Flight is the process by which an object moves, through an atmosphere (the air in the case of earth) or beyond it (as in the case of spaceflight) without direct support from any surface. This can be achieved by generating aerodynamic lift, propulsive thrust, aerostatically using buoyancy, or by ballistic movement.
The engineering aspects of flight are studied in aerospace engineering which is subdivided into aeronautics, the study of vehicles that travel through the air, and astronautics, the study of vehicles that travel through space, and in ballistics, the study of the flight of projectiles.
Types of flight
Humans have managed to construct lighter than air vehicles that raise off the ground and fly, due to their buoyancy in air.
An aerostat is a system that remains aloft primarily through the use of buoyancy to give an aircraft the same overall density as air. Aerostats include free balloons, airships, and moored balloons. An aerostat's main structural component is its envelope, a lightweight skin containing a lifting gas to provide buoyancy, to which additional components are attached.
Aerostats are so named because they use "aerostatic" lift, a buoyant force that doesn't require lateral movement through the surrounding air mass. By contrast, aerodynes primarily use aerodynamic lift, which requires the lateral movement of at least a few part of the aircraft through the surrounding air mass.
Unpowered flight versus powered flight
Some things that fly don't generate propulsive thrust through the air, for example, the flying squirrel. This is termed gliding. Some additional things can exploit rising air to climb such as raptors (when gliding) and man-made sailplane gliders. This is termed soaring. However most additional birds and all powered aircraft need a source of propulsion to climb. This is termed powered flight.
The only groups of living things that use powered flight are birds, insects, and bats, while a large number of groups have evolved gliding. The extinct Pterosaurs, an order of reptiles contemporaneous with the dinosaurs, were additionally quite successful flying animals. Each of these groups' wings evolved independently. The wings of the flying vertebrate groups are all based on the forelimbs, but differ significantly in structure; those of insects are hypothesised to be highly modified versions of structures that form gills in most additional groups of arthropods.
Bats are the only mammals capable of sustaining level flight. Notwithstanding there are several gliding mammals which are able to glide from tree to tree using fleshy membranes between their limbs; a few can travel hundreds of metres in this way with quite little loss in height. Flying frogs use greatly enlarged webbed feet for a similar purpose, and there are flying lizards which fold out their mobile ribs into a pair of flat gliding surfaces. "Flying" snakes additionally use mobile ribs to flatten their body into an aerodynamic shape, with a back and forth motion much the same as they use on the ground.
Flying fish can glide using enlarged wing-like fins, and have been observed soaring for hundreds of meters. It is thought that this ability was chosen by natural selection because it was an effective means of escape from underwater predators. The longest recorded flight of a flying fish was 45 seconds.
Most birds fly (see bird flight), with a few exceptions. The largest birds, the ostrich and the emu, are earthbound, as were the now-extinct dodos and the Phorusrhacids, which were the dominant predators of South America in the Cenozoic era. The non-flying penguins have wings adapted for use under water and use the same wing movements for swimming that most additional birds use for flight. Most small flightless birds are native to small islands, and lead a lifestyle where flight would offer little advantage.
Mechanical flight is the use of a machine to fly. These machines include aircraft such as airplanes, gliders, helicopters, autogyros, airships, balloons, ornithopters as well as spacecraft. Gliders are capable of unpowered flight. An Additional form of mechanical flight is para-sailing where a parachute-like object is pulled by a boat. In an airplane, lift is created by the wings; the shape of the wings of the aeroplane are designed specially for the type of flight desired. There are different types of wings: tempered, semi-tempered, sweptback, rectangular and elliptical. An aircraft wing is at times called an airfoil, which is a device that creates lift when air flows across it.
Supersonic flight is flight faster than the speed of sound. Supersonic flight is associated with the formation of shock waves that form a sonic boom that can be heard from the ground, and is frequently startling. This shockwave takes quite a lot of energy to create and this makes supersonic flight generally less efficient than subsonic flight at about 85 percent of the speed of sound.
Hypersonic flight is quite high speed flight where the heat generated by the compression of the air due to the motion through the air causes chemical changes to the air. Hypersonic flight is achieved by reentering spacecraft such as the Space Shuttle and Soyuz.
Some things generate little or no lift and move only or mostly under the action of momentum, gravity, air drag and in a few cases thrust. This is termed ballistic flight. Examples include balls, arrows, bullets, fireworks etc.
Essentially an extreme form of ballistic flight, spaceflight is the use of space technology to achieve the flight of spacecraft into and through outer space. Examples include ballistic missiles, orbital spaceflight etc.
Spaceflight is used in space exploration, and additionally in commercial activities like space tourism and satellite telecommunications. Additional non-commercial uses of spaceflight include space observatories, reconnaissance satellites and additional earth observation satellites.
A spaceflight typically begins with a rocket launch, which provides the initial thrust to overcome the force of gravity and propels the spacecraft from the surface of the Earth. Once in space, the motion of a spacecraft—both when unpropelled and when under propulsion—is covered by the area of study called astrodynamics. Some spacecraft remain in space indefinitely, a few disintegrate throughout atmospheric reentry, and others reach a planetary or lunar surface for landing or impact.
George Cayley studied flight scientifically in the first half of the nineteenth century, and in the second half of the nineteenth century Otto Lilienthal made over 200 gliding flights and was additionally one of the first to understand flight scientifically. His work was replicated and extended by the Wright brothers who made gliding flights and finally the first controlled and extended, manned powered flights.
Spaceflight, particularly human spaceflight became a reality in the twentieth Century following theoretical and practical breakthroughs by Konstantin Tsiolkovsky and Robert H. Goddard. The first orbital spaceflight was in 1957 and Yuri Gagarin was carried aboard the first manned orbital spaceflight in 1961.
There are different approaches to flight. If an object has a lower density than air, then it is buoyant and is able to float in the air without using energy. A heavier than air craft, known as an aerodyne, includes flighted animals and insects, fixed-wing aircraft and rotorcraft. Because the craft is heavier than air, it must generate lift to overcome its weight. The wind resistance caused by the craft moving through the air is called drag and is overcome by propulsive thrust except in the case of gliding.
Finally, momentum dominates the flight of ballistic flying objects.
Forces relevant to flight are
- Propulsive thrust: (except in gliders)
- Lift: created by the reaction to an airflow
- Drag: created by aerodynamic friction
- Weight: (created by gravity)
- Buoyancy: for lighter than air flight
These forces must be balanced for stable flight to occur.
In the context of an air flow relative to a flying body, the lift force is the component of the aerodynamic force that's perpendicular to the flow direction. Aerodynamic lift results when the wing causes the surrounding air to be deflected - the air then causes a force on the wing in the opposite direction, in accordance with Newton's third law of motion.
Lift is commonly associated with the wing of an aircraft, although lift is additionally generated by rotors on rotorcraft (which are effectively rotating wings, performing the same function without requiring that the aircraft move forward through the air). While common meanings of the word "lift" suggest that lift opposes gravity, aerodynamic lift can be in any direction. When an aircraft is cruising for example, lift does oppose gravity, but lift occurs at an angle when climbing, descending or banking. On high-speed cars, the lift force is directed downwards (called "down-force") to keep the car stable on the road.
Lift can additionally occur in a different way if the air isn't still, especially if there's an updraft due to heat ("thermals") or wind blowing along sloping terrain or additional meteorological conditions. This form of lift permits soaring and is particularly important for gliding. It is used by birds and gliders to stay in the air for long periods with little effort.
For a solid object moving through a fluid, the drag is the component of the net aerodynamic or hydrodynamic force acting opposite to the direction of the movement. Therefore, drag opposes the motion of the object, and in a powered vehicle it must be overcome by thrust. The process which creates lift additionally causes a few drag. The lift-to-drag ratio will vary depending on the use for which the aircraft is intended.
Air pressure acting up against an object in air is greater than the pressure above pushing down. The buoyancy, in both cases, is equal to the weight of fluid displaced - Archimedes' principle holds for air just as it does for water.
A cubic metre of air at ordinary atmospheric pressure and room temperature has a mass of about 1.2 kilograms, so its weight is about 12 newtons. Therefore, any 1-cubic-meter object in air is buoyed up with a force of 12 newtons. If the mass of the 1-cubic-meter object is greater than 1.2 kilogrammes (so that its weight is greater than 12 newtons), it falls to the ground when released. If an object of this size has a mass less than 1.2 kilograms, it rises in the air. Any object that has a mass that's less than the mass of an equal volume of air will rise in air - in additional words, any object less dense than air will rise.
When lift is created by the motion of an object through the air, this deflects the air, and this is the source of lift. For sustained level flight, lift must be equal to weight.
However, this lift inevitably causes a few drag also, and it turns out that the efficiency of lift creation can be associated with a lift-to-drag ratio for a vehicle; the lift-to-drag ratios are approximately constant over a wide range of speeds.
Lift-to-drag ratios can be determined by flight test, by calculation or by testing in a wind tunnel. Lift-to-drag ratios for practical aircraft vary from about 4:1 up to 60:1 or more. The lower ratios are generally for vehicles and birds with relatively short wings, and the higher ratios are for vehicles with quite long wings, such as gliders. In general, long wings permit a large amount of air to be deflected and accelerated by a small amount, rather than a small amount of air by a large amount. Since energy is a square law on deflection speed, whereas lift is a linear relation, it takes less energy, and less lift-induced drag is created, with longer wings.
Thrust to weight ratio
Thrust-to-weight ratio is, as its name suggests, the ratio of instantaneous thrust to weight (where weight means weight at the Earth's standard acceleration ). It is a dimensionless parameter characteristic of rockets and additional jet engines and of vehicles propelled by such engines (typically space launch vehicles and jet aircraft).
If the thrust-to-weight ratio is greater than the local gravity strength (expressed in gs), then flight can occur without any forward motion or any aerodynamic lift being required.
If the thrust-to-weight ratio times the lift-to-drag ratio is greater than local gravity then takeoff using aerodynamic lift is possible.
Flight dynamics is the science of air and space vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of mass, known as pitch, roll and yaw (See Tait-Bryan rotations for an explanation).
The control of these dimensions can involve a horizontal stabilizer (i.e. "a tail"), ailerons and additional movable aerodynamic devices which control angular stability i.e. flight attitude (which in turn affects altitude, heading). Wings are often angled slightly upwards- they have "positive dihedral angle" which gives inherent roll stabilization.
To create thrust so as to be able to gain height, and to push through the air to overcome the drag associated with lift all takes energy. Different objects and creatures capable of flight vary in the efficiency of their muscles, motors and how well this translates into forward thrust.
Propulsive efficiency determines how much energy vehicles generate from a unit of fuel.
The range that powered flight articles can achieve is ultimately limited by their drag, as well as how much energy they can store on board.
All animals and devices capable of sustained flight need relatively high power-to-weight ratios to be able to generate enough lift and/or thrust to achieve take off.
Takeoff and landing
Vehicles that can fly can have different ways to takeoff and land. Conventional aircraft accelerate along the ground until sufficient lift is generated for takeoff, and reverse the process for landing. Some aircraft can take off at low speed; this is called a short takeoff. Some aircraft such as helicopters and Harrier jump jets can take off and land vertically. Rockets additionally usually take off and land vertically, but a few designs can land horizontally.
Guidance, navigation and control
In aircraft, successful air navigation involves piloting an aircraft from place to place without getting lost, breaking the laws applying to aircraft, or endangering the safety of those on board or on the ground.
The techniques used for navigation in the air will depend on whether the aircraft is flying under the visual flight rules (VFR) or the instrument flight rules (IFR). In the latter case, the pilot will navigate exclusively using instruments and radio navigation aids such as beacons, or as directed under radar control by air traffic control. In the VFR case, a pilot will largely navigate using dead reckoning combined with visual observations (known as pilotage), with reference to appropriate maps. This might be supplemented using radio navigation aids.
A guidance system is a device or group of devices used in the navigation of a ship, aircraft, missile, rocket, satellite, or additional moving object. Typically, guidance is responsible for the calculation of the vector (i.e., direction, velocity) toward an objective.
A conventional fixed-wing aircraft flight control system consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft engine controls are additionally considered as flight controls as they change speed.
In the case of aircraft, air traffic is controlled by air traffic control systems.
Collision avoidance is the process of controlling spacecraft to try to prevent collisions.
Air safety is a term encompassing the theory, investigation and categorization of flight failures, and the prevention of such failures through regulation, education and training. It can additionally be applied in the context of campaigns that inform the public as to the safety of air travel.