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To meet this goal, an aircraft must be "stealthy" in many areas.
1. It must be very hard to detect on radar.
2. The hot emissions from the engines must be minimal.
3. It must be quiet
4. Its engines should not produce contrails or exhaust smoke in cold atmosphere
5. It should be hard to see with the human eye
Click on the buttons to the left to learn more about the different aspects of Stealth. Find out yourself how planes and especially the F/A-22 can become almost invisible for enemy radar, what does a stealthy airframe look like and what may the future have in store?
Have a good time discovering the secrets of technology!
Detection
1.0 Radar technology
2.0 Detection techniques
2.1 Direct echo's
2.2 Jet wake detection
2.3 Heat detection
2.4 Turbulence detection
2.5 Visual detection
2.6 Acoustic detection
1.0 Radar technology
Currently the way to detect and even identify aircraft, is the use of radar. This system, invented during world war II, simply works by constantly sending bursts of radio waves of certain frequencies and measure the echo's of each burst.
Parts of the energy of radio waves are being reflected by objects. This can be a plane, but also a cloud or a bird. Depending on the material the object is made of, this echo is stronger or weaker, but there is an echo. By measuring the reflected energy as a function of position and time, computers can calculate what it is that reflects the energy, where it is in 3D space and also in what direction it moves.
To get a proper overview of an area with radar, the transmitting and receiving antenna should rotate in angles of 360 degrees. This is why you always see these rotating antenna's at for instance airports and ships. To protect the antenna's from damage, they are often mounted in a radio wave transparent dome, which you will probably already have seen somewhere.
2.0 Detection techniques
There are a number of causes for planes or other flying objects like missiles, giving away the fact that they are there. Radar or in other cases laser technology enables the searching party to detect the flying object and act upon detection.
2.1 Direct echo's
Once radar waves hit a plane, a part of the radar energy is bounced back to the sending source. The amount of bounced back energy highly depends on the shape of the object and the material it is made of.
The returned echo can be deteced, giving away the position and speed of the object.
2.2 Jet wake
The parameter determining radar return from a jet wake is the ionization present. Return from resistive particles, such as carbon, is seldom a significant factor. The very strong ion-density dependency on maximum gas temperature quickly leads to the conclusion that the radar return from the jet wake of an engine running in dry power is insignificant, while that from an after burning wake could be dominant.
2.3 Heat detection
Another way of detecting if an aircraft is flying somewhere is by measuring the heat it radiates. Normally this heat is produced by the planes engines. There are two significant sources of infrared radiation from air-breathing propulsion systems: hot parts and jet wakes.
By using modern heat image sensors (read InfraRed sensors) the difference can be seen between a flying object itself and the surrounding cold air.
This is the same for the jet engine exhaust gases
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The ideal case would be that the plane body and exhaust fumes have the same temperature as the surrounding air, making it blend with its background (seen from the detectors point of view.
Heat detection is often used in missiles which can lock themselves on the hot jet-engine exhaust and thus flying themselves directly into the planes most vital part. The Sidewinder is a good example of such a missile.
2.4 Turbulence detection
Shape also has a lot to do with the `invisibility' of stealth planes. Extreme aerodynamics keep air turbulence to a minimum. Rumors are heard about sophisticated laser controlled turbulence sensors, which can measure paths of disturbed air, generated by an aircraft which just passed.
2.5 Visual detection
Reducing smoke in the exhaust is accomplished by improving the efficiency of the combustion chambers. Getting rid of contrails - the white line in the sky caused by high flying planes - is a harder task however. More about that later.
2.6 Acoustic detection
A very obvious source of detection is the noise, generated by jet engines. Several systems have been designed in the meantime to reduce the sound of jet engine exhausts to a minimum, making them harder to detect by just measuring sound waves. But often it is already too late if you can hear the plane...
Get Stealthy
| 1.0 How to get Stealthy 1.1 Ingredients of Stealth 1.2 Radar Cross section RCS 2.0 Getting invisible 2.1 Echo scattering 2.2 Radar absorbtion 2.3 Echo cancellation 3.0 Heat radiation reduction 4.0 Turbulence reduction 5.0 Visual detection reduction 5.1 Hiding smoke contrails 5.2 Low visibility 5.3 Low level flight |
1.1 Ingredients of Stealth technology
To make a stealthy aircraft, designers had to consider five key ingredients:
- reducing the imprint on radar screens / stifling radio transmissions
- turning down the heat of its infrared picture
- Improve aerodynamics
- making the plane less visible.
- muffling noise
To understand more about each ingredient, here is some theory first.
1.2 Radar Cross section (RCS)
The first goal is to cut down the size of the aircraft's radar image, called its "radar cross section," or RCS. This normally involves using radical design features and some nonmetallic materials.
A conventional fighter aircraft has an Radar Cross Section (RCS) in the region of 6 square metres. The much larger B-2B bomber, using the latest stealth technology, displays an RCS of only 0.75 square metres. By comparison, a bird in flight displays an RCS of 0.01 square metres.
Stealth plane designers have to take in account that the used materials (for instance composites) may not be transparant to radar, but they are also not completely reflective. In other words, the parts behind the skin of the plane may be invisible for the eye, but they are not for radar waves, thus causing echos.
2.0 Getting invisible
This section explains more about what radar echos look like and how they can be prevented to reach the radar receiver again after hitting the plane.
2.1 Echo scattering
Curving surfaces on conventional aerodynamic bodies act as scatterers, reflecting radar waves from any angle and giving the radar operator a clear signal. The right-angled surfaces at the wing and tail roots also reflect radar signals straight back to their source.
Scintillation is a measure of how rapidly the size of the return varies with the angle. The greater this variation, the more difficult a target is to track. The lower the number of lobes and the narrower the lobes, the lower the probability of detecting any return.
Panels on planes are angled so that radar is scattered and no signal goes back to base.
The F-117 airframe for instance has a large number of faceted surfaces, not unlike a crystal.
The facets are presumed to reflect radar energy away from the aircraft in any other direction than that of the radar emitter.
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A flat plate at right angles to an impinging radar wave has a very large radar signal, and a cavity, similarly located, also has a large return. Thus the inlet and exhaust systems of a jet aircraft would be expected to be dominant contributors to radar cross section in the nose-on and tall-on viewing directions, and the vertical tail dominates the side-on signature.
2.2 Radar absorbtion
A second way of stopping radar reflections is by coating the plane with material that soaks up radar energy.
These typically consist of carbon, carbon fibre componsites, or magnetic ferrite-based substance.
The result is that for instance the B-2 is reported to have the same RCS as a child's tricycle!
Flight-control surface can be made from honeycombed materials which reflect incoming radar waves internally rather than back to the radar. Radar-absorbing coatings can be applied to the surface of the body which effectively drain the energy of the radar signal.
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2.3 Echo cancellation
The key dimension of a quarter wavelength can vary in practice from millimeter to one meter. Although the coating designer will frequently try to use materials whose dielectric constant varies in a way that maintains a constant wavelength independent of frequency, the reality is that a number of different coatings and absorbers are needed to cover the required bandwidth.
Imagine a low frequency absorber that might be made of glass fiber hex-cell material. Its resistance is graded from front to back so that the edge is initially electro-magnetically soft and gradually becomes more attenuating as the wave passes through. This approach is particularly taken when, for practical reasons, the layer cannot be as deep as a quarter wavelength. The inner absorber is covered by a high-frequency ferromagnetic coating, which completes the frequency coverage.
Metal components such as the engine, which produce significant radar reflections, can be shielded using a metal and plastic sandwich whose layers are spaced in such a way as to create a standing wave, cancelling out any radar reflections.
3.0 Heat radiation reduction
Infrared radiation (heat) should be minimized by a combination of temperature reduction and masking, although there is no point in doing these past the point where the hot parts are no longer the dominant terms in the radiation equation. The main body of the airplane has its own radiation, heavily dependent on speed and altitude, and the jet plume can be a most significant factor, particularly in afterburning operation.
Air has a very low emissivity, carbon particles have a high broadband emissivity, and water vapor emits in very specific bands. Infrared seekers have mixed feelings about water-vapor wavelengths, because, while they help in locating jet plumes, they hinder in terms of the general attenuation due to moisture content in the atmosphere. There is no reason, however, why smart seekers shouldn't be able to make an instant decision about whether conditions were favorable for using water-vapor bands for detection.
4.0 Turbulence reduction
By optimizing the aerodynamics of the stealth plane, the for the eye invisible turbulence trail in the air, can be kept to a minimum. This way it becomes harder for the very special laser equipment to detect the trail and trace it back all the way to the plane which created it.
5.0 Visual detection reduction
5.1 Hiding smoke contrails (jet wake)
Reducing smoke in the exhaust is accomplished by improving the efficiency of the combustion chambers. Getting rid of contrails - that distinct white line in the sky caused by high flying jets - is a harder task.
Tests have been done using exotic chemicals to be inserted into the engine outlet gases to modify infrared signature as well as to force water molecules in the exhaust plume to break up into much finer particles, thus reduce or even eliminate contrails. One of the chemical used for this was chloro-fluoro-sulphonic acid. Several other acids were tested too, but the result was that the chemicals were too corrosive and the system was waved.
5.2 Low visibility
An aircraft at low to medium altitudes tends to be a black dot against the background of the sky. To avoid this, the plane a given a special medium gray color.
The gray, when combined with light scattering at low to medium altitudes ensures about as low observability as can be possible, or a reduction to 30% in visibility.
5.3 Low level flight
Another technique used by aircraft to avoid radar is to fly at very low levels where there is a great deal of 'ground clutter' ... radar reflections given off by buildings and other objects. Low-level aircraft can go undetected by most radar systems.
The latest ground-defence systems however are designed to discriminate between ground-clutter and hostile planes. In addition, ground-clutter is partly avoided by using 'look down' radar systems, which track aircraft from other aircraft flying above.
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1.0 Stealth features of the F/A-22
Taking a look at the F/A-22, quickly reveals the fundamental principles of a stealthy design as discussed earlier.
2.0 Continuous curves
The F/A-22 uses a combination of different ways to keep radar waves from bouncing back to their origin. The most sophisticated system is the use of so-called continuous curvature.
Many of of the surface shapes of the F/A-22 are curves with constantly changing radii. These scatter radar beams in all directions instead of back to the radar source. There are no right angles on the exterior of the design.
In order to calculate the curves and the effect they have on radar reflections form any point in 3D space, requires a tremendous computing power.
The first plane using this technology extensively is the B-2 stealth bomber, also known as the flying wing.
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Since computer- and software development has sky-rocketed over the past 20 years, prediction models can now be calculated quite precisely ,taking in account radar reflection versus the shape of the plane, while supporting more naturally aerodynamic shapes.3.0 Planform alignment
The second way to keep radar waves from returning to the sending antenna, the leading and trailing edges of the wing and tail have identical sweep angles (a design technique called planform alignment).
The fuselage and canopy have sloping sides. The vertical tails are canted. The engine face is deeply hidden by a serpentine inlet duct and weapons are carried internally.
4.0 Saw-toothed edges
The F/A-22 has a low height triangle appearance from the front. This physical cross sectional view ensures a small signature from the front and low observability touches such as paint and materials, as well as little "W" shapes where straight lines might have appeared, all tend to break up the signature by absorption or redirection.
The "W" shapes are found at numerous places on the stealth aircraft. For instance, in the forefront of the cockpit glass, there is a very apparent "W" shape. This reduces the radar energy reflected during a head-on pass to the radar emitter. The "W" shape is also found on landing gear doors, engine inlets and outlets, as well as other openings.
5.0 Engine nozzles
Reduction of radar cross section of nozzles is also very important, and is complicated by high material temperatures.
The approach taken at Lockheed is to use ceramic materials.
The ceramics may be either lightweight, parasitic sheets mounted on conventional nozzle structures or heavier structural materials forming saw-toothed edges.
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The pilot's head, complete with helmet, is a major source of radar return. This effect is amplified by the returns of internal bulkheads and frame members. The solution is to design the cockpit so that its external shape conforms to good low radar cross section design rules, and then plate the glass with a film similar to that used for temperature control in commercial buildings. Here, the requirements are more stringent: it should pass at least 85% of the visible energy and reflect essentially all of the radar energy. At the same time, one would prefer not to have noticeable instrument-panel reflection during night flying.
7.0 Antennas
On-board antennas and radar systems are a major potential source of high radar visibility for two reasons. One is that it is obviously difficult to hide something that is designed to transmit with very high efficiency, so the so-called in-band radar cross section is liable to be significant. The other is that even if this problem is solved satisfactorily, the energy emitted by these systems can normally be readily detected. The work being done to reduce these signatures is classified.
8.0 Paint scheme
In order to make the F/A-22 disappear for the human eye on the ground, when in flight, special camouflage schemes have been developed. This way the plane will blend with the background sky as much as possible viewed from the bottom and disappear in the ground texture when seen from above.
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9.0 Conclusion
The result of all these as well as a number of un-disclosed or non mentioned measures is the F/A-22's BVR capability, meaning that it can detect, engage and kill an opponent fighter, while staying invisible itself.
The future of Stealth
Imagine you can electronically change the color of a given surface in such a way it can match the terrain below it. Looking from above, the surface appears to match the terrain. Fly over forest, and the surface takes on a green like hue. A cloudy day, add clouds to match what sensors see underneath and the aircraft becomes a chameleon and disappears.
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This may sound like Science Fiction, but then think of the LCD display of notebooks and it may not seem so far fetched all of a sudden. Recent breakthroughs in chemical polymer technology have made it possible to create polymer (plastic) color displays. In other words, mold the polymer in any shape you like and with the additional control electronics you can make it virtually invisible from any point of view.
You can try it yourself with your own computer. Take a look at this website, containing a tutorial on how to make your computer screen transparent, which of course is just an illusion...
This is not a new idea, in fact several military fiction writers have already come up with the idea, in one particular instance having the aircraft continually modifying top and bottom like a magician's mirror box making the aircraft totally invisible.
More technologies are currently under development and will be closely monitored to be found here. But likewise the F-117, we may not hear about that until the first smart-bomb coming out of nowhere has made a successful hit!
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