Aerodynamics

Aerodynamics is the science of airflow over airplanes, cars, buildings, and other objects. Aerodynamic principles are used to find the best ways in which airplanes produce lift, reduce drag, and remain stable (by controlling the shape and size of the wing, the angle at which it is positioned with respect to the airstream, and the flight speed). The flight characteristics change at higher altitudes as the surrounding air becomes colder and thinner. The behavior of the airflow also changes dramatically at flight speeds close to, and beyond, the speed of sound. The explosion in computational capability has made it possible to understand and exploit the concepts of aerodynamics and to design improved wings for airplanes. Increasingly sophisticated wind tunnels are also available to test new models.

Airflow is governed by the principles of fluid dynamics that deal with the motion of liquids and gases in and around solid surfaces. The viscosity, density, compressibility, and temperature of the air determine how the air will flow around a building or a plane. The viscosity of a fluid is its resistance to flow. Even though air is 55 times less viscous than water , viscosity is important near a solid surface because air, like all other fluids, tends to stick to the surface and slow down the flow. A fluid is compressible if its density can be increased by squeezing it into a smaller volume. At flow speeds less than 220 mph (354 kph), one third the speed of sound, we can assume that air is incompressible for all practical purposes. At speeds closer to that of sound (660 mph [1,622 kph]) however, the variation in the density of the air must be taken into account. The effects of temperature change also become important at these speeds. A regular commercial airplane, after landing, will feel cool to the touch. The Concorde jet, which flies at twice the speed of sound, can feel hotter than boiling water.

Flow patterns of the air may be laminar or turbulent. In laminar or streamlined flow, air, at any point in the flow, moves with the same speed in the same direction at all times so that smoke in the flow appears to be smooth and regular. The smoke then changes to turbulent flow, which is cloudy and irregular, with the air continually changing speed and direction.

Laminar flow, without viscosity, is governed by Bernoulli's principle that states that the sum of the static and dynamic pressures in a fluid remains the same. A fluid at rest in a pipe exerts static pressure on the walls. If the fluid starts moving, some of the static pressure is converted to dynamic pressure, which is proportional to the square of the speed of the fluid. The faster a fluid moves, the greater its dynamic pressure and the smaller the static pressure it exerts on the sides.

Bernoulli's principle works very well far from the surface. Near the surface, however, the effects of viscosity must be considered since the air tends to stick to the surface, slowing down the flow nearby. Thus, a boundary layer of slow-moving air is formed on the surface of an airplane or automobile. This boundary layer is laminar at the beginning of the flow, but it gets thicker as the air moves along the surface and becomes turbulent after a point.

Airflow is determined by many factors, all of which work together in complicated ways to influence flow. Very often, the effects of factors such as viscosity, speed, and turbulence cannot be separated. Engineers have found ingenious ways to get around the difficulty of treating such complex situations. They have defined some characteristic numbers, each of which tells us something useful about the nature of the flow by taking several different factors into account.

One such number is the Reynolds number, which is greater for faster flows and denser fluids and smaller for more viscous fluids. The Reynolds number is also higher for flow around larger objects. Flows at lower Reynolds numbers tend to be slow, viscous, and laminar. As the Reynolds number increases, there is a transition from laminar to turbulent flow. The Reynolds number is a useful similarity parameter. This means that flows in completely different situations will behave in the same way as long as the Reynolds number and the shape of the solid surface are the same. If the Reynolds number is kept the same, water moving around a small stationary airplane model will create exactly the same flow patterns as a full-scale airplane of the same shape, flying through the air. This principle makes it possible to test airplane and automobile designs using small-scale models in wind tunnels.

At speeds greater than 220 mph (354 kph), the compressibility of air cannot be ignored. At these speeds, two different flows may not be equivalent even if they have the same Reynolds number. Another similarity parameter, the Mach number, is needed to make them similar. The Mach number of an airplane is its flight speed divided by the speed of sound at the same altitude and temperature. This means that a plane flying at the speed of sound has a Mach number of one.

The drag coefficient and the lift coefficient are two numbers that are used to compare the forces in different flow situations. Aerodynamic drag is the force that opposes the motion of a car or an airplane. Lift is the upward force that keeps an airplane afloat against gravity . The drag or lift coefficient is defined as the drag or lift force divided by the dynamic pressure, and also by the area over which the force acts. Two objects with similar drag or lift coefficients experience comparable forces, even when the actual values of the drag or lift force, dynamic pressure, area, and shape are different in the two cases.

There are several sources of drag. The air that sticks to the surface of a car creates a drag force due to skin friction. Pressure drag is created when the shape of the surface changes abruptly, as at the point where the roof of an automobile ends. The drop from the roof increases the space through which the air stream flows. This slows down the flow and, by Bernoulli's principle, increases the static pressure. The air stream is unable to flow against this sudden increase in pressure and the boundary layer gets detached from the surface creating an area of low-pressure turbulent wake or flow. Because the pressure in the wake is much lower than the pressure in front of the car, a net backward drag or force is exerted on the car. Pressure drag is the major source of drag on blunt bodies. Car manufacturers experiment with vehicle shapes to minimize the drag. For smooth or "streamlined" shapes, the boundary layer remains attached longer, producing only a small wake. For such bodies, skin friction is the major source of drag, especially if they have large surface areas. Skin friction comprises almost 60% of the drag on a modern airliner.

An airfoil is the two-dimensional cross-section of the wing of an airplane as one looks at it from the side. It is designed to maximize lift and minimize drag. The upper surface of a typical airfoil has a curvature greater than that of the lower surface. This extra curvature is known as camber. The straight line, joining the front tip or the leading edge of the airfoil to the rear tip or the trailing edge, is known as the chord line. The angle of attack is the angle that the chord line forms with the direction of the air stream.

The stagnation point is the point at which the stream of air moving toward the wing divides into two streams, one flowing above and the other flowing below the wing. Air flows faster above a wing with greater camber since the same amount of air has to flow through a narrower space. According to Bernoulli's principle, the faster flowing air exerts less pressure on the top surface, so that the pressure on the lower surface is higher, and there is a net upward force on the wing, creating lift. The camber is varied, using flaps and slats on the wing in order to achieve different degrees of lift during takeoff, cruise, and landing.

Because the air flows at different speeds above and below the wing, a large jump in speed will tend to arise when the two flows meet at the trailing edge, leading to a rearward stagnation point on top of the wing. Wilhelm Kutta (1867–1944) discovered that a circulation of air around the wing would ensure smooth flow at the trailing edge. According to the Kutta condition, the strength of the circulation, or the speed of the air around the wing, is exactly as much as is needed to keep the flow smooth at the trailing edge.

Increasing the angle of attack moves the stagnation point down from the leading edge along the lower surface so that the effective area of the upper surface is increased. This results in a higher lift force on the wing. If the angle is increased too much, however, the boundary layer is detached from the surface, causing a sudden loss of lift. This is known as a stall; the angle at which this occurs for an airfoil of a particular shape is known as the stall angle.

The airfoil is a two-dimensional section of the wing. The length of the wing in the third dimension, out to the side, is known as the span of the wing. At the wing tip at the end of the span, the high-pressure flow below the wing meets the low-pressure flow above the wing, causing air to move up and around in wing-tip vortices. These vortices are shed as the plane moves forward, creating a downward force or down-wash behind it. The downwash makes the airstream tilt downward and the resulting lift force tilt backward so that a net backward force or drag is created on the wing. This is known as induced drag or drag due to lift. About one third of the drag on a modern airliner is induced drag.

In addition to lift and drag, the stability and control of an aircraft in all three dimensions is important since an aircraft, unlike a car, is completely surrounded by air. Various control devices on the tail and wing are used to achieve this. Ailerons, for instance, control rolling motion by increasing lift on one wing and decreasing lift on the other.

Flight at speeds greater than that of sound are supersonic. Near a Mach number of one, some portions of the flow are at speeds below that of sound, while other portions move faster than sound. The range of speeds from Mach number 0.8 to 1.2 is known as transonic. Flight at Mach numbers greater than five is hypersonic.

The compressibility of air becomes an important aerodynamic factor at these high speeds. The reason for this is that sound waves are transmitted through the successive compression and expansion of air. The compression due to a sound wave from a supersonic aircraft does not have a chance to get away before the next compression begins. This pile up of compression creates a shock wave, which is an abrupt change in pressure, density, and temperature. The shock wave causes a steep increase in the drag and loss of stability of the aircraft. Drag due to the shock wave is known as wave drag. The familiar "sonic boom" is heard when the shock wave touches the surface of the earth.

Temperature effects also become important at transonic speeds. At hypersonic speeds above a Mach number of five, the heat causes nitrogen and oxygen molecules in the air to break up into atoms and form new compounds by chemical reactions. This changes the behavior of the air and the simple laws relating pressure, density, and temperature become invalid.

The need to overcome the effects of shock waves has been a formidable problem. Swept-back wings have helped to reduce the effects of shock. The supersonic Concorde that cruises at Mach 2 and several military airplanes have delta or triangular wings. The supercritical airfoil designed by Richard Whitcomb of the NASA Langley Laboratory has made air flow around the wing much smoother and has greatly improved both the lift and drag at transonic speeds. It has only a slight curvature at the top and a thin trailing edge. The proposed hypersonic aerospace plane is expected to fly partly in air and partly in space and to travel from Washington to Tokyo within two hours. The challenge for aerodynamicists is to control the flight of the aircraft so that it does not burn up like a meteor as it returns to Earth at several times the speed of sound.

See also Atmosphere; Atmospheric circulation; Atmospheric composition and structure; Atmospheric pressure; Aviation physiology; Bernoulli's equation; Meteorology; Physics; Space physiology; Wind shear

Aerodynamics

Aerodynamics is the study of air flow over airplanes, cars, and other objects. Airplanes fly because of the way in which air flows over their wings and around their bodies, so a knowledge of aerodynamics is crucial to the design and construction of airplanes. The efficiency with which automobiles use fuel is also a function of air flow. Even stationary objects are affected by aerodynamics. Winds blowing past a tall building, for example, may cause windows to pop out if they are not properly designed and installed.

Factors affecting air flow

Four properties of air affect the way in which it flows past an object: viscosity, density, compressibility, and temperature. Viscosity is the resistance of a fluid to flow. Molasses is very viscous because is flows slowly, while water is less viscous because it flows readily. The viscosity of air is important in aerodynamics because air tends to stick to any surface over which it flows, slowing down the motion of the air.

The density and compressibility of air are important factors at high speeds. As an object travels rapidly through air, it causes air to become compressed and more dense. As a result, other properties of air then change.

The effects of temperature change on air flow also become important at high speeds. A regular commercial airplane, after landing, will feel cool to the touch. But the Concorde jet, which flies at twice the speed of sound, will feel hotter than boiling water.

Laminar and turbulent flow

Air can travel over a surface following patterns of flow referred to as either laminar or turbulent. In laminar or streamlined flow, air moves with the same speed in the same direction at all times. The flow appears to be smooth and regular. Bernoulli's principle applies during laminar flow. Bernoulli's principle states that a fluid (such as air) traveling over the surface of an object exerts less pressure than if the fluid were still. Airplanes fly because of Bernoulli's principle. When an airplane takes off, air rushes over the top surface of its wing, reducing pressure on the upper surface of the wing. Normal pressure below the wing pushes the wing upward, carrying the airplane upward along with it.

Turbulent flow is chaotic and unpredictable. It consists of irregular eddies (circular currents) of air that push on a surface in unexpected ways. The bumpy ride you may have experienced on a commercial airplane could have resulted from the development of turbulent flow over the airplane's wings.

Skin friction and pressure drag

Drag is any force that tends to prevent an object from moving forward. One source of drag in airplanes and automobiles is skin friction. As air passes over the surface of either vehicle, friction between air and surface tends to slow the plane or automobile down. One of the goals of transportation engineers is to find a shape that has the least amount of skin friction, thus reducing the amount of drag on the vehicle. Sleek, tear-drop-shaped cars became popular in the middle to late 1990s because they had so little skin friction.

Engineers also work to reduce pressure drag. Pressure drag is caused by abrupt changes in the shape of a car or airplane. The point at which the roof of a car ends, as an example, is a point of high pressure drag. A car designed with a smooth transition from roof to trunk will have less pressure drag and, therefore, will travel more smoothly.

Words to Know

Airfoil: The cross section of an airplane wing parallel to (or running in the same direction as) the length of the plane.

Angle of attack: The angle that the length of the airfoil forms with an oncoming airstream.

Camber: The additional curvature of the upper surface of the airfoil relative to the lower surface.

Induced drag: Also known as drag due to lift; the drag on the airplane due to vortices (whirling patterns of air) on the wingtips.

Stall: A sudden loss of lift on the airplane wing when the angle of attack increases beyond a certain value.

Supersonic speed: A speed greater than that of sound.

Airfoil

An airfoil is a two-dimensional cross section of the wing of an airplane as viewed from the side. Engineers seek to design airfoils that will have the greatest amount of lift and the least amount of drag. One factor important in the design of an airfoil is the curvature, or camber, of the upper side of the wing. The greater the camber, the faster air moves over the upper surface and the greater the lift to the wing. The amount of camber in a wing is changed by means of flaps and slats in the wing that produce different amounts of lift during take-off, cruise, and landing.

Another factor affecting lift is the angle of attack: the position of the wing in comparison to the ground. As the forward edge of the wing is tipped downward, the amount of lift on the wing is increased. Increasing the angle of attack too much, however, may result in a sudden loss of lift, causing the airplane to lose all lift and go into a stall.

Induced drag

Air movement around an airplane wing can also cause drag effects. At the very tip of the wing, air traveling above and below the wing meet and produce whirlpool-like patterns known as vortices. These vortices tend to pull the wing downward, thus producing drag forces on the wing. These forces are known as induced drag.

Bernoulli's Principle

What do jet airplanes and a baseball pitcher's curveball have in common? They both depend on Bernoulli's principle for their operation. Bernoulli's principle states that the faster a fluid (such as air or water) flows over a surface, the less pressure the fluid exerts on that surface.

In the case of the jet airplane, air travels faster over the top of the wing than across the bottom of the wing. Since the pressure on top of the wing is reduced, the airplane is pushed upward. A pitcher can cause a baseball to curve by making it spin. On one side of the ball, air carried along by the spinning ball rushes past in the same direction the ball is traveling. On the other side of the ball, air is pushed in the opposite direction. On the side of the ball where air is traveling faster, pressure is reduced. Higher air pressure on the opposite side of the ball pushes it out of a straight path, causing it to curve.

Stability and control

Aerodynamics is applied to other problems of air flight as well. Airplanes have tendencies to rotate in one of three directions: in a horizontal back-and-forth motion around the center of the airplane, in a front-over-back motion, or in a rolling fashion, wing-over-wing. Special controls must be developed to prevent loss of control in any of these directions, any one of which could cause the airplane to crash. Ailerons (pronounced AYL-uh-ronz) are one such control. They prevent a rolling action by increasing the lift on one wing while decreasing it on the other.

Supersonic flight

Flight at speeds greater than that of sound present special problems for engineers. One reason is the importance of the compressibility of air at these speeds. Sound waves produced by an aircraft moving through the air travel more slowly than the aircraft itself. This produces a shock wave characterized by an abrupt change in temperature, pressure, and density that causes a steep increase in the drag and loss of stability of the aircraft. The loud sonic boom one hears is the wave hitting Earth's surface.

The need to overcome the effects of shock waves has been a major problem for engineers. Swept-back wings are one way of reducing the effects of shock. The delta or triangular wings of the supersonic Concorde and several military airplanes are another solution. The most advanced airplane designs actually call for the complete retraction of the wings during the fastest part of a flight, converting them into a rocket for part of the trip.

[See also Aircraft; Balloon; Fluid dynamics ]

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 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 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 , the Coanda effect , and Newton's third law of motion . 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 .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 (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 , 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, a branch of the science of pneumatics which deals with air and other gases in motion and with their mechanical effects. In its maritime connection it can be used to explain how a wind produces forward motion in a sailing vessel even when it blows from before the vessel's beam.

When a wind strikes a surface at an angle, its force can be resolved into two components, one acting at right angles to the surface and the other along the surface. If this surface is the sail of a boat, the component blowing along the sail can be disregarded, as it is providing no force on the sail, but the component at right angles to the sail does exert a force. That component can now similarly be resolved into two more components, not in relation to the angle of the sail but to the fore-and-aft line of the boat. The larger of these two components exerts a force which tries to blow the boat directly to leeward, and the smaller of them, blowing along the fore-and-aft line, is all that is left of the wind to drive the boat forward. It is at this point that the boat's keel, or the centreboard in the case of dinghies, comes into play. It provides a lateral grip on the water which offers considerable resistance to the larger component and very little resistance to the smaller, so that the boat moves forward and makes only a small amount of leeway.

The aerodynamic forces on the sail, described above, arise from the flow of air over the sail and the changes in the pressures acting on the sail which this produces. In plain view, a section of a sail is roughly parabolic in form, with the steepest part of the curve at its luff. When an airstream strikes such a surface at an angle, it accelerates over the upper surface, thereby reducing pressure there. Accelerations are not so great over the lower surface so the overall effect is to create a force acting on the sail to leeward. Resolving this force into longitudinal and lateral components provides the driving and heeling/sideslipping forces mentioned above.

The greater the speed of flow over the upper surface of the sail, the greater the aerodynamic force, and the faster the yacht moves forward. With this in mind, modern sail plans increase the speed of the airflow by means of a foresail (or sails) so aligned to produce a slot or ‘funnel’ along the luff of the mainsail. The slot increases the flow by acting as a form of venturi (duct). In designs where the clew of the foresail overlaps the luff of the mainsail, the airflow is funneled with even greater speed over the steepest part of the mainsail's curve, thereby increasing the aerodynamic force.

Therefore, when a vessel is sailing close-hauled, with the wind blowing forward of the beam, the aerodynamic forces resulting from pressure changes over the sails allow it to move against the wind. These also cause heel, which is resisted by the crew (of a sailing dinghy) or the ballast keel (of a displacement yacht), and leeway which is resisted by hydrodynamic forces generated by the centreboard or keel.

aer·o·dy·nam·ics / ˌe(ə)rōdīˈnamiks/ • pl. n. [treated as sing.] the study of the properties of moving air, and esp. of the interaction between the air and solid bodies moving through it. ∎  the properties of a solid object regarding the manner in which air flows around it. ∎  [treated as pl.] these properties insofar as they result in maximum efficiency of motion. DERIVATIVES: aer·o·dy·nam·i·cist / -ˈnaməsist/ n.

aerodynamics Science of gases in motion and the forces acting on objects, such as aircraft, in motion through the air. An aircraft designer must consider four main factors and their interrelationships: weight of the aircraft and the load it will carry; lift to overcome the pull of gravity; drag, or the forces that retard motion; and thrust, the driving force. Air resistance (drag) increases as the square of an object's speed and is minimized by streamlining. Engineers use the wind tunnel and computer systems to predict aerodynamic performance.

aerodynamics n.
1. the study of the properties of moving air, and especially of the interaction between the air and solid bodies moving through it.

2. the properties of a solid object regarding the manner in which air flows around it.