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Natural flight: a "hummingbird
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Human-invented flight: a "Royal Jordanian Airlines "Boeing 787

Flight is the process by which an "object "moves, through an "atmosphere or beyond it, as in the case of "spaceflight. This can be achieved by generating "aerodynamic lift, "propulsive thrust, "aerostatically using "buoyancy, or by "ballistic movement.

Many things fly, from natural aviators such as "birds, "bats and "insects to human inventions such as "missiles, "aircraft such as "airplanes, "helicopters and "balloons, to "rockets such as "spacecraft.

The engineering aspects of flight are the purview of "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

### Buoyant flight

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An airship flies because the upward force, from air displacement, is equal or greater than the force of gravity

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 that encloses a volume of "lifting gas[1][2] to provide "buoyancy, to which other components are attached.

Aerostats are so named because they use "aerostatic" lift, a "buoyant force that does not require lateral movement through the surrounding air mass to effect a lifting force. By contrast, "aerodynes primarily use "aerodynamic "lift, which requires the lateral movement of at least some part of the "aircraft through the surrounding air mass.

### Aerodynamic flight

#### 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 other things can exploit rising air to climb such as "raptors (when gliding) and "man-made sailplane gliders. This is termed "soaring. However most other birds and all "powered aircraft need a source of "propulsion to climb. This is termed powered flight.

#### Animal flight

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Female "mallard duck
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Tau emerald "dragonfly
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"Kea

The only groups of "living things that use powered flight are "birds, "insects, and "bats, while many groups have evolved gliding. The extinct "Pterosaurs, an "order of reptiles contemporaneous with the "dinosaurs, were also very 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 hypothesized to be highly modified versions of structures that form gills in most other groups of "arthropods.[3]

"Bats are the only "mammals capable of sustaining level flight.[4] However, there are several "gliding mammals which are able to glide from tree to tree using fleshy membranes between their limbs; some can travel hundreds of meters in this way with very 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 also 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.[5]

Most "birds fly (see "bird flight), with some 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 other birds use for flight.["citation needed] Most small flightless birds are native to small islands, and lead a lifestyle where flight would offer little advantage.

Among living animals that fly, the "wandering albatross has the greatest wingspan, up to 3.5 meters (11 feet); the "great bustard has the greatest weight, topping at 21 kilograms (46 pounds).[6]

Most species of "insects can fly as adults. "Insect flight makes use of either of two basic aerodynamic models: creating a leading edge vortex, found in most insects, and using "clap and fling, found in very small insects such as "thrips.[7][8]

#### Mechanical

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Mechanical flight: A "Robinson R22 Beta "helicopter

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. Another 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 airplane 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 sometimes called an "airfoil, which is a device that creates lift when air flows across it.

##### Supersonic

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,[9] 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% of the speed of sound.

##### Hypersonic

Hypersonic flight is very 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.

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### Ballistic

#### Atmospheric

Some things generate little or no lift and move only or mostly under the action of momentum, gravity, air drag and in some cases thrust. This is termed ballistic flight. Examples include "balls, "arrows, "bullets, "fireworks etc.

#### Spaceflight

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 also in commercial activities like "space tourism and "satellite telecommunications. Additional non-commercial uses of spaceflight include "space observatories, "reconnaissance satellites and other "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.[10] 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, some disintegrate during "atmospheric reentry, and others reach a planetary or lunar surface for landing or impact.

## History

Many human cultures have built devices that fly, from the earliest projectiles such as stones and spears,[11][12] the "boomerang in "Australia, the hot air "Kongming lantern, and "kites.

### Aviation

"George Cayley studied flight scientifically in the first half of the 19th century,[13][14][15] and in the second half of the 19th century "Otto Lilienthal made over 200 gliding flights and was also 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.[16]

### Spaceflight

Spaceflight, particularly "human spaceflight became a reality in the 20th Century following theoretical and practical breakthroughs by "Konstantin Tsiolkovsky and "Robert H. Goddard. The "first orbital spaceflight was in 1957[17] and "Yuri Gagarin was carried aboard the first manned orbital spaceflight in 1961.[18]

## Physics

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Lighter-than-air "airships are able to fly without any major input of energy

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 expending 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.

Some vehicles also use thrust for flight, for example "rockets and "Harrier Jump Jets.

Finally, "momentum dominates the flight of ballistic flying objects.

### Forces

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Main forces on a heavier-than-air aircraft

Forces relevant to flight are[19]

These forces must be balanced for stable flight to occur.

#### Thrust

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Forces on an "aerofoil cross section

A "fixed-wing aircraft generates forward thrust when air is pushed in the direction opposite to flight. This can be done in several ways including by the spinning blades of a "propeller, or a rotating "fan pushing air out from the back of a "jet engine, or by ejecting hot gases from a "rocket engine.[20] The forward thrust is proportional to the "mass of the airstream multiplied by the difference in "velocity of the airstream. Reverse thrust can be generated to aid braking after landing by reversing the pitch of variable-pitch propeller blades, or using a "thrust reverser on a jet engine. "Rotary wing aircraft and "thrust vectoring "V/STOL aircraft use engine thrust to support the weight of the aircraft, and vector sum of this thrust fore and aft to control forward speed.

#### Lift

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Lift is defined as the component of the total aerodynamic force perpendicular to the flow direction, and drag is the component parallel to the flow direction

In the context of an "air flow relative to a flying body, the lift force is the "component of the "aerodynamic force that is "perpendicular to the flow direction.[21] 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 also 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 also occur in a different way if the air is not still, especially if there is an updraft due to heat ("thermals") or wind blowing along sloping terrain or other 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.

#### Drag

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.[22][23][24][25] Therefore, drag opposes the motion of the object, and in a powered vehicle it must be overcome by "thrust. The process which creates lift also causes some drag.

#### Lift-to-drag ratio

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Speed and drag relationships for a typical flight article

Aerodynamic lift is created by the motion of an aerodynamic object (wing) through the air, which due to its shape and angle deflects the air. For sustained straight and level flight, lift must be equal and opposite to weight. In general, long narrow wings are able deflect a large amount of air at a slow speed, whereas smaller wings need a higher forward speed to deflect an equivalent amount of air and thus generate an equivalent amount of lift. Large cargo aircraft tend to use longer wings with higher angles of attack, whereas supersonic aircraft tend to have short wings and rely heavily on high forward speed to generate lift.

However, this lift (deflection) process inevitably causes a retarding force called drag. Because lift and drag are both aerodynamic forces, the ratio of lift to drag is an indication of the aerodynamic efficiency of the airplane. The lift to drag ratio is the L/D ratio, pronounced "L over D ratio." An airplane has a high L/D ratio if it produces a large amount of lift or a small amount of drag. The lift/drag ratio is determined by dividing the lift coefficient by the drag coefficient, CL/CD. [26]

The lift coefficient Cl is equal to the lift L divided by the (density r times half the velocity V squared times the wing area A). [Cl = L / (A * .5 * r * V^2)] The lift coefficient is also affected by the compressibility of the air, which is much greater at higher speeds, so velocity V is not a linear function. Compressibility is also affected by the shape of the aircraft surfaces. [27]

The drag coefficient Cd is equal to the drag D divided by the (density r times half the velocity V squared times the reference area A). [Cd = D / (A * .5 * r * V^2)] [28]

Lift-to-drag ratios for practical aircraft vary from about 4:1 for vehicles and birds with relatively short wings, up to 60:1 or more for vehicles with very long wings, such as gliders. A greater angle of attack relative to the forward movement also increases the extent of deflection, and thus generates extra lift. However a greater angle of attack also generates extra drag.

Lift/drag ratio also determines the glide ratio and gliding range. Since the glide ratio is based only on the relationship of the aerodynamics forces acting on the aircraft, aircraft weight will not affect it. The only effect weight has is to vary the time that the aircraft will glide for – a heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time. [29]

#### Buoyancy

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 meter 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 kilograms (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 is less than the mass of an equal volume of air will rise in air - in other words, any object less dense than air will rise.

#### 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 ${\displaystyle g_{0}}$).[30] It is a dimensionless parameter characteristic of "rockets and other 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

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"Pitch
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"Yaw
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"Roll
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The upward tilt of the wings and tailplane of an aircraft, as seen on this "Boeing 737, is called dihedral angle

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 other 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.

### Energy efficiency

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.[31][32]

### Range

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 and how efficiently they can turn that energy into propulsion.[33]

For powered aircraft the useful energy is determined by their "fuel fraction- what percentage of the takeoff weight is fuel, as well as the "specific energy of the fuel used.

### Power-to-weight ratio

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 also usually take off and land vertically, but some designs can land horizontally.

"Navigation is the systems necessary to calculate current position (e.g. "compass, "GPS, "LORAN, "star tracker, "inertial measurement unit, and "altimeter).

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 may be supplemented using radio navigation aids.

### Guidance

A guidance system is a device or group of devices used in the "navigation of a "ship, "aircraft, "missile, "rocket, "satellite, or other moving object. Typically, guidance is responsible for the calculation of the vector (i.e., direction, velocity) toward an objective.

### Control

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 also considered as flight controls as they change speed.

#### Traffic

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.

## Flight safety

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 also be applied in the context of campaigns that inform the public as to the safety of "air travel.

## References

Notes
1. ^ Walker 2000, p. 541. Quote: the gas-bag of a balloon or airship.
2. ^ Coulson-Thomas 1976, p. 281. Quote: fabric enclosing gas-bags of airship.
3. ^ Averof, Michalis. "Evolutionary origin of insect wings from ancestral gills." Nature, Volume 385, Issue 385, February 1997, pp. 627–630.
4. ^ World Book Student. Chicago: World Book. Retrieved: April 29, 2011.
5. ^ "BBC article and video of flying fish." BBC, May 20, 2008. Retrieved: May 20, 2008.
6. ^ "Swan Identification." The Trumpeter Swan Society. Retrieved: January 3, 2012.
7. ^ Wang, Z. Jane (2005). "DISSECTING INSECT FLIGHT" (pdf). Annual Review of Fluid Mechanics. Annual Reviews. 37: 183–210. "Bibcode:2005AnRFM..37..183W. "doi:10.1146/annurev.fluid.36.050802.121940.
8. ^ Sane, Sanjay P. (2003). "The aerodynamics of insect flight" (PDF). The Journal of Experimental Biology. 206 (23): 4191–4208. "PMID 14581590. "doi:10.1242/jeb.00663.
9. ^ Bern, Peter. "Concorde: You asked a pilot." BBC, October 23, 2003.
10. ^ Spitzmiller, Ted (2007). Astronautics: A Historical Perspective of Mankind's Efforts to Conquer the Cosmos. Apogee Books. "ISBN "9781894959667.
11. ^ "Archytas of Tar entum." Archived December 26, 2008, at the "Wayback Machine. Technology Museum of Thessaloniki, Macedonia, Greece/ Retrieved: May 6, 2012.
12. ^ "Ancient history." Automata. Retrieved:May 6, 2012.
13. ^ "Sir George Cayley". Flyingmachines.org. Retrieved 26 July 2009. Sir George Cayley is one of the most important people in the history of aeronautics. Many consider him the first true scientific aerial investigator and the first person to understand the underlying principles and forces of flight.
14. ^ "The Pioneers: Aviation and Airmodelling". Retrieved 26 July 2009. Sir George Cayley, is sometimes called the 'Father of Aviation'. A pioneer in his field, he is credited with the first major breakthrough in heavier-than-air flight. He was the first to identify the four aerodynamic forces of flight – weight, lift, drag, and thrust – and their relationship and also the first to build a successful human carrying glider.
15. ^ "U.S. Centennial of Flight Commission – Sir George Cayley.". Retrieved 10 September 2008. Sir George Cayley, born in 1773, is sometimes called the Father of Aviation. A pioneer in his field, Cayley literally has two great spurts of aeronautical creativity, separated by years during which he did little with the subject. He was the first to identify the four aerodynamic forces of flight – weight, lift, drag, and thrust and their relationship. He was also the first to build a successful human-carrying glider. Cayley described many of the concepts and elements of the modern aeroplane and was the first to understand and explain in engineering terms the concepts of lift and thrust.
16. ^ "Orville Wright's Personal Letters on Aviation." Shapell Manuscript Foundation, (Chicago), 2012.
17. ^ https://history.nasa.gov/sputnik/sputorig.html
18. ^ "Gagarin anniversary." NASA. Retrieved: May 6, 2012.
19. ^ "Four forces on an aeroplane." NASA. Retrieved: January 3, 2012.
20. ^ http://www.grc.nasa.gov/WWW/k-12/airplane/newton3.html
21. ^ "Definition of lift." NASA. Retrieved: May 6, 2012.
22. ^ French 1970, p. 210.
23. ^ "Basic flight physics." Berkeley University. Retrieved: May 6, 2012.
24. ^ "What is Drag?" NASA. Retrieved: May 6, 2012.
25. ^ "Motions of particles through fluids." lorien.ncl.ac. Retrieved: May 6, 2012.
26. ^ The Beginner's Guide to Aeronautics - NASA Glenn Research Center https://www.grc.nasa.gov/www/k-12/airplane/ldrat.html
27. ^ The Beginner's Guide to Aeronautics - NASA Glenn Research Center https://www.grc.nasa.gov/www/k-12/airplane/liftco.html
28. ^ The Beginner's Guide to Aeronautics - NASA Glenn Research Center https://www.grc.nasa.gov/www/k-12/airplane/dragco.html
29. ^ The Beginner's Guide to Aeronautics - NASA Glenn Research Center https://www.grc.nasa.gov/www/k-12/airplane/ldrat.html
30. ^ Sutton and Biblarz 2000, p. 442. Quote: "thrust-to-weight ratio F/W0 is a dimensionless parameter that is identical to the acceleration of the rocket propulsion system (expressed in multiples of g0) if it could fly by itself in a gravity free vacuum."
31. ^ ch10-3 "History." NASA. Retrieved: May 6, 2012.
32. ^ Honicke et al. 1968["page needed]
33. ^ http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node98.html
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