later. Various discoveries were made during the 18th and 19th centuries that contributed to a better understanding of the factors influencing the movement of solid bodies through air. The relationship of resistance to the viscous properties of a fluid, for example, was perceived in part by the early 1800s, and the experiments of the British physicist Osborne Reynolds in the 1880s brought into clearer view the significance of viscous effects. Modern aerodynamics emerged about the time that the Wright brothers made their first powered flight (1903). Several years after their historic effort, Frederick W. Lanchester, a British engineer, proposed a circulation theory of lift of an airfoil of infinite span and a vortex theory of the lift of a wing of finite span. The German physicist Ludwig Prandtl, commonly regarded as the father of modern aerodynamics, arrived independently at the same hypotheses as Lanchester and developed the mathematical treatment. Prandtl's work, refined and expanded by subsequent investigators, formed the theoretical foundation of the field. Among others who played a prominent role in the development of modern aerodynamics was the Hungarian-born engineer Theodore von Krmn, whose contributions led to major advances in such areas as turbulence theory and supersonic flight.