Aeronautics

Summary

Aeronautics is the science of atmospheric flight. It is sometimes used interchangeably with "aviation," although that term is often distinguished as specifically referring to the design, development, production, and operation of heavier-than-air flight vehicles. Aeronautics also includes lighter-than-air vehicles and ballistics. Aerospace engineering extends these efforts to space vehicles. Transonic airliners, airships, space launch vehicles, helicopters, and fighter planes are all applications of aeronautics or aerospace engineering.

Definition and Basic Principles

Aeronautics is the science of atmospheric flight. The term (aero- referring to flight and -nautics referring to ships or sailing) originated from the activities of pioneers who aspired to navigate the sky. These early engineers designed, tested, and flew their own creations, many of which were lighter-than-air balloons. Modern aeronautics encompasses the science and engineering of designing and analyzing all areas associated with flying machines.

The concept of aviation (based on the Latin word for “bird”) originated with the idea of flying like birds using heavier-than-air vehicles. “Aviation” refers to the field of operating aircraft, while the term “aeronautics” has largely been superseded by “aerospace engineering,” which specifically includes the science and engineering of spacecraft in the design, development, production, and operation of flight vehicles.

A fundamental tenet of aerospace engineering is to test and extend boundaries using established precepts, such as the laws of physics and mathematical proofs. As an example, lighter-than-air airships are based on the principle of buoyancy, which derives from the law of gravity. An object that weighs less than the equivalent volume of air experiences a net upward force as the air sinks around it.

Two basic principles that enable the design of heavier-than-air flight vehicles are those of aerodynamic lift and propulsion. Both arise from Sir Isaac Newton's second and third laws of motion. Aerodynamic lift is a force perpendicular to the direction of motion, generated from the turning of flowing air around an object. In propulsion, the reaction to the acceleration of a fluid generates a force that propels an object, whether in air or in the vacuum of space. Understanding these principles allowed engineers to design vehicles that could fly despite being heavier than the air they displaced and allowed rocket scientists to develop vehicles that could accelerate in space. Spaceflight occurs at speeds so high that the vehicle's kinetic energy is comparable to the potential energy due to gravitation. Here the principles of orbital mechanics derive from the laws of dynamics and gravitation and extend to the regime of relativistic phenomena. The engineering sciences of building vehicles that can fly, keeping them stable, controlling their flight, navigating, communicating, and ensuring the survival, health, and comfort of occupants, draw on every field of science.

Background and History

The intrepid balloonists of the nineteenth century were followed by aviators who used the principles of aerodynamics to fly unpowered gliders. The Wright brothers demonstrated sustained, controlled, powered aerodynamic flight of a heavier-than-air aircraft in 1903. The increasing altitude, payload, and speed capabilities of airplanes made them powerful weapons in World War I. Such advances improved flying skills, designs, and performance, though at a terrible cost in lives.

The monoplane design replaced the fabric-and-wire biplane and triplane designs of World War I. The helicopter was developed during World War II and quickly became an indispensable tool for medical evacuation and search and rescue. The jet engine was developed in the 1940s. The technology was employed on warplanes such as the Messerschmitt 262 and Junkers aircraft by the Luftwaffe, and the Gloster Meteor by the British. Jet engines enabled flight in the stratosphere at speeds sufficient to generate enough lift to climb in the thin air. Such innovations led to smooth, long-range and comfortable flights in pressurized cabins. Fatal crashes of the de Havilland Comet airliner in 1953 and 1954 focused attention on the science of metal fatigue.

The Boeing 707 opened up intercontinental air travel. This model was soon joined by others such as the Boeing 747, the supersonic Concorde, and the EADS Airbus A380. A series of manned experimental aircraft, designated with an X prefix, pushed the performance boundaries of flight regimes. They drove the development of supporting technologies sudh as wind tunnels and high-altitude simulation chambers. Drawing on German World War II wartime designs, and aided by captured German scientists, both the US and Soviet established ballistic missile programs. These programs became facets of the Cold War as well as a space race that resulted. This culminated with the United States being the first to land humans on the Moon in 1969 and return them safely to the Earth. At the same time, an offshoot of combat-aircraft development enabled advances that resulted in safer and more efficient airliners.

How It Works

Force Balance in Flight. Five basic forces act on a vehicle while it is in flight. These are aerodynamic lift, gravity, thrust, drag, and centrifugal force. For a vehicle in steady level flight in the atmosphere, lift and thrust balance gravity (weight) and aerodynamic drag. Centrifugal force due to moving steadily around the Earth is too weak at most airplane flight speeds but is strong for a maneuvering aircraft. Aircraft turn by rolling the lift vector toward the center of curvature of the desired flight path, balancing the centrifugal reaction due to inertia. In the case of a vehicle in space beyond the atmosphere, centrifugal force and thrust counter gravitational force.

Aerodynamic Lift. Aerodynamics deals with the forces due to the motion of air and other gaseous fluids relative to flight surfaces on aircraft, such as its wings. Aerodynamic lift is generated perpendicular to the direction of the free stream as the reaction to the rate of change of momentum of air turning around an object, and, at high speeds, to compression of air by the object. Flow turning is accomplished by changing the angle of attack of the surface, by using the camber of the surface in subsonic flight, or by generating vortices along the leading edges of swept wings.

Propulsion. Propulsive force is generated as a reaction to the rate of change of momentum of a fluid moving through and out of the vehicle. Rockets carry all of the propellant onboard and accelerate it out through a nozzle using chemical heat release, other heat sources, or electromagnetic fields. Jet engines “breathe” air and accelerate it is combined with fuel and ignited in a chamber. Rotors, propellers, and fans exert lift force on the air and generate thrust from the reaction to this force. Solar sails use the pressure of solar radiation to push large, ultralight surfaces.

Static Stability. An aircraft is statically stable if a small disturbance in its attitude causes a restoring aerodynamic moment that erases the disturbance. Typically, the aircraft center of gravity must be ahead of the center of pressure for longitudinal stability. The tails or canards help provide stability about the different axes. Rocket engines are said to be stable if the rate of generation of gases in the combustion chamber does not depend on pressure stronger than by a direct proportionality, such as a pressure exponent of 1.

Flight Dynamics and Controls. Static stability is not the whole story, as every pilot discovers when the airplane drifts periodically up and down instead of holding a steady altitude and speed. Flight dynamics studies the phenomena associated with aerodynamic loads and the response of the vehicle to control surface deflections and engine-thrust changes. The study begins with writing the equations of motion of the aircraft resolved along the six degrees of freedom: linear movement along the longitudinal, vertical and sideways axes, and roll, yaw, and pitch rotations about them. Maneuvering aircraft must deal with coupling between the different degrees of freedom, so that roll accompanies yaw, and so on.

The autopilot system was an early flight-control achievement. Terrain-following systems combine information about the terrain with rapid updates, enabling military aircraft to fly close to the ground, much faster than a human pilot could do safely. Modern flight-control systems achieve such feats as reconfiguring control surfaces and fuel to compensate for damage and engine failures; or enabling autonomous helicopters to detect, hover over, and pick up small objects and return; or sending a space probe at thousands of kilometers per hour close to a planetary moon or landing it on an asteroid and returning it to Earth. This field makes heavy use of ordinary differential equations and transform techniques, along with simulation software.

Orbital Missions. The rocket equation attributed to Russian scientist Konstantin Tsiolkovsky related the speed that a rocket-powered vehicle gains to the amount and speed of the mass that it ejects. A vehicle launched from Earth's surface goes into a trajectory where its kinetic energy is exchanged for gravitational potential energy. At low speeds, the resulting trajectory intersects the Earth, so that the vehicle falls to the surface. At high enough speeds, the vehicle travels a sufficient distance with enough velocity that its trajectory remains in space and takes the shape of a continuous ellipse around Earth. At even higher kinetic energy levels, the vehicle goes into a hyperbolic trajectory, escaping Earth's orbit into the solar system. The key is thus to achieve enough tangential speed relative to Earth. Most rockets rise rapidly through the atmosphere so that the acceleration to high tangential speed occurs well above the atmosphere, thus minimizing air-drag losses.

Hohmann Transfer. Theoretically, the most efficient way to impart kinetic energy to a vehicle is impulsive launch, expending all the propellant instantly so that no energy is wasted lifting or accelerating propellant with the vehicle. In theory, this would destroy any vehicle other than a cannonball. Thus, large rockets use gentle accelerations of no more than 1.4 to 3 times the acceleration due to gravity. The advantage of impulsive thrust is used in the Hohmann transfer maneuver between different orbits in space. A rocket is launched into a highly eccentric elliptical trajectory. At its highest point, more thrust is quickly added. This sends the vehicle into a circular orbit at the desired height or into a new orbit that takes it close to another heavenly body. Reaching the same final orbit using continuous, gradual thrust would require roughly twice as much expenditure of energy. However, continuous thrust is still an attractive option for long missions in space, because a small amount of thrust can be generated using electric propulsion engines that accelerate propellant to extremely high speeds compared with the chemical engines used for the initial ascent from Earth.

Applications and Products

Aerospace Structures. Aerospace engineers always seek to minimize the mass required to build the vehicle but still ensure its safety and durability. Unlike buildings, bridges, or even automobiles, aircraft cannot be made safer merely by making them more massive. This is because they must also be able to overcome Earth's gravity. This characteristic has driven development of new materials and detailed, accurate methods of analysis, measurement, and construction. The first aircraft were built mostly from wood frames and fabric skins. These were superseded by all-metal craft, constructed using the monocoque concept. This is where the outer skin bears most of the stresses. The British Mosquito bomber in World War II reverted to wood construction for better performance. Woodworkers learned to align the grain (fiber direction) along the principal stress axes. Metal offers the same strength in all directions for the same thickness. Composite structures allow fibers with high tensile strength to be placed along the directions where strength is needed, bonding different layers together.

Aeroelasticity. Aeroelasticity is the study of the response of structurally elastic bodies to aerodynamic loads. Early in the history of aviation, several mysterious and fatal accidents occurred wherein pieces of wings or tails failed in flight, under conditions where the steady loads should have been well below the strength limits of the structure. The research to address these disasters showed that beyond certain flight speeds, small changes in lift, such as those due to a gust or a maneuver, would cause the structure to respond in a resonant bending-twisting oscillation. This aerodynamic disturbance caused the amplitude of the aircraft to rapidly rise in a “flutter” mode until structural failure occurred. Predicting such aeroelastic instabilities demanded a highly mathematical approach to understand and apply the theories of unsteady aerodynamics and structural dynamics. Modern aircraft are designed so that the flutter speed is well above any possible speed achieved. In the case of helicopter rotor blades and gas turbine engine blades, the problems of ensuring aeroelastic stability are still the focus of leading-edge research. Related advances in structural dynamics have enabled development of composite structures and of highly efficient turbo machines that use counter-rotating stages, such as those in the F135 engines used in the F-35 Joint Strike Fighter. Such advances also made it possible for earthquake-surviving high-rise buildings to be built in cities such as San Francisco, Tokyo, and Los Angeles. In these cases a number of sensors, structural-dynamics-analysis software, and actuators allow the correct response to dampen the effects of earth movements even on the upper floors.

Smart Materials. Various composite materials such as carbon fiber and metal matrix composites have come to find application even in primary aircraft structures. The Boeing 787 is the first to use a composite main spar in its wings. Research on nanomaterials promises the development of materials with hundreds of times as much strength per unit mass as steel. Another leading edge of research in materials is in developing high-temperature or very low-temperature (cryogenic) materials. These are used inside jet and rocket engines, the spinning blades of turbines, and the impeller blades of liquid hydrogen pumps in rocket engines. Single crystal turbine blades enabled the development of jet engines with very high turbine inlet temperatures and, thus, high thermodynamic efficiency. Ceramic designs that are not brittle are pushing turbine inlet temperatures even higher. Other materials are “smart,” meaning they respond actively in some way to inputs. Examples include piezoelectric materials.

Wind Tunnels and Other Physical Test Facilities. Wind tunnels, used by the Wright brothers to develop airfoil shapes with desirable characteristics, are still used heavily in developing concepts and proving the performance of new designs. They are also used to investigate causes of problems and to develop solutions and data to validate computational prediction techniques. Generally, a wind tunnel has a fan or a high-pressure reservoir to add work to the air and raise its stagnation pressure. The air then flows through means of reducing turbulence and is accelerated to the maximum speed in the test section, where models and measurement systems operate.

The power required to operate a wind tunnel is proportional to the mass flow rate through the tunnel and to the cube of the flow speed achieved. Low-speed wind tunnels have relatively large test sections and can operate continuously for several minutes at a time. Supersonic tunnels generally operate with air blown from a high-pressure reservoir for short durations. Transonic tunnels are designed with ventilating slots to operate in the difficult regime where there may be both supersonic waves and subsonic flow over the test configuration. Hypersonic tunnels require heaters to avoid liquefying the air and to simulate the high stagnation temperatures of hypersonic flight and operate for millisecond durations. Shock tubes generate a shock from the rupture of a diaphragm, allowing high-energy air to expand into stationary air in the tube. They are used to simulate the extreme conditions across shocks in hypersonic flight. Many other specialized test facilities are used in structural and materials testing, developing jet and rocket engines, and designing control systems.

Avionics and Navigation. Condensed from the term “aviation electronics,” the term “avionics” has come to include the generation of intelligent software systems and sensors to control unmanned aerial vehicles (UAVs), which may operate autonomously. Avionics also deals with various subsystems such as radar and communications, as well as navigation equipment, and is closely linked to the disciplines of flight dynamics, controls, and navigation.

During World War II, pilots on long-range night missions would navigate celestially. The gyroscopes in their aircrafts would spin at high speed so that their inertia allowed them to maintain a reference position as the aircraft changed altitude or accelerated. Most modern aircraft use the Global Positioning System (GPS), Galileo, or GLONASS satellite constellations to obtain accurate updates of position, altitude, and velocity. The ordinary GPS signal determines position and speed with fair accuracy. Much greater precision and higher rates of updates are available to authorized vehicle systems through the differential GPS signal and military frequencies.

Gravity Assist Maneuver. Yuri Kondratyuk, the Ukrainian scientist whose work paved the way for the first human mission to the moon, suggested in 1918 that a spacecraft could use the gravitational attraction of the moons of planets to accelerate and decelerate at the two ends of a journey between planets. The Soviet Luna 3 probe used the gravity of the Moon when photographing its far side in 1959. American mathematician Michael Minovitch pointed out that the gravitational pull of planets along the trajectory of a spacecraft could be used to accelerate the craft toward other planets. The Mariner 10 probe used this “gravitational slingshot” maneuver around Venus to reach Mercury at a speed small enough to go into orbit around Mercury. The Voyager missions used the rare alignment of the outer planets to receive gravitational assists from Jupiter and Saturn to travel to Uranus and Neptune. It later performed another slingshot maneuver around Jupiter and Saturn to escape the solar system. Gravity assist has become part of the mission planning for all exploration missions and even for missions near Earth, where the gravity of the Moon is used.

Careers and Course Work

Aerospace engineers work on problems that push the frontiers of technology. Typical employers in this industry are manufacturers of aircraft or their parts and subsystems, airlines, government agencies and laboratories, and the defense services. Many aerospace engineers are also sought by financial services and other industries seeking those with excellent quantitative (mathematical and scientific) skills and talents.

University curricula generally start with a year of mathematics, physics, chemistry, computer graphics, computer science, language courses, and an introduction to aerospace engineering. Second-year courses are in basic statics, dynamics, materials, and electrical engineering. Other core courses include low-speed and high-speed aerodynamics, linear systems analysis, thermodynamics, propulsion, and structural analysis. These are followed by instruction in composite materials, vehicle performance, stability, control theory, avionics, orbital mechanics, aeroelasticity and structural dynamics. This course of study can culminate in a two-semester sequence on capstone design of flight vehicles. High school students aiming for such careers should take courses in mathematics, physics, chemistry and natural sciences, and computer graphics. Aerospace engineers are frequently required to write clear reports and present complex issues to skeptical audiences, which demands excellent communication skills. Taking flying lessons or getting a private pilot license should be considered if one desires a career as a pilot or an astronaut.

Social Context and Future Prospects

Aeronautics, along with aviation and aerospace engineering, remains an important field with significant social and economic impact. This will remain the case as long as humans maintain an interest in flight. With the value of air and space travel only expected to grow as society progresses, there will likely always be demand for aeronautic expertise.

Various developments have impacted cutting-edge research and other trends in aeronautics. The defense industry has increasingly embraced unmanned aerial vehicles (UAVs, also widely known as drones), which do not need a human crew onboard and often can perform beyond the limits of what a human can survive. Aircraft approach, landing, traffic management, emergency response, and collision avoidance systems may soon become fully automated and will require maneuvering responses that are beyond what a human pilot can provide in time and accuracy. UAVs still require engineers, designers, manufacturers, and controllers, however, even fully automated flight systems likewise need sophisticated development and implementation.

An unfortunate, but enduring, characteristic of aeronautical development is that major advances occur during the urgency presented by wartime need. Following the Russian invasion of Ukraine in 2022, the Russians began employing hypersonic weapons against Ukrainian targets. This highlighted the need for countries such as the United States to spur its own development of such weapons.

The rise of the commercial space industry played an important role in aerospace engineering in the early twenty-first century. The NASA space shuttle program ended in 2011, but other manned efforts have taken lead. These include many commercial ventures, such as the American company SpaceX, which provided launch and space vehicles. SpaceX has also built private launch facilities in locations such as Texas.

While some scientists initially expressed reservations about the impact of commercialization on space science, the influx of private funding has been critical to aerospace engineering research at times when federal budgets were often limited. Also unlike government efforts, those conducted by private businesses need to be commercially profitable. This helps spur more efficient use of resources and is a powerful incentive to maintain constant technological advancements.

The twenty-first century could see the return of human beings to the Moon and a manned exploration of Mars. These efforts will be made possible only through aeronautical innovation.

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