Aerodynamics

Aerodynamics is the branch of fluid dynamics that studies gases (especially air) in motion and related forces. It is of great importance in the design and use of moving objects such as aircraft and automobiles and stationary objects such as skyscrapers.

Overview

Aerodynamics is a branch of fluid dynamics. It studies the movement of bodies through gases such as air or the movement of these gases around stationary bodies; it is crucial in understanding the movement and safe operation of ground vehicles and aircraft and to the building and maintenance of skyscrapers.

Without a knowledge of aerodynamic principles, heavier-than-air flight would not be possible. The Italian Renaissance architect, artist, and engineer Leonardo da Vinci studied birds in flight in the late fifteenth century and conceived of devices such as birdlike wings and helicopter-like machines that would enable human beings to fly. All of his proposed inventions failed, however, because he had no knowledge of aerodynamics. In fact, aerodynamics did not develop enough to engender heavier-than-air flight until the early twentieth century, when Wilbur and Orville Wright used aerodynamics principles to design the first manned aircraft, a biplane with two fixed wings.

In the case of heavier-than-air aircraft, before the vehicle can climb into the air in controlled flight, the force of gravity must be conquered. Three main forces are involved. The first is thrust (or thrust force). Thrust force is caused by the propeller-driven or jet-powered engines of an aircraft. It enables an aircraft to move forward as long as the aircraft's design allows the applied thrust to exceed the force (called "drag") caused by the viscosity of the air through which the aircraft moves. Drag diminishes the speed of any moving vehicle because of air resistance. The thrust-to-drag ratio can be greatly increased by streamlining the aircraft, or shaping it so that the drag experienced is minimized.

A knowledge of the forces of thrust and drag are enough to design a very fast-moving ground vehicle such as a race car, but the key to achieving flight is the third of the aerodynamic forces: lift. A thorough understanding of lift is necessary to conceptualize and actualize aircraft. Lift enables aircraft to rise into the air and move upward, at a right angle to the forward motion of the aircraft. It is supplied by the aircraft's airfoils (mostly by wings but also via tail assemblies). Airfoils are designed with a rounded leading (front) edge and a sharp trailing (rear) edge, so that the angle at which they meet the air causes air to flow much more rapidly past the upper airfoil surface than past its lower surface. This makes the air pressure above the airfoil lower than the air pressure below it, producing the amount of lift needed to raise an aircraft into flight. This asymmetrical airflow is produced by the curved shape of the airfoil (its camber) and its design, which allows it to meet approaching air at an angle (the angle of attack). In addition, the airflow from above and below an airfoil must merge smoothly as the air leaves its trailing edge, producing what is known as the "Kutta condition."

The importance of the angle of attack of airfoils is shown by pilot use and misuse of the angle during flight. An aircraft's angle of attack can be changed by altering its position in space. Up to 15 degrees, increasing the angle of attack enhances the lift produced by aircraft airfoils, enabling faster climbing rates though slowing airspeed. Once the angle of attack becomes too steep, air eddies form atop the airfoil and cause large decreases in lift, which make the aircraft drop toward the ground in a stall. When a pilot's misjudgment produces a stall, the aircraft will crash unless the pilot quickly decreases the angle of attack to a safe value.

To produce appropriate lift, an airfoil must move through the air above a minimum speed that is associated with the aircraft to which it is attached. However, during landing and takeoff, safety concerns make it desirable to fly as slowly as possible. Because of these conflicting demands, aircraft have special assemblies (or parts) called high-lift devices. Two important high-lift devices are flaps and slats. A flap is the hinged portion of the back of each airfoil. In flight, it fits smoothly, in line with the airfoil. However, an aircraft's flaps may be lowered on takeoff or landing. The aircraft's camber increases when this is done. Lowered flaps furnish the extra lift needed on takeoff or a slower aircraft ground speed upon landing. The slats in aircraft airfoils are hinged sections that are located at the front tip of each airfoil. Slat design causes them to automatically move forward when an aircraft slows down, increasing the craft's lift by adding to the camber of each airfoil.

To create an operational aircraft design, the airplane's aerodynamics are analyzed to ensure that its overall composition, including fuselage, airfoils, and high-lifting devices will allow the aircraft to fly well and do so at a cost that makes its use economically feasible. The aircraft's body (or fuselage) is streamlined as much as possible, and the airfoil ability of the aircraft's wings and tail are modified as needed by changing their size, shape, and high-lift device content to optimum dimensions. Optimization is facilitated by studying scaled-down aircraft models in wind tunnels and via computer simulations.

Another aerodynamic concept aircraft manufacturers must take into account is the boundary layer of air surrounding an aircraft. The boundary layer is the portion of the air closest to the aircraft's surface. This is the region where the very strongest effects of turbulence caused by air resistance occur. Minimization of this turbulence is essential for both the optimization of passenger comfort and aircraft longevity. Tools used to minimize air turbulence in the boundary layer include streamlining the aircraft and making all aircraft surfaces as smooth as possible.

Aircraft are designed somewhat differently for operation at subsonic, supersonic, and hypersonic airspeeds. These airspeeds are, respectively, below the speed of sound, between the speed of sound and twice that airspeed, and about five times the speed of sound or faster. Hypersonic aircraft are largely space vehicles. Subsonic aircraft are propeller-driven airplanes, and supersonic aircraft are jet planes.

The speed of sound varies with the density of the air through which it travels. Near the earth's surface, sound moves more slowly than high in the atmosphere where the air is thinner. Aircraft designers use the ratio of the airspeed attained to the speed of sound to more accurately designate aircraft speed. This ratio, created by Austrian engineer Ernst Mach is called the Mach number of an aircraft. All subsonic flight (below the speed of sound) therefore occurs at speeds under Mach 1. Aircraft flying at subsonic speeds are in a milieu where the air is an incompressible fluid of unchanging density. The aerodynamics of flight for these vessels is relatively simple. At Mach 1 and above, additional problems associated with flight and aircraft design occur. Air density, air pressure, and air temperature begin to have effects and complicate aerodynamic issues. To counter these effects, supersonic aircraft fly at much higher altitudes than subsonic aircraft. The main advantage of high-altitude flight is the much lower air density that the aircraft encounters. However, any high-altitude flight requires additional design changes such as pressurization of the aircraft cabin.

The increase of airspeed from just below Mach 1 to just above it is called breaking the sound barrier, a term originated by World War II pilots. Breaking this barrier causes variable shock waves that throw aircraft about and require additional aircraft modifications to minimize in-flight positional instability. Fortunately supersonic jets travel well above Mach 1 and do not have this turbulence problem because the shock waves that are associated with their flight speeds are more constant and therefore more manageable.

Another interesting aerodynamic property of aircraft relates to the air-pressure disturbances they create because of airflow around airfoils and fuselages. These pressure disturbances move away from the aircraft at the speed of sound. Therefore, aircraft in subsonic flight create pressure disturbances that precede them, and people on the ground hear these aircraft approaching and getting louder. In contrast, an aircraft in supersonic flight produces pressure disturbances that most often become sonic booms (or bangs) heard only after they pass by. These air-pressure disturbances can damage and even destroy aircraft that are not designed to withstand them. The effects of air-pressure disturbances are factored into aircraft design. In the early days of supersonic aircraft, the absence of alterations in aircraft design to accommodate the effects of air-pressure disturbances and shock waves led to numerous crashes.

Applications

Aerodynamics can be applied to understanding the fabrication, the movement, and the safe operation of ground vehicles and aircraft and the construction of durable skyscrapers. A basic understanding of aerodynamics was necessary before flight was possible. The first heavier-than-air manned aircraft, developed by Wilbur and Orville Wright in 1903, was flown four times for a maximum flight time of one minute before it was destroyed in a crash. This biplane, which had paired airfoils joined by braces, was much slower than desired. However, it formed the basis for the much faster, more advanced biplane versions designed to better employ aerodynamic principles that were widely used for the next twenty years.

In the early 1920s, single-wing aircraft (or monoplanes) began to replace the biplanes. All modern aircraft, from propeller-driven to jet airplanes, are monoplanes. The main initial modifications of monoplanes were alterations of airfoil size, shape, and camber and the streamlining of their engines and fuselages.

The site where airfoils are attached varies greatly. One of the most common designs, low-wing aircraft, have twin airfoils located at the fuselage low point. Whether in a propeller-driven or jet aircraft, this arrangement is preferred because it engenders the fastest version of an aircraft whose speed is determined solely by airfoil position. It also gives pilots and passengers the greatest safety margin in crashes. However, airfoils may be put anywhere on the fuselage. For example, in seaplanes, where high speed is not the main determinant of aircraft function, the airfoils may consist of a single parasol wing connected to the fuselage top by braces.

Another important variant among aircraft has been the number and type of engines used to power them. Most aircraft flown before the 1960s were propeller driven, and their thrust derived from one to eight engines. In single-engine aircraft, the propeller-driven engines are put in the front of the vehicle's fuselage. In multiengine aircraft, up to four propeller-driven engines are located in each airfoil. Usually, four engines power even large aircraft, and eight engines are reserved for huge aircraft such as the largest bombers. These propeller-driven aircraft gradually began to be replaced by aircraft with jet engines, and by the early 1960s, jet engines had become very common. By the end of the twentieth century, almost all new aircraft except for small personal models used jet engines for the sake of speed. The fastest propeller-driven aircraft cannot exceed 650 miles per hour. Jet aircraft can exceed Mach 1, and rocket-propelled aircraft can approach or reach hypersonic speed (Mach 5). The increasingly powerful engines are necessitated by the horsepower needed to propel an aircraft at higher speeds. For example, if it takes 5,000 horsepower to fly a given aircraft at 500 miles per hour, attaining 600 and 1,000 miles per hour will require 20,000 and 70,000 horsepower, respectively.

Once the propulsion system has been designed to produce the desired thrust, aircraft manufacturers apply appropriate aerodynamic principles to airfoil design, streamlining, and high-lift devices to optimize performance. Airfoil shape is often deemed the most important of these factors. Besides the familiar straight wing, airfoils take the shape of gull wings, which extend upward at a steep angle and then bend at a chosen point to become parallel to the ground. Sometimes the airfoils are raked backward and combine camber with streamlining to maximize lift force. In some very fast supersonic jets and many hypersonic aircraft, airfoils are triangular delta wings, deemed superior for streamlining purposes. Tail airfoil assemblies also vary widely. Often one fuselage makes up the entire body of an aircraft. In some aircraft—for example, Lockheed's Lightning and some of the newest jet aircraft—portions of the fuselage behind the airfoils have been split for added stability. However, this is quite uncommon, perhaps because it leads to complications in sound aerodynamic aircraft design.

Manufacturers also strive to make every aircraft as light as possible in order to minimize the amount of fuel needed to fly the aircraft for a given length of time. Although the weight factor is not strictly an aerodynamic concern, it is very important to streamlining and all the other aspects of aircraft design because it determines the structural materials that can be used for airfoils, fuselages, and high-lift devices.

Another interesting aspect of aerodynamics is the development of vertical takeoff and landing (VTOL) aircraft. Interest in this type of aircraft has developed for a number of reasons, especially the overcrowding of modern airports. Some VTOL aircraft have been designed that have engines or airfoils that can tilt away from the horizontal and provide the needed thrust direction for vertical takeoff and landing. Once the aircraft have taken off, their engines or airfoils move to the appropriate position for horizontal flight. VTOL aircraft pose several aerodynamic problems associated with streamlining, airfoil design, and fuselage design, all of which have not been well enough solved to make VTOL aircraft ready for common, widespread use.

Context

The first reported air flight was made by the Greek Archytas of Tarentum in the fourth century BCE. He is supposed to have flown a gas-powered model of a bird for fifty feet. Modern interest in aerodynamics is said to have begun with Italian Leonardo da Vinci's fifteenth century study of bird flight. However, its first solid conceptualization had to await Sir Isaac Newton's seventeenth century theory of air resistance, which explained the behavior of forces between objects and fluids. Also crucial in the development of aerodynamics was the work of Newton's Swiss contemporary Daniel Bernoulli. Bernoulli showed that the pressure of a fluid decreases as its speed increases.

It was not until the middle of the nineteenth century when Otto Lilienthal and others in Europe developed gliders that the first valuable application of aerodynamic principles was made. Aerodynamicists such as American Samuel Langley produced small models of functional aircraft in the latter half of the nineteenth century, but it was not until 1903 that the first manned aircraft was flown. This was Wilbur and Orville Wright's propeller-driven aircraft, a biplane with two fixed wings.

The use of aircraft in World Wars I and II led to many advances in aerodynamics, including a switch first to single-wing propeller-driven aircraft and then to jet aircraft. In order for this to happen, the theory and practice of aerodynamics had to develop further. For example, with the advent in the 1940's of the jet aircraft, aerodynamicists and aircraft engineers had to explore the technology needed to conquer supersonic flight. The first supersonic flight was made by Charles E. Yeager in a Bell XS¹ jet aircraft in 1947.

Among the advances developed to engender faster and better supersonic aircraft were their streamlined, daggerlike fuselage noses and variable-sweep airfoils, which could be adjusted to straight or swept-back positions in flight. By the late 1960's, these devices often enhanced or replaced earlier modes of streamlining and the use of wing camber and high-lift devices. The result was better lift force utilization, higher safe airspeeds, and safer landings. By the mid-1980's, design advances in large jet aircraft made the supersonic airliner widely available and led to efforts to build aerospace aircraft that could take off from any of the world's conventional airports, travel at hypersonic speeds, enter orbits around the earth, and then return to land at any chosen conventional airport.

Several aerodynamic issues demand attention. Among these is the minimization of sonic booms, which startle and annoy people on the ground and can cause damage (such as broken windows) to buildings in a supersonic aircraft's flight path. Aerodynamicists also seek to devise ways to quiet or better muffle jet engines to reduce noise levels at and around airports. Furthermore, the development of aircraft that are more economical to fly would make air travel more accessible to the average individual. Finally, because of airport crowding, practical vertical takeoff and landing aircraft may one day need to be developed.

Principal terms

airfoil: the wing or other devices (for example, the tail) used in enabling heavier-than-air aircraft to fly

angle of attack: the angle that an airfoil makes with the air flowing past it; changing this angle will increase or decrease an aircraft's lift force

Bernoulli principle: the discovery that the pressure of a fluid decreases as its speed increases, an important principle enabling the flight of heavier-than-air aircraft

boundary layer: a thin layer of gas (such as the air) immediately adjacent to an aircraft or some other body moving through a fluid

drag: the aerodynamic force that counters lift, slowing aircraft and diminishing ability to remain in flight

hypersonic flight: flight at very high airspeeds, usually at Mach 5 or above

lift: the force produced by the motion of an airfoil, which gives an aircraft the ability to leave the ground and holds it up during flight

Mach number: the number obtained by dividing an aircraft's airspeed by the speed of sound, under the conditions of the flight in progress

sonic boom: the bang or booming sound heard after a supersonic aircraft passes, caused by shock waves produced by the aircraft

subsonic flight: flight in which aircraft speeds are below that of sound (Mach 1)

supersonic flight: flight in which airspeeds are above Mach 1 but not at hypersonic speeds

thrust: the force provided by propeller-driven engines or jet engines of an aircraft; it enables forward aircraft motion

Bibliography

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Anderson, John D. An Introduction to Flight. 9th ed., McGraw-Hill, 2024.

"Guide to Aerodynamics." Glenn Research Center, NASA, www1.grc.nasa.gov/beginners-guide-to-aeronautics/learn-about-aerodynamics/. Accessed 21 Nov. 2024.

Kuchemann, Dietrich. Aerodynamic Design of Aircraft. McGraw-Hill, 1978.

Kuethe, Arnold M., and Chow Chuen-Yen. Foundations of Aerodynamics: Basis of Aerodynamic Design. 5th ed. John Wiley & Sons, 1997.

Snow, Theodore P., and Michael J. Shull. Physics. West, 1986.

Wegener, Peter. What Makes Airplanes Fly? History, Science, and Applications. 2nd ed. Springer, 1997.