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INDEX

Topics page no.

1 Introduction2 Principles2.1 Radar cross-section (RCS) reductions2.1.1 Vehicle shape2.1.2 Non-metallic airframe2.1.3 Radar-absorbing material2.1.4 Radar stealth countermeasures and limits2.1.4.1 Low-frequency radar

2.1.4.2 Multiple emitters2.1.4.3 Moore's law2.1.4.4 Ship's wakes and spray2.2 Acoustics2.3 Visibility2.4 Infrared2.5 Reducing radio frequency (RF) emissions3 Measuring4 Tactics5 Research

6 List of stealth aircraft7 List of stealth ships7.1 Fully stealth types7.2 Reduced RCS types8 See also9 References9.1 Bibliography9.2 Notes10 External links

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INTRODUCTION TO STEALTH TECHNLOGY

F-117 stealth attack plane

Stealth technology also termed LO technology (low observabletechnology) is a sub-discipline of military tactics and passive electronic

countermeasures,[1] which cover a range of techniques used withpersonnel, aircraft, ships, submarines, and missiles, to make them lessvisible (ideally invisible) to radar, infrared,[2] sonar and other detectionmethods.

Development in the United States occurred in 1958,[3][4] whereearlier attempts in preventing radar tracking of its U-2 spy planes duringthe Cold War by the Soviet Union had been unsuccessful.[5] Designersturned to develop a particular shape for planes that tended to reducedetection, by redirecting electromagnetic waves from radars.[6] Radar-absorbent material was also tested and made to reduce or block radarsignals that reflect off from the surface of planes. Such changes toshape and surface composition form stealth technology as currentlyused on the Northrop Grumman B-2 Spirit "Stealth Bomber".[4] Theconcept of stealth is to operate or hide without giving enemy forces anyindications as to the presence of friendly for.

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Principles

Stealth technology (or LO for "low observability") is not a singletechnology. It is a combination of technologies that attempt to greatly

reduce the distances at which a person or vehicle can be detected; inparticular radar cross section reductions, but also acoustic, thermal, andother aspects:

Radar cross-section (RCS) reductions

Radar cross section is the measure of a target's ability to reflect radar signals in

the direction of the radar receiver, i.e. it is a measure of the ratio of backscatterpower per steradian (unit solid angle) in the direction of the radar (from the

target) to the power density that is intercepted by the target. The RCS of a target

can be viewed as a comparison of the strength of the reflected signal from a

target to the reflected signal from a perfectly smooth sphere of cross sectional

area of 1 m2 as shown in Figure.

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The conceptual definition of RCS includes the fact that not all of the radiated

energy falls on the target. A targets RCS( ) is most easily visualized as the

product of three factors:

= Projected cross section x Reflectivity x Directivity.

Reflectivity: The percent of intercepted power reradiated (scattered) by thetarget.

Directivity: The ratio of the power scattered back in the radar's direction tothe power that would have been backscattered had the scattering been uniform

in all directions (i.e. isotropically).

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For a sphere, the RCS, = r2 , where is the radius of the sphere. The RCS of a

sphere is independent of frequency if operating at sufficiently high frequencies

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(4)3 R4

(4R)2

Therefore,Pj or Pj Note: jammer transmission line loss is combined with

the jammer antenna gain to obtainGt .

Thus the Deductions can be made from the figure given below. This shows an

example of the effects of RCS reduction. Thus if the RCS of an aircraft is

reduced to 0.75 (75%) of its original value, then the jammer power required to

achieve the same effectiveness would be 0.75 (75%) of the original value (or

-1.25 dB). Likewise, If Jammer power is held constant, then burn-through range

is 0.87 (87%) of its original value (-1.25 dB), and the detection range of the

radar for the smaller RCS target (jamming not considered) is 0.93 (93%) of its

original value (-1.25 dB)

Reduction of RCS Affects Radar Detection, Burn-through, and Jammer Power

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RADAR STEALTH

There are two broad aspects of RCS minimization techniques. One falls under

the effort

to restructure the frame, and covers the geometric design considerations that are

taken into account when aiming for a low RCS. The other principle is referred

to as radar absorbent materials and is concerned with the materials that help

to reduce the reflectivity of the airframe, as well as the structures that will

support these materials and integrate them into the airframe often referred to as

Radar-absorbent structures. These two axes are of course not taken in

isolation during the design; trade-offs often have to be made between them.

Vehicle shape

The possibility of designing aircraft in such a manner as to reducetheir radar cross-section was recognized in the late 1930s, when the firstradar tracking systems were employed, and it has been known since atleast the 1960s that aircraft shape makes a significant difference indetectability. The Avro Vulcan, a British bomber of the 1960s, had aremarkably small appearance on radar despite its large size, andoccasionally disappeared from radar screens entirely. It is now knownthat it had a fortuitously stealthy shape apart from the vertical element of

the tail. In contrast, the Tupolev 95 Russian long range bomber (NATOreporting name 'Bear') appeared especially well on radar. It is nowknown that propellers and jet turbine blades produce a bright radarimage the Bear had four pairs of large (5.6 meter diameter) contra-rotating propellers.

Another important factor is internal construction. Some stealthaircraft have skin that is radar transparent or absorbing, behind whichare structures termed re-entrant triangles. Radar waves penetrating theskin get trapped in these structures, reflecting off the internal faces and

losing energy. This method was first used on the Blackbird series (A-12 /YF-12A / SR-71).

The most efficient way to reflect radar waves back to the emittingradar is with orthogonal metal plates, forming a corner reflectorconsisting of either a dihedral (two plates) or a trihedral (threeorthogonal plates). This configuration occurs in the tail of a conventional

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aircraft, where the vertical and horizontal components of the tail are setat right angles. Stealth aircraft such as the F-117 use a differentarrangement, tilting the tail surfaces to reduce corner reflections formedbetween them. A more radical method is to eliminate the tail completely,as in the B-2 Spirit.

In addition to altering the tail, stealth design must bury the engineswithin the wing or fuselage, or in some cases where stealth is applied toan extant aircraft, install baffles in the air intakes, so that the turbineblades are not visible to radar. A stealthy shape must be devoid ofcomplex bumps or protrusions of any kind; meaning that weapons, fueltanks, and other stores must not be carried externally. Any stealthyvehicle becomes un-stealthy when a door or hatch opens.

Planform alignment is also often used in stealth designs. Planformalignment involves using a small number of surface orientations in theshape of the structure. For example, on the F-22A Raptor, the leadingedges of the wing and the tail surfaces are set at the same angle.Careful inspection shows that many small structures, such as the airintake bypass doors and the air refueling aperture, also use the sameangles. The effect of planform alignment is to return a radar signal in avery specific direction away from the radar emitter rather than returninga diffuse signal detectable at many angles.

Stealth airframes sometimes display distinctive serrations on someexposed edges, such as the engine ports. The YF-23 has suchserrations on the exhaust ports. This is another example in the use of re-entrant triangles and planform alignment, this time on the externalairframe.

Shaping requirements have strong negative influence on theaircraft's aerodynamic properties. The F-117 has poor aerodynamics, isinherently unstable, and cannot be flown without a fly-by-wire controlsystem.

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K32 HMS Helsingborg, a stealth ship

Ships have also adopted similar methods. The Skjold class patrol boatwas the first stealth ship to enter service, though the earlier ArleighBurke class destroyer incorporated some signature-reduction features.

Non-metallic airframe

Dielectric composites are more transparent to radar, whereaselectrically conductive materials such as metals and carbon fibers reflect

electromagnetic energy incident on the material's surface. Compositesmay also contain ferrites to optimize the dielectric and magneticproperties of a material for its application.

RAS

RAS or Radar absorbent surfaces are the surfaces on the aircraft,which can deflect the incoming radar waves and reduce the detectionrange. RAS works due to the angles at which the structures on the

aircraft's fuselage or the fuselage itself are placed. These structures canbe anything from wings to a refueling boom on the aircraft. Theextensive use of RAS is clearly visible in the F-117 "Night Hawk". Due tothe facets (as they are called) on the fuselage, most of the incomingradar waves are reflected to another direction. Due to these facets onthe fuselage, the F-117 is a very unstable aircraft.

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The concept behind the RAS is that of reflecting a light beam from atorch with a mirror. The angle at which the reflection takes place is alsomore important. When we consider a mirror being rotated from 0o to90o, the amount of light that is reflected in the direction of the light beamis more. At 90o, maximum amount of light that is reflected back to samedirection as the light beam's source. On the other hand when the mirroris tilted above 90o and as it proceeds to 180o, the amount of lightreflected in the same direction decreases drastically. This makes theaircraft like F-117 stealthy.

Radar-absorbing material

Radar-absorbent material, or RAM, is a class of materials used instealth technology to disguise a vehicle or structure from radar detection. A

material's absorbency at a given frequency of radar wave depends upon its

composition. RAM cannot perfectly absorb radar at any frequency, but any

given composition does have greater absorbency at some frequencies than

others; there is no one RAM that is suited to absorption of all radar frequencies.

A common misunderstanding is that RAM makes an object invisible to

radar. A radar absorbent material can significantly reduce an object's radar

cross-section in specific radar frequencies, but it does not result in "invisibility"

on any frequency. Bad weather may contribute to deficiencies in stealthcapability. A particularly disastrous example occurred during the Kosovo war,

in which moisture on the surface of an F-117 Nighthawk allowed long-

wavelength radar to track and shoot it down. RAM is only a part of achieving

stealth.

RAMs are one of four ways of reducing the radar cross-section of an

object, which is a measure of the reflection of radar waves by an object. A

larger radar cross-section (RCS) of an object corresponds to a longer detection

range and a higher signal-to-noise ratio for the observing radar operator. A 747

would have a huge RCS, whereas a bumblebee would have an insignificant

RCS. Other ways of reducing RCS include passive cancellation, incorporating

an echo source which by design cancels another echo source for a certain

frequency and angle, active cancellation, incorporating a sensor and emitter

which cooperate to radiate waves which interfere with incident radar waves,

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and by geometric shaping and design modifications. Only the last will be

discussed, as the former two are rather impractical and are less dependent on

material or process properties.

Dielectric and magnetic RAMs are the two main types (along with variouscombinations of these) of RAMs in current operational use; these will be

explored in further detail as we go along.

Properties if RAMs based on angle of incidence

Types of RAM

IRON BALL PAINT

One of the most commonly known types of RAM is iron ball paint. It

contains tiny spheres coated with carbonyl iron or ferrite. Radar waves induce

molecular oscillations from the alternating magnetic field in this paint, which

leads to conversion of the radar energy into heat. The heat is then transferred to

the aircraft and dissipated. The iron particles in the paint are obtained by

decomposition of iron pentacarbonyl and may contain traces of carbon, oxygen

and nitrogen.

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A related type of RAM consists of neoprene polymer sheets with ferrite

grains or carbon black particles (containing about 30% of crystalline graphite)

embedded in the polymer matrix. The tiles were used on early versions of the F-

117A Nighthawk, although more recent models use painted RAM. The painting

of the F-117 is done by industrial robots with the plane covered in tiles glued to

the fuselage and the remaining gaps filled with iron ball paint.

The United States Air Force introduced a radar absorbent paint made

from both ferrofluidic and non-magnetic substances. By reducing the reflection

of electromagnetic waves, this material helps to reduce the visibility of RAM

painted aircraft on radar.

The Israeli firm Nanoflight has also made a radar-absorbing paint that uses

nanoparticles.

Foam absorber

Foam absorber is used as lining of anechoic chambers for electromagnetic

radiation measurements. This material typically consists of a fireproofed

urethane foam loaded with carbon black, and cut into long pyramids. The length

from base to tip of the pyramid structure is chosen based on the lowest expected

frequency and the amount of absorption required. For low frequency damping,

this distance is often 24 inches, while high frequency panels are as short as 3-

4 inches. Panels of RAM are installed with the tips pointing inward to the

chamber. Pyramidal RAM attenuates signal by two effects: scattering andabsorption. Scattering can occur both coherently, when reflected waves are in-

phase but directed away from the receiver, or incoherently where waves are

picked up by the receiver but are out of phase and thus have lower signal

strength. This incoherent scattering also occurs within the foam structure, with

the suspended carbon particles promoting destructive interference. Internal

scattering can result in as much as 10dB of attenuation. Meanwhile, the

pyramid shapes are cut at angles that maximize the number of bounces a wave

makes within the structure. With each bounce, the wave loses energy to the

foam material and thus exits with lower signal strength. Other foam absorbers

are available in flat sheets, using an increasing gradient of carbon loadings in

different layers.

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Jaumann absorber

A Jaumann absorber or Jaumann layer is a radar absorbent device. When first

introduced in 1943, the Jaumann layer consisted of two equally-spaced

reflective surfaces and a conductive ground plane. One can think of it as a

generalized, multi-layered Salisbury screen as the principles are similar.

Being a resonant absorber (i.e. it uses wave interfering to cancel the reflected

wave), the Jaumann layer is dependent upon the /4 spacing between the first

reflective surface and the ground plane and between the two reflective surfaces

(a total of /4 + /4 ).

Because the wave can resonate at two frequencies, the Jaumann layer produces

two absorption maxima across a band of wavelengths (if using the two layers

configuration). These absorbers must have all of the layers parallel to each other

and the ground plane that they conceal.

More elaborate Jaumann absorbers use series of dielectric surfaces that separate

conductive sheets. The conductivity of those sheets increases with proximity tothe ground plane.

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Freeofluid used as a RAM material Iron carbonyl RAM Iron ball paint

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Radar stealth countermeasures and limits

Low-frequency radar

Shaping offers far fewer stealth advantages against low-frequencyradar. If the radar wavelength is roughly twice the size of the target, ahalf-wave resonance effect can still generate a significant return.However, low-frequency radar is limited by lack of available frequencies-many are heavily used by other systems, by lack of accuracy of thediffraction-limited systems given their long wavelengths, and by theradar's size, making it difficult to transport. A long-wave radar maydetect a target and roughly locate it, but not provide enough informationto identify it, target it with weapons, or even to guide a fighter to it.

Noise poses another problem, but that can be efficientlyaddressed using modern computer technology; Chinese "Nantsin" radar

and many older Soviet-made long-range radars were modified this way.It has been said that "there's nothing invisible in the radar frequencyrange below 2 GHz".

Multiple emitters

Much of the stealth comes from reflecting radar emissions in directionsdifferent than a direct return. Thus, detection can be better achieved ifemitters are separate from receivers. One emitter separate from one

receiver is termed bistatic radar; one or more emitters separate frommore than one receiver is multitatic. Proposals exist to use reflectionsfrom emitters such as civilian radio transmitters, including cellulartelephone radio towers.

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Moore's law

By Moore's law the processing power behind radar systems isrising over time. This will erode the ability of physical stealth to hide

vehicles.

Ship's wakes and spray

Synthetic Aperture sidescan radars can be used to detect thelocation and heading of ships from their wake patterns.These may bedetectable from orbit. When a ship moves through a seaway it throws upa cloud of spray which can be detected by radar.

Acoustics

Acoustic stealth plays a primary role in submarine stealth as wellas for ground vehicles. Submarines use extensive rubber mountings toisolate and avoid mechanical noises that could reveal locations tounderwater passive sonar arrays.Early stealth observation aircraft used slow-turning propellers to avoidbeing heard by enemy troops below. Stealth aircraft that stay subsoniccan avoid being tracked by sonic boom. The presence of supersonic and

jet-powered stealth aircraft such as the SR-71 Blackbird indicates thatacoustic signature is not always a major driver in aircraft design,although the Blackbird relied more on its extremely high speed andaltitude.

Visibility

The simplest stealth technology is simply camouflage; the use ofpaint or other materials to color and break up the lines of the vehicle or

person.Most stealth aircraft use matte paint and dark colors, and operate only atnight. Lately, interest in daylight Stealth (especially by the USAF) hasemphasized the use of gray paint in disruptive schemes, and it isassumed that Yehudi lights could be used in the future to mask shadowsin the airframe (in daylight, against the clear background of the sky, dark

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tones are easier to detect than light ones) or as a sort of activecamouflage. The original B-2 design had wing tanks for a contrail-inhibiting chemical, alleged by some to be chlorofluorosulfonic acid,[24]but this was replaced in the final design with a contrail sensor fromOther that alerts the pilot when he should change altitude[25] andmission planning also considers altitudes where the probability of theirformation is minimized.

InfraredPassive IR detection techniques rely on the fact that every atom of matter

continuously

sends electromagnetic radiation at an IR wavelength which corresponds to its

temperature. IR detectors identify an aircraft by discriminating its IR radiation

with that of the background; hence it is desirable to have an IR emission fromthe aircraft close to the background radiation. Since controlling an IR emission

during a Military operation is not always feasible; IR emission control has to be

incorporated at the design stage of the aircraft itself. The major IR signature

contributors are the airframe, engine casing and the plume. The amount of

incident IR radiation in the detectors band depends upon the amount of

radiation emitted by the source, its position with respect to the detector, and the

amount of radiation that is attenuated (absorbed and scattered) by the

atmosphere on its way to the detector. It is not possible to always operate in a

position that results in minimum amount of incident IR on the detector in itsband. Also it is not possible to control the amount of atmospheric attenuation

ofthe IR emitted by the source in the direction of detector.

Hence the only operation that remains is to control the IR intensity emitted by

the source. Infrared Signatures Suppression Systems (IRSS) like

BlackHoleOcarina,

Film cooled tail pipe and Centre Body tail pipe; are some of the popular IR

countermeasures adopted. Further, in order to avoid IR seeking missiles,

countermeasures such as infrared jamming systems, infrared flares or decoys

are frequently employed.

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Electromagnetic Radiation Spectrum

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Thermal Radiation

The total amount of radiation emitted is dependent on emissivity and the

fourth power of absolute temperature as given by Stefan Boltzmann Law, e = .

. T4 , where, Stefan Boltzmann constant, = 5.67 x 10-8 W/m2K4 From

electromagnetic considerations, Plancks Law gives the monochromaticemissive power of a black surface as

eb= 2C1 5[e(C2/T)-1] , whereC1 andC2 are constants whose values are

0.596 x 10-16 W/m2 and 0.014387mK respectively.

For a non black surface, monochromatic emissive power is given by,

e= . Eb

The emissive power within a specified band of wavelengths is obtained by

integrating the Plancks law within that wavelength interval. The total radiant

emittance increases rapidly with temperature. The wavelength of maximumspectral radial emittance shifts towards shorter wavelengths with the increase in

temperature. Individual curves never cross one another and hence higher the

temperature, higher will be the radial emittance at all wavelengths.

Generalized IR System

Every typical IR systems components are designed to optimize the system

performance for specific wavelength region, for maximum detectivity, for highresolution,and

depending upon the type of source to be detected and the kind of information

the system is required to furnish. Consider a model of a generalized IR system,

in order to help us understand the principles underlying the many IR systems.

Every IR system model is composed of basic building blocks and every IR

system

whether active or passive, is composed of most if not all of these building

blocks. For example, all IR systems include a source or target, a background, an

atmosphere or environment, optics and a detector. With the aid of thisgeneralized model, the path of IR radiation from its sources can be analyzed,

step by step through the various modifications necessary for its final

presentation in some form of display

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Elements of a passive IR system Elements of an active IR system

Reducing radio frequency (RF) emissions

In addition to reducing infrared and acoustic emissions, a stealthvehicle must avoid radiating any other detectable energy, such as fromonboard radars, communications systems, or RF leakage fromelectronics enclosures. The F-117 uses passive infrared and low lightlevel television sensor systems to aim its weapons and the F-22 Raptorhas an advanced LPI radar which can illuminate enemy aircraft withouttriggering a radar warning receiver response.

Measuring

The size of a target's image on radar is measured by the radarcross section or RCS, often represented by the symbol and expressedin square meters. This does not equal geometric area. A perfectlyconducting sphere of projected cross sectional area 1 m2 (i.e. adiameter of 1.13 m) will have an RCS of 1 m2. Note that for radarwavelengths much less than the diameter of the sphere, RCS is

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independent of frequency. Conversely, a square flat plate of area 1 m2will have an RCS of = 4 A2 / 2 (where A=area, =wavelength), or13,982 m2 at 10 GHz if the radar is perpendicular to the flat surface.[27]At off-normal incident angles, energy is reflected away from the receiver,reducing the RCS. Modern stealth aircraft are said to have an RCScomparable with small birds or large insects,[28] though this varieswidely depending on aircraft and radar.If the RCS was directly related to the target's cross-sectional area, theonly way to reduce it would be to make the physical profile smaller.Rather, by reflecting much of the radiation away or by absorbing it, thetarget achieves a smaller radar cross section.[29]

Tactics

Stealthy strike aircraft such as the F-117, designed by LockheedMartin's famous Skunk Works, are usually used against heavilydefended enemy sites such as Command and Control centers orsurface-to-air missile (SAM) batteries. Enemy radar will cover theairspace around these sites with overlapping coverage, making

undetected entry by conventional aircraft nearly impossible. Stealthyaircraft can also be detected, but only at short ranges around the radars,so that for a stealthy aircraft there are substantial gaps in the radarcoverage. Thus a stealthy aircraft flying an appropriate route can remainundetected by radar. Many ground-based radars exploit Doppler filter toimprove sensitivity to objects having a radial velocity component withrespect to the radar. Mission planners use their knowledge of enemyradar locations and the RCS pattern of the aircraft to design a flight paththat minimizes radial speed while presenting the lowest-RCS aspects ofthe aircraft to the threat radar. To be able to fly these "safe" routes, it isnecessary to understand an enemy's radar coverage (see ElectronicIntelligence). Airborne or mobile radar systems such as AWACS cancomplicate tactical strategy for stealth operation.

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Research

Negative index metamaterials are artificial structures whichrefractive index has a negative value for some frequency range, such as

in microwave, infrared, or possibly optical.[30] These offer another wayto reduce detectability, and may provide electromagnetic near-invisibilityin designed wavelengths.Plasma stealth is a phenomenon proposed to use ionized gas (plasma)to reduce RCS of vehicles. Interactions between electromagneticradiation and ionized gas have been studied extensively for manypurposes, including concealing vehicles from radar. Various methodsmight form a layer or cloud of plasma around a vehicle to deflect orabsorb radar, from simpler electrostatic to RF more complex laserdischarges, but these may be difficult in practice.[31]

Several technology research and development efforts exist to integratethe functions of aircraft flight control systems such as ailerons, elevators,elevons, flaps, and flaperons into wings to perform the aerodynamicpurpose with the advantages of lower RCS for stealth via simplergeometries and lower complexity (mechanically simpler, fewer or nomoving parts or surfaces, less maintenance), and lower mass, cost (upto 50% less), drag (up to 15% less during use) and, inertia (for faster,stronger control response to change vehicle orientation to reducedetection). Two promising approaches are flexible wings, and fluidics.

In flexible wings, much or all of a wing surface can change shape inflight to deflect air flow. The X-53 Active Aeroelastic Wing is a NASAeffort. The Adaptive Compliant Wing is a military and commercial effort.[32][33][34]In fluidics, fluid injection is being researched for use in aircraft to controldirection, in two ways: circulation control and thrust vectoring. In both,larger more complex mechanical parts are replaced by smaller, simplerfluidic systems, in which larger forces in fluids are diverted by smaller

jets or flows of fluid intermittently, to change the direction of vehicles.In circulation control, near the trailing edges of wings, aircraft flightcontrol systems are replaced by slots which emit fluid flows.[35][36][37]In thrust vectoring, in jet engine nozzles, swiveling parts are replaced byslots which inject fluid flows into jets to divert thrust.[38] Tests show thatair forced into a jet engine exhaust stream can deflect thrust up to 15degrees. The U.S. FAA has conducted a study about civilizing 3Dmilitary thrust vectoring to help jetliners avoid crashes. According to this

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study, 65% of all air crashes can be prevented by deploying thrustvectoring means.

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