Modern aerodynamics only dates back to the seventeenth century, but aerodynamic forces have been harnessed by humans for thousands of years in sailboats and windmills. The formal study of aerodynamics began in the modern sense in the eighteenth century, although observations of fundamental concepts such as aerodynamic drag were recorded much earlier. Most of the early efforts in aerodynamics were directed toward achieving heavier-than-air flight, which was first demonstrated by Otto Lilienthal in 1891.
In 1726, Sir Isaac Newton became the first person to develop a theory of air resistance. Daniel Bernoulli followed in 1738 with Hydrodynamica in which he described a fundamental relationship between pressure, density, and flow velocity for incompressible flow known today as Bernoulli's principle. In 1757, Leonhard Euler published the more general Euler equations which could be applied to both compressible and incompressible flows. The Euler equations were extended to incorporate the effects of viscosity in the first half of the 1800s, resulting in the Navier–Stokes equations.
In 1799, Sir George Cayley became the first person to identify the four aerodynamic forces of flight: weight, lift, drag, and thrust. In 1871, Francis Herbert Wenham constructed the first wind tunnel, allowing precise measurements of aerodynamic forces. Otto Lilienthal was also the first to propose thin, curved airfoils that would produce high lift and low drag. Building on these developments, the Wright brothers flew the first powered airplane on December 17, 1903.
As aircraft speed increased, designers began to encounter challenges associated with air compressibility at speeds near the speed of sound. The ratio of the flow speed to the speed of sound was named the Mach number after Ernst Mach. Theodore von Kármán and Hugh Latimer Dryden introduced the term transonic to describe flow speeds between the critical Mach number and Mach 1 where drag increases rapidly. The sound barrier was broken in 1947 using the Bell X-1 aircraft.
Understanding the motion of air around an object enables the calculation of forces and moments acting on the object. In many aerodynamics problems, the forces of interest are the fundamental forces of flight: lift, drag, thrust, and weight. Of these, lift and drag are aerodynamic forces. Calculation of these quantities is often founded upon the assumption that the flow field behaves as a continuum. Continuum flow fields are characterized by properties such as flow velocity, pressure, density, and temperature.
Flow velocity is used to classify flows according to speed regime. Subsonic flows are flow fields in which the air speed field is always below the local speed of sound. Transonic flows include both regions of subsonic flow and regions in which the local flow speed is greater than the local speed of sound. Supersonic flows are defined to be flows in which the flow speed is greater than the speed of sound everywhere. A fourth classification, hypersonic flow, refers to flows where the flow speed is much greater than the speed of sound. Compressible flow accounts for varying density within the flow. Subsonic flows are often idealized as incompressible, while transonic and supersonic flows are compressible.
The assumption of a fluid continuum allows problems in aerodynamics to be solved using fluid dynamics conservation laws. Three conservation principles are used: conservation of mass, conservation of momentum, and conservation of energy. Together, the equations are known as Navier–Stokes equations. Euler equations are a set of similar conservation equations which neglect viscosity and may be used in cases where the effect of viscosity is expected to be small. Additionally, Bernoulli's equation is a solution in one dimension to both the momentum and energy conservation equations.
Aerodynamic problems are classified by the flow environment or properties of the flow, including flow speed, compressibility, and viscosity. External aerodynamics is the study of flow around solid objects of various shapes. Internal aerodynamics is the study of flow through passages in solid objects. Aerodynamic problems can also be classified according to whether the flow speed is below, near or above the speed of sound, leading to subsonic, transonic, supersonic, and hypersonic classifications.
Aerodynamics is a significant element of vehicle design, including road cars and trucks where the main goal is to reduce the vehicle drag coefficient, and racing cars, where in addition to reducing drag the goal is also to increase the overall level of downforce. It is also used in the design of mechanical components such as hard drive heads. Structural engineers resort to aerodynamics when calculating wind loads in the design of large buildings, bridges, and wind turbines.
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