# aerodynamics

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## aerodynamics,

study of gases in motion. As the principal application of aerodynamics is the design of aircraft, air is the gas with which the science is most concerned. Although aerodynamics is primarily concerned with flight, its principles are also used in designing automobile and train bodies for minimum drag and in computing wind stresses on bridges, buildings, smokestacks, trees, and other structures. It is also used in charting flows of pollutants in the atmosphere and in determining frictional effects in gas ducts. The wind tunnel**wind tunnel,**

apparatus for studying the interaction between a solid body and an airstream. A wind tunnel simulates the conditions of an aircraft in flight by causing a high-speed stream of air to flow past a model of the aircraft (or part of an aircraft) being tested.

**.....**Click the link for more information. is one of the aerodynamicist's basic experimental tools; however in recent years, it has been supplanted by the simulation of aerodynamic forces during the computer-aided design

**computer-aided design**

(CAD) or

**computer-aided design and drafting**

(CADD), form of automation that helps designers prepare drawings, specifications, parts lists, and other design-related elements using special graphics- and calculations-intensive computer programs.

**.....**Click the link for more information. of aircraft and automobiles.

### The Basic Forces of Thrust, Drag, and Lift

There are three basic forces to be considered in aerodynamics: thrust, which moves an airplane forward; drag, which holds it back; and lift, which keeps it airborne. Lift is generally explained by three theories: Bernoulli's principle**Bernoulli's principle,**

physical principle formulated by Daniel Bernoulli that states that as the speed of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases.**.....** Click the link for more information. , the Coanda effect**Coanda effect**

or **wall-attachment effect,**

the tendency of a moving fluid, either liquid or gas, to attach itself to a surface and flow along it. As a fluid moves across a surface a certain amount of friction (called "skin friction") occurs between the fluid and the**.....** Click the link for more information. , and Newton's third law of motion**motion,**

the change of position of one body with respect to another. The rate of change is the speed of the body. If the direction of motion is also given, then the velocity of the body is determined; velocity is a vector quantity, having both magnitude and direction, while speed**.....** Click the link for more information. . Bernoulli's principle states that the pressure of a moving gas decreases as its velocity increases. When air flows over a wing having a curved upper surface and a flat lower surface, the flow is faster across the curved surface than across the plane one; thus a greater pressure is exerted in the upward direction. This principle, however, does not fully explain flight; for example, it does not explain how an airplane can fly upside down. Scientists have begun suggesting that the Coanda effect is at least partially responsible for how planes fly. Regardless of the shape of a plane's wing, the Coanda effect, in which moving air is attracted to and flows along the surface of the wing, and the tilt of the wing, called the angle of attack, cause the air to flow downward as it leaves the wing. The greater the angle of attack, the greater the downward flow. In obedience to Newton's third law of motion, which requires an equal and opposite reaction, the airplane is deflected upward. At the same time, a force that retards the forward motion of the aircraft is developed by diverting air in this way and is known as drag due to lift. Another kind of drag is caused by the slowing of air very near to the aircraft's surface; this can be reduced by making the surface area of the craft as small as possible. At low speeds (below Mach**Mach number**

[for E. Mach], ratio between the speed of an object and the speed of sound in the medium in which the object is traveling. An airplane that has the velocity of Mach 3.0 is traveling at three times the speed of sound as measured in the prevailing atmospheric conditions.**.....** Click the link for more information. .7) the ratio between lift and drag decreases with gains in speed; accordingly, aerodynamic development for many years stressed increases in thrust over real reductions in drag.

### Creation of Shock Waves

Above speeds of Mach .7 the air flowing over the wing accelerates above the speed of sound, causing a shock wave**shock wave,**

wave formed of a zone of extremely high pressure within a fluid, especially the atmosphere, that propagates through the fluid at a speed in excess of the speed of sound.**.....** Click the link for more information. (also known as a *sonic boom*) as the airplane compresses air molecules faster than they can move away from the airplane. The danger of this shock wave is its effect on control surfaces and fragile wing members, and for many years it was thought to represent a near-solid barrier to faster flight. The problems associated with this shock wave were ultimately conquered through the use of swept-back wings and the moving of critical control surfaces out of the wave's direct path. Chuck Yeager**Yeager, Chuck**

(Charles Elwood Yeager), 1923–, American aviator. A fighter pilot during World War II, he was a test pilot during the early postwar years. Among other records, he was the first person to fly faster than the speed of sound (1947) and set a world speed record**.....** Click the link for more information. , in 1947, was the first to fly at sustained supersonic speed. Other troublesome phenomena associated with supersonic flight are the shock waves that build up at engine air intakes, and the much larger wave that trails after the craft.

### Effect of Hypersonic Speeds

Recently, intense research has gone into the development of planes that can fly at hypersonic speeds, approximately five times or more than the speed of sound. At these speeds the properties of air change radically; there is a rapid increase in temperature associated with the air flowing at such speeds along a plane's surface. The U.S. Air Force is working to develop an aircraft that could travel at 13,000 mph (21,000 kph), a speed that would generate temperatures greater than 3,500°F; (2,000°C;).

### Bibliography

See A. M. Kuethe and C. Y. Chow, *Foundations of Aerodynamics* (5th ed. 1997); D. Anderson and S. Eberhardt, *Understanding Flight* (2001); G. Craig, *Introduction to Aerodynamics* (2003); D. Bloor, *The Enigma of the Aerofoil: Rival Theories in Aerodynamics, 1909–1930* (2011).

## Aerodynamics

The applied science that deals with the dynamics of airflow and the resulting interactions between this airflow and solid boundaries. The solid boundaries may be a body immersed in the airflow, or a duct of some shape through which the air is flowing. Although, strictly speaking, aerodynamics is concerned with the flow of air, in modern times the term has been liberally interpreted as dealing with the flow of gases in general.

Depending on its practical objectives, aerodynamics can be subdivided into external and internal aerodynamics. External aerodynamics is concerned with the forces and moments on, and heat transfer to, bodies moving through a fluid (usually air). Examples are the generation of lift, drag, and moments on airfoils, wings, fuselages, engine nacelles, and whole airplane configurations; wind forces on buildings; the lift and drag on automobiles; and the aerodynamic heating of high-speed aerospace vehicles such as the space shuttle. Internal aerodynamics involves the study of flows moving internally through ducts. Examples are the flow properties inside wind tunnels, jet engines, rocket engines, and pipes. In short, aerodynamics is concerned with the detailed physical properties of a flow field and also with the net effect of these properties in generating an aerodynamic force on a body immersed in the flow, as well as heat transfer to the body. *See* Aerodynamic force, Aerothermodynamics

Aerodynamics can also be subdivided into various categories depending on the dominant physical aspects of a given flow. In low-density flow the characteristic size of the flow field, or a body immersed in the flow, is of the order of a molecular mean free path (the average distance that a molecule moves between collisions with neighboring molecules); while in continuum flow the characteristic size is much greater than the molecular mean free path. More than 99% of all practical aerodynamic flow problems fall within the continuum category. *See* Rarefied gas flow

Continuum flow can be subdivided into viscous flow, which is dominated by the dissipative effects of viscosity (friction), thermal conduction, and mass diffusion; and inviscid flow, which is, by definition, a flow in which these dissipative effects are negligible. Both viscous and inviscid flows can be subdivided into incompressible flow, in which the density is constant, and compressible flow, in which the density is a variable. In low-speed gas flow, the density variation is small and can be ignored. In contrast, in a high-speed flow the density variation is keyed to temperature and pressure variations, which can be large, so the flow must be treated as compressible. *See* Compressible flow, Fluid flow, Incompressible flow, Viscosity

In turn, compressible flow is subdivided into four speed regimes: subsonic flow, transonic flow, supersonic flow, and hypersonic flow. These regimes are distinguished by the value of the Mach number, which is the ratio of the local flow velocity to the local speed of sound.

A flow is subsonic if the Mach number is less than 1 at every point. Subsonic flows are characterized by smooth streamlines with no discontinuity in slope. The flow over light, general-aviation airplanes is subsonic.

A transonic flow is a mixed region of locally subsonic and supersonic flow. The flow far upstream of the airfoil can be subsonic, but as the flow moves around the airfoil surface it speeds up, and there can be pockets of locally supersonic flow over both the top and bottom surfaces of the airfoil.

In a supersonic flow, the local Mach number is greater than 1 everywhere in the flow. Supersonic flows are frequently characterized by the presence of shock waves. Across shock waves, the flow properties and the directions of streamlines change discontinuously, in contrast to the smooth, continuous variations in subsonic flow. *See* Supersonic flow

Hypersonic flow is a regime of very high supersonic speeds. A conventional rule is that any flow with a Mach number equal to or greater than 5 is hypersonic. Examples include the space shuttle during ascent and reentry into the atmosphere, and the flight of the X-15 experimental vehicle. The kinetic energy of many hypersonic flows is so high that, in regions where the flow velocity decreases, kinetic energy is traded for internal energy of the gas, creating high temperatures. Aerodynamic heating is a particularly severe problem for bodies immersed in a hypersonic flow.

## Aerodynamics

a branch of hydromechanics that deals with the laws of air motion and the forces generated on the surfaces of bodies relative to which air motion occurs. In aerodynamics, motion at subsonic velocities, or velocities up to 340 m/sec (1,200 km/hr), is examined.

One of the basic tasks of aerodynamics is to predict the behavior of aircraft by calculating the aerodynamic forces acting upon them. In order to determine performance in the process of designing an airplane, helicopter, or other aircraft, so-called aerodynamic calculations are made which result in the determination of such data as the maximum, cruising, and landing speeds; the rate of climb and maximum flight altitude or “ceiling”; the range of flight; and the payload.

A special branch of aerodynamics, known as aircraft aerodynamics, is concerned with working out methods of calculation and determination of aerodynamic forces and moments acting on the aircraft as a whole and on its parts such as its wings, fuselage, and tail unit. This usually includes calculation of aircraft stability and balance as well as propeller theory. Questions linked with varying nonstationary conditions of aircraft movement are considered in a special branch of the field called flight dynamics.

Aerodynamics arose as an independent science at the beginning of the 20th century in connection with the demands of aviation. In this new field, a theory and methods were required for calculating the lift force of a wing, the aerodynamic drag of an aircraft and its parts and the thrust of a propeller. One of the first theoretical investigations of these problems in world science is contained in the works of the Russian scientists K. E. Tsiolkovskii (*On the Question of Winged Flight*, 1891) and N. E. Zhukovskii (*On a Theory of Flight*, 1891). A theory for calculating the lift force of a wing of infinite span was worked out at the beginning of the 20th century in Russia by N. E. Zhukovskii and S. A. Chaplygin, in Germany by W. Kytta, and in England by F. Lanchester. In 1912 the works of N. E. Zhukovskii appeared, which set forth the vortex theory of the propeller. A theory of cascades consisting of wing airfoils, worked out by N. E. Zhukovskii and S. A. Chaplygin, made it possible to take into account the mutual influence of propeller blades and became the basis for designing the wheels and guide cascades of turbomachines. N. E. Zhukovskii’s memoir *On the Soaring of Birds* (1892) should be considered the first work on flight dynamics. This work gives the theoretical basis for the “dead loop” first executed by the Russian aviator P. N. Nesterov in 1913.

Simultaneously with the development of a theory of flight for obtaining numerical values for aerodynamic characteristics, specialized aerodynamic laboratories were established and then became the basis for experimental aerodynamics. The pioneers in this field were N. E. Zhukovskii, the French scientist A. G. Eiffel, and the German scientist L. Prandtl. In 1902, N. E. Zhukovskii founded the Moscow State University aerodynamic laboratory and, in 1904, the aerodynamic institute at Kuchino. In 1909, A. G. Eiffel founded an aerodynamic laboratory in Paris, and soon after, L. Prandtl established one in Göttingen. In accordance with the proposal of N. E. Zhukovskii, the Central Aerodynamic and Hy-drodynamic Institute (TsAGI) was founded in 1918; it is at present one of the largest centers of research in aerodynamics in the world.

In addition to N. E. Zhukovskii and S. A. Chaplygin, other scientists have contributed greatly to the development of aerodynamics, including the Soviet scientists V. P. Vetchin-kin, A. A. Dorodnitsyn, M. V. Keldysh, M. A. Lavrent’ev, G. I. Petrov, L. I. Sedov, A. N. Tupolev, S. A. Kristia-novich, and B. N. Iur’ev; the German scholars L. Prandtl, H. Schlichting, and A. Busemann; the English scholars H. Glauert, F. Lanchester, and A. Fage; and the American scholars T. Von Kármán, H. Dryden, and G. Taylor.

A distinction is made between theoretical and experimental aerodynamics according to the methods applied in solving the problems which arise. Theoretical aerodynamics seeks solutions by means of a theoretical analysis of the fundamental laws of hydroaeromechanics which were formulated into equations by L. Euler, J. Lagrange, L. M. H. Navier, and G. Stokes, among others. The solution, or integration, of these equations for the majority of important practical problems is possible, even today, only on the assumption that the viscosity of the air is zero; that is, replacing the air with an “ideal” gas. However, the solution of equations simplified in this manner gives results which contradict those obtained from experiments. For example, the aerodynamic drag of a sphere turns out to be zero (the d’Alembert-Euler paradox). This contradiction was to a certain extent resolved by L. Prandtl, who proposed that the space in which disturbances caused by a moving body are observed be divided into two regions: the region close to the surface of the body called the boundary layer, where effects of viscosity are felt, and the region outside the boundary layer, where the air may be considered an ideal gas.

Prandtl’s hypothesis and the equations he evolved in 1904 for the motion of gas in the boundary layer were further developed in the research of many scientists, including such Soviet scientists as L.G. Loitsianskii and A. A. Dorodnitsyn; this work made it possible to solve a great number of problems. The proposed scheme does not fully correspond to the reality of existing flows; in addition, these methods do not permit theoretical calculation of the flow in the case of a turbulent boundary layer or for bodies of complex form. In these cases it is necessary to apply empirical methods worked out in the experimental study of models of the flow under consideration. By analyzing the fundamental air flow laws of theoretical aerodynamics, questions are worked out which deal with the similarity theory and modeling; these permit determination of the aerodynamic forces acting on an aircraft from results obtained on a small-scale model of the aircraft. The modeling theory makes possible as well the determination of the conditions under which the model should be tested. This area of theoretical aerodynamics forms the basis of experimental aerodynamics, the main purpose of which is to obtain numerical values for the aerodynamic forces acting on the aircraft by testing a model in special kinds of equipment. Experimental aerodynamics makes wide use of the law of motion reversal, in which a force acting on a body moving with velocity *ν* is equal to the force acting on the same body when stationary and struck by an air current with identical velocity v.

Wind tunnels, in which forces and moments acting on a stationary body are measured, are a main part of the experimental equipment of aerodynamic laboratories. Methods of aerodynamic measurement permit a detailed investigation of the forces acting on a model, and likewise the determination of values for velocity, density, and the temperature of the air in front of and behind the model.

When the flight velocity is increased and approaches the speed of sound, it becomes necessary to consider the compressibility of the medium. Supersonic flight of a body is characterized by a number of peculiarities. Shock waves are generated which increase the aerodynamic drag, and the flying body becomes heated from air friction and by the radiation of gas behind the shock wave. During flight at great supersonic velocities, dissociation and ionization of the gas in the shock waves occur. All these questions linked to the supersonic motion of a body are usually related to the branch of hydroaeromechanics called gas dynamics.

Much of the nonaviation application of aerodynamics is in the field of industrial aerodynamics. In this field questions are examined which relate to the design of blowers, wind motors, jet blowers or ejectors, and ventilation technology (especially airconditioning), as well as questions linked with aerodynamic forces in land transportation (automobiles, trains) and with wind stress on buildings and installations.

In the USSR, in addition to TsAGI, other institutions carry on scientific research in the field of aerodynamics. These include the Central Institute of Aircraft Engine Construction (TsIAM); the scientific research institutes of the Academy of Sciences of the USSR; branches of scientific research institutes at the Moscow, Leningrad, and other universities; the Moscow and Kharkov aviation institutes; Moscow Higher Technical School (MVTU); N. E. Zhukovskii Air Force Engineering Academy; and other institutions of higher learning. In the USA most aerodynamic investigation is done by the National Aeronautics and Space Administration (NASA), which has large laboratory centers at Moffett Field in California and Langley Field in Virginia. Others involved in such work are the California and Massachusetts institutes of technology, the air force, the navy, and the laboratories of large corporations which produce aircraft, rockets, and armaments. There are also large research centers in Great Britain, France, Japan, and other countries.

The results of scientific investigations are published in the periodicals *Izvestiia Akademii Nauk SSSR: Mekhanika zhid-kosti i gaza* (since 1966), *Zhurnal prikladnoi mekhaniki i tekhnicheskoi fiziki* (since 1960), *AIAA Journal* (New York since 1963, translated into Russian), *Journal of the Royal Aeronautical Society* (London, since 1897), and *Technique et science aéronautiques et spatiales* (Paris, since 1943).

### REFERENCES

Fabrikant, N.*la. Aerodinamika*, part 1. Moscow-Leningrad, 1962.

Prandtl, L.

*Gidrodinamika*, 2nd ed. Moscow, 1951. (Translated from German.)

Martynov, A. K.

*Eksperimental’naia aerodinamika*, 2nd. ed. Moscow, 1958.

Pyshnov, V. S.

*Aerodinamika samoleta*. Moscow, 1943.

Ostoslavskii, I. V., and V. M. Titov.

*Aerodinamicheskii raschet samoleta*. Moscow, 1947.

Glauert, H.

*Osnovy teorii kryl’ev i vinta*. Moscow-Leningrad, 1931. (Translated from English.)

M. IA. IUDELOVICH

## aerodynamics

[‚e·rō·dī′nam·iks]## Aerodynamics

The applied science that deals with the dynamics of airflow and the resulting interactions between this airflow and solid boundaries. The solid boundaries may be a body immersed in the airflow, or a duct of some shape through which the air is flowing. Although, strictly speaking, aerodynamics is concerned with the flow of air, in modern times the term has been liberally interpreted as dealing with the flow of gases in general.

Depending on its practical objectives, aerodynamics can be subdivided into external and internal aerodynamics. External aerodynamics is concerned with the forces and moments on, and heat transfer to, bodies moving through a fluid (usually air). Examples are the generation of lift, drag, and moments on airfoils, wings, fuselages, engine nacelles, and whole airplane configurations; wind forces on buildings; the lift and drag on automobiles; and the aerodynamic heating of high-speed aerospace vehicles such as the space shuttle. Internal aerodynamics involves the study of flows moving internally through ducts. Examples are the flow properties inside wind tunnels, jet engines, rocket engines, and pipes. In short, aerodynamics is concerned with the detailed physical properties of a flow field and also with the net effect of these properties in generating an aerodynamic force on a body immersed in the flow, as well as heat transfer to the body. *See* Aerodynamic force, Aerothermodynamics

Aerodynamics can also be subdivided into various categories depending on the dominant physical aspects of a given flow. In low-density flow the characteristic size of the flow field, or a body immersed in the flow, is of the order of a molecular mean free path (the average distance that a molecule moves between collisions with neighboring molecules); while in continuum flow the characteristic size is much greater than the molecular mean free path. More than 99% of all practical aerodynamic flow problems fall within the continuum category.

Continuum flow can be subdivided into viscous flow, which is dominated by the dissipative effects of viscosity (friction), thermal conduction, and mass diffusion; and inviscid flow, which is, by definition, a flow in which these dissipative effects are negligible. Both viscous and inviscid flows can be subdivided into incompressible flow, in which the density is constant, and compressible flow, in which the density is a variable. In low-speed gas flow, the density variation is small and can be ignored. In contrast, in a high-speed flow the density variation is keyed to temperature and pressure variations, which can be large, so the flow must be treated as compressible.

In turn, compressible flow is subdivided into four speed regimes: subsonic flow, transonic flow, supersonic flow, and hypersonic flow. These regimes are distinguished by the value of the Mach number, which is the ratio of the local flow velocity to the local speed of sound.

A flow is subsonic if the Mach number is less than 1 at every point. Subsonic flows are characterized by smooth streamlines with no discontinuity in slope. The flow over light, general-aviation airplanes is subsonic. *See* Subsonic flight

A transonic flow is a mixed region of locally subsonic and supersonic flow. The flow far upstream of the airfoil can be subsonic, but as the flow moves around the airfoil surface it speeds up, and there can be pockets of locally supersonic flow over both the top and bottom surfaces of the airfoil. *See* Transonic flight

In a supersonic flow, the local Mach number is greater than 1 everywhere in the flow. Supersonic flows are frequently characterized by the presence of shock waves. Across shock waves, the flow properties and the directions of streamlines change discontinuously, in contrast to the smooth, continuous variations in subsonic flow. *See* Supersonic flight

Hypersonic flow is a regime of very high supersonic speeds. A conventional rule is that any flow with a Mach number equal to or greater than 5 is hypersonic. Examples include the space shuttle during ascent and reentry into the atmosphere, and the flight of the X‐15 experimental vehicle. The kinetic energy of many hypersonic flows is so high that, in regions where the flow velocity decreases, kinetic energy is traded for internal energy of the gas, creating high temperatures. Aerodynamic heating is a particularly severe problem for bodies immersed in a hypersonic flow. *See* Hypersonic flight

## aerodynamics

**i**. The science that deals with the motion of air and other gaseous fluids and of the forces acting on bodies when the bodies move through such fluids, or when such fluids move against or around the bodies. It also studies the qualities required for fast and efficient movement through the air and methods of reducing resistance and drag to the body in its movement.

**ii**. The actions and forces resulting from the movement or flow of gaseous fluids against or around bodies, as for example, the aerodynamics of a wing in supersonic flight.

**iii**. The application of the principles of gaseous fluid flows and of their actions against and around bodies to the design and construction of bodies intended to move through such fluids, such as a design used in aerodynamics.