STEALTH - RCS comparisons - Materials - Techniques

[DBdev]

Stealth is too important of a subject to be discussed just under KAAN or any other forum. So I think it deserves it's very own section.

One of the most simplified hopefully accurate information on STEALTH, regarding RCS comparisons.
Did you guys know even the ancient stealth airplane F-117 attacked 1600 targets inside HEAVILY AD armed Saddam's Iraq without ever being close to be in danger?
Will we be in any diffierent of a situation if finally impending war breaks out because of our countless potential conflict issues with Americans?

Radar cross section: The measure of stealth​

Blog
March 31, 2017

RAY ALDERMAN​

VITA Standards Organization

WARFARE EVOLUTION BLOG: The primary measure of stealth, or low observability (LO), is the radar cross section (RCS) of the target, whether it?s aircraft, missiles, or ships. The radar pulse goes out from the transmitter, hits the target, and bounces back. The radar receiver measures the energy in the return signal in decibel (db) units, but that?s a hard way for normal people to visualize the size of a target. So, we must convert db to square meters (m2) to get the picture.​

Here’s all the the conversion formulas you'll need if you enjoy lots of digits to the right of the decimal point. <http://www.alternatewars.com/BBOW/Radar/Decibels_Radars.htm>

Instead of laboriously converting the db values for all the different fighter jets mentioned in my previous article, there's a nifty table with most of the conversions done for us. <http://www.globalsecurity.org/military/world/stealth-aircraft-rcs.htm> Using this table will also allow me to escape the clutches of the unwritten laws of journalism too. They state that readership will decline by 50 percent for every formula in an article, and by 75 percent if you show any calculations. Many good papers and articles exist on the web that expose the intricacies of RCS if you want to dig deeper into this topic. For this article, we’ll just use the m2 values from the table and skip the joys of the math.

To read more Warfare Evolution Blogs by Ray Alderman, click here.

Before we get into this, other complications need to be mentioned. The RCS of an aircraft depends on its aspect, the orientation of the target to the radar source. Any aircraft will have a smaller RCS from the front, and show-up bigger from the side or the rear. Looking at the table values, their numbers seem to be based on the frontal aspect, the lowest RCS of the aircraft. Also, some fighter jets have their largest RCS from the rear, due to the exhaust nozzles. And, RCS depends on the wavelength (frequency) of the radar signal and how far away the target is. But we’ll ignore these pesky details for now. Just consider the numbers we use here as the best RCS values for each aircraft. For comparison, an average man has an RCS of about 1m2.

- In my last blog, we talked about Russia’s older 4G MIG-XX, 4.5G SU-34/35, and 5G PAK-FA (T-50) fighter jets. The older 4G MIG platforms have an RCS of 15m2, down to 3m2 for the MIG-21. The SU-34s RCS is 1m2. SU-35s are 1m2 to 3m2, according to web sources. The PAK-FA (T-50) has an RCS of 0.5m2, about the same as a Tomahawk cruise missile. None of those planes are truly stealthy. Most of them are big fat targets to our advanced radars.

- China’s fighter planes are mostly the older 4G J-7s, which were derived from the Russian MIG-21 airframe. So, they probably show the same RCS of about 3m2 best case. Their J-10s have an RCS of 0.5 to 1.5m2. Their newer J-20 fighter has some flaws in its design, so it has an RCS about the same as older 4G fighters (about 1m2 to 3m2). We don’t have much data on their new J-31 (FC-31) fighters, but they did create some rippled exhaust nozzles on their engines that will reduce its rear-aspect reflections. If I had to guess, since the J-31 looks like the offspring of the F-35 and the PAK-FA (T-50), It might have an RCS in the range of 0.5 to 0.1m2, being generous. And, if China is doing so well with their stealth fighter designs, why did they just take delivery of four Russian SU-35s in December 2016? The frontal RCS of their new J-31 is certainly much better than the SU-35, if my guess is accurate, so they must want access to the Russian radar and on-board systems.

- Among the U.S. fighter planes, the 4G F-15 has an RCS of 25m2, not very impressive and bigger than the older Russian MIG fighters. The 4G F16 has an RCS of 5m2, better but still not great. The 4.5G F/A-18 Hornet Navy fighter jet has an RCS of 1m2, about the same as the Russian SU-34/35 and the Chinese J-20. The 5G F-35 has an RCS of 0.005m2, about the size of a golf ball. However, from the rear, it looks much bigger because of the exhaust nozzles, the same problem we saw with the 5G Russian PAK-FA (T-50). For comparison, the 5G F-22 has an RCS of 0.0001m2, about the size of a bumble bee. The U2 and SR-71 spy planes have an RCS of 0.01m2, about the size of a small bird. Our first stealth fighter/bomber was the F-117 Nighthawk from the 1980s. It has an RCS of 0.003m2, about the size of a hummingbird, and those F-117s hit more than 1,600 targets without being molested by Iraqi air defenses during the 1991 Gulf War.

We didn’t discuss long range bomber aircraft in the previous article, but it’s worth throwing those values in here for your reading enjoyment. The B-52 has an RCS of about 100m2. The B-1 bomber is 10m2. The B-2 bomber has an RCS of 0.0001m2, the same as the F-22, the size of a bumble bee. The new B-21 bomber, now being built by Northrop Grumman, is virtually invisible to UHF/VHF radar. It shows up about the size of a mosquito. I am forced to use one formula and do the calculations here, since the return signal is -70db for the B-21, and the RCS in square meters was not on the web anywhere: RCSsm=10db/10 = 10-70/10 = 10-7, or about 0.000001m2. If you ever wondered about it, that’s the size of a mosquito on radar.

Our major enemies fly terribly old and antiquated non-stealthy bomber aircraft. The Russians are still using propeller-driven aircraft like the TU-95 Bear. By comparison again, the RCS of a World War II B-25 prop-driven bomber is 3,100m2. The Russians also have their TU-160 jet-powered bombers in operation, and the Chinese have a similar elderly H-6 jet bomber. There are no good numbers for these antiquated airframes on the web, so I think they would have about the same RCS as a Hilton Hotel.

As you can tell from all these numbers, good stealth design requires that all weapons (missiles, bombs) be enclosed inside the airframe. Also, external auxiliary fuel tanks under the wings will make the aircraft light-up a radar screen since they increase RCS dramatically. And, you must pay attention to the exhaust nozzles of the engines if you want to remain unnoticed. Neither Russia nor China have been able to equal what the U.S. has done with stealth design and radar-absorbant coatings on fighter jets. And both are way behind our stealth bomber designs. Additionally, I must give my usual disclaimer here: there are multiple RCS numbers on the web. I chose the only ones available, the ones found the most in articles, or calculated them from the published db values.

Next time, we’ll take a look at aircraft carriers, and why we don't need them anymore. That should make the Navy nervous. And, sorry about the formulas and calculations. The readership numbers on this piece in the Mil-Embedded.com web stats will verify if the unwritten journalism laws are true.

Radar cross section: The measure of stealth - Military Embedded Systems

WARFARE EVOLUTION BLOG: The primary measure of stealth, or low observability (LO), is the radar cross section (RCS) of the target, whether it?s aircraft, missiles, or ships. The radar pulse goes out from the transmitter, hits the target, and bounces back. The radar receiver measures the energy...

militaryembedded.com

 

[Radonsider]

Stealth is too important of a subject to be discussed just under KAAN or any other forum. So I think it deserves it's very own section.

One of the most simplified hopefully accurate information on STEALTH, regarding RCS comparisons.
Did you guys know even the ancient stealth airplane F-117 attacked 1600 targets inside HEAVILY AD armed Saddam's Iraq without ever being close to being any danger?
Will we be in any diffierent of a situation if finally impending war breaks out because of our countless potential conflict issues with Americans?

Radar cross section: The measure of stealth​

Blog
March 31, 2017

RAY ALDERMAN​

VITA Standards Organization

WARFARE EVOLUTION BLOG: The primary measure of stealth, or low observability (LO), is the radar cross section (RCS) of the target, whether it?s aircraft, missiles, or ships. The radar pulse goes out from the transmitter, hits the target, and bounces back. The radar receiver measures the energy in the return signal in decibel (db) units, but that?s a hard way for normal people to visualize the size of a target. So, we must convert db to square meters (m2) to get the picture.​

Here’s all the the conversion formulas you'll need if you enjoy lots of digits to the right of the decimal point. <http://www.alternatewars.com/BBOW/Radar/Decibels_Radars.htm>

Instead of laboriously converting the db values for all the different fighter jets mentioned in my previous article, there's a nifty table with most of the conversions done for us. <http://www.globalsecurity.org/military/world/stealth-aircraft-rcs.htm> Using this table will also allow me to escape the clutches of the unwritten laws of journalism too. They state that readership will decline by 50 percent for every formula in an article, and by 75 percent if you show any calculations. Many good papers and articles exist on the web that expose the intricacies of RCS if you want to dig deeper into this topic. For this article, we’ll just use the m2 values from the table and skip the joys of the math.

To read more Warfare Evolution Blogs by Ray Alderman, click here.

Before we get into this, other complications need to be mentioned. The RCS of an aircraft depends on its aspect, the orientation of the target to the radar source. Any aircraft will have a smaller RCS from the front, and show-up bigger from the side or the rear. Looking at the table values, their numbers seem to be based on the frontal aspect, the lowest RCS of the aircraft. Also, some fighter jets have their largest RCS from the rear, due to the exhaust nozzles. And, RCS depends on the wavelength (frequency) of the radar signal and how far away the target is. But we’ll ignore these pesky details for now. Just consider the numbers we use here as the best RCS values for each aircraft. For comparison, an average man has an RCS of about 1m2.

- In my last blog, we talked about Russia’s older 4G MIG-XX, 4.5G SU-34/35, and 5G PAK-FA (T-50) fighter jets. The older 4G MIG platforms have an RCS of 15m2, down to 3m2 for the MIG-21. The SU-34s RCS is 1m2. SU-35s are 1m2 to 3m2, according to web sources. The PAK-FA (T-50) has an RCS of 0.5m2, about the same as a Tomahawk cruise missile. None of those planes are truly stealthy. Most of them are big fat targets to our advanced radars.

- China’s fighter planes are mostly the older 4G J-7s, which were derived from the Russian MIG-21 airframe. So, they probably show the same RCS of about 3m2 best case. Their J-10s have an RCS of 0.5 to 1.5m2. Their newer J-20 fighter has some flaws in its design, so it has an RCS about the same as older 4G fighters (about 1m2 to 3m2). We don’t have much data on their new J-31 (FC-31) fighters, but they did create some rippled exhaust nozzles on their engines that will reduce its rear-aspect reflections. If I had to guess, since the J-31 looks like the offspring of the F-35 and the PAK-FA (T-50), It might have an RCS in the range of 0.5 to 0.1m2, being generous. And, if China is doing so well with their stealth fighter designs, why did they just take delivery of four Russian SU-35s in December 2016? The frontal RCS of their new J-31 is certainly much better than the SU-35, if my guess is accurate, so they must want access to the Russian radar and on-board systems.

- Among the U.S. fighter planes, the 4G F-15 has an RCS of 25m2, not very impressive and bigger than the older Russian MIG fighters. The 4G F16 has an RCS of 5m2, better but still not great. The 4.5G F/A-18 Hornet Navy fighter jet has an RCS of 1m2, about the same as the Russian SU-34/35 and the Chinese J-20. The 5G F-35 has an RCS of 0.005m2, about the size of a golf ball. However, from the rear, it looks much bigger because of the exhaust nozzles, the same problem we saw with the 5G Russian PAK-FA (T-50). For comparison, the 5G F-22 has an RCS of 0.0001m2, about the size of a bumble bee. The U2 and SR-71 spy planes have an RCS of 0.01m2, about the size of a small bird. Our first stealth fighter/bomber was the F-117 Nighthawk from the 1980s. It has an RCS of 0.003m2, about the size of a hummingbird, and those F-117s hit more than 1,600 targets without being molested by Iraqi air defenses during the 1991 Gulf War.

We didn’t discuss long range bomber aircraft in the previous article, but it’s worth throwing those values in here for your reading enjoyment. The B-52 has an RCS of about 100m2. The B-1 bomber is 10m2. The B-2 bomber has an RCS of 0.0001m2, the same as the F-22, the size of a bumble bee. The new B-21 bomber, now being built by Northrop Grumman, is virtually invisible to UHF/VHF radar. It shows up about the size of a mosquito. I am forced to use one formula and do the calculations here, since the return signal is -70db for the B-21, and the RCS in square meters was not on the web anywhere: RCSsm=10db/10 = 10-70/10 = 10-7, or about 0.000001m2. If you ever wondered about it, that’s the size of a mosquito on radar.

Our major enemies fly terribly old and antiquated non-stealthy bomber aircraft. The Russians are still using propeller-driven aircraft like the TU-95 Bear. By comparison again, the RCS of a World War II B-25 prop-driven bomber is 3,100m2. The Russians also have their TU-160 jet-powered bombers in operation, and the Chinese have a similar elderly H-6 jet bomber. There are no good numbers for these antiquated airframes on the web, so I think they would have about the same RCS as a Hilton Hotel.

As you can tell from all these numbers, good stealth design requires that all weapons (missiles, bombs) be enclosed inside the airframe. Also, external auxiliary fuel tanks under the wings will make the aircraft light-up a radar screen since they increase RCS dramatically. And, you must pay attention to the exhaust nozzles of the engines if you want to remain unnoticed. Neither Russia nor China have been able to equal what the U.S. has done with stealth design and radar-absorbant coatings on fighter jets. And both are way behind our stealth bomber designs. Additionally, I must give my usual disclaimer here: there are multiple RCS numbers on the web. I chose the only ones available, the ones found the most in articles, or calculated them from the published db values.

Next time, we’ll take a look at aircraft carriers, and why we don't need them anymore. That should make the Navy nervous. And, sorry about the formulas and calculations. The readership numbers on this piece in the Mil-Embedded.com web stats will verify if the unwritten journalism laws are true.

Radar cross section: The measure of stealth - Military Embedded Systems

WARFARE EVOLUTION BLOG: The primary measure of stealth, or low observability (LO), is the radar cross section (RCS) of the target, whether it?s aircraft, missiles, or ships. The radar pulse goes out from the transmitter, hits the target, and bounces back. The radar receiver measures the energy...

militaryembedded.com

Plain US propaganda

 

[DBdev]

This is the most scary part for me. Imagine B-21s attack completely invisible to all our radars including supposedly "anti stealth" UHF/VHF radar of s400. It would be devistating day after day without any way to stop them.

"B-21 bomber, now being built by Northrop Grumman, is virtually invisible to UHF/VHF radar. It shows up about the size of a mosquito. "
"RCSsm=10db/10 = 10-70/10 = 10-7, or about 0.000001m2" !!!

 

[DBdev]

Plain US propaganda

Saddam must have thought the same regarding American stealth technology of the 1980s. Sticking our heads in to the sand is not the answer I am afraid. Stealth RCS values are backed by multiple research papers. B-21 especially is unstoppable, if it is immune to all radar frequencies as their research, trillions in development of multi-spectrum stealth to this new level suggests.

 

[Huelague]

Saddam must have thought the same regarding American stealth technology of the 1980s. Sticking our heads in to the sand is not the answer I am afraid. Stealth RCS values are backed by multiple research papers. B-21 especially is unstoppable, if it is immune to all radar frequencies as their research, trillions in development of multi-spectrum stealth to this new level suggests.

Is Aselsan working on quantum Radar?

 

[BaburKhan]

Is Aselsan working on quantum Radar?

See here:

Quantum Technology (Radar, Compass, Network, Quantum Computer and Nano-Satellite) in Türkiye

TÜBİTAK BİLGEM Technology magazine in May-2020 issue It has been announced that work has begun on Quantum Network technologies, Quantum Radar, Quantum compass and Quantum nano satellite projects.

defencehub.live

 

[DBdev]

Theoretical RCS value of Kizilelma was around 0.24 m2 without Ram paint according to software calculations.

https://twitter.com/i/web/status/1596576470306263040

Now it's relatively lower RCS was field tested against Turkish F-16s and it indeed detects them earlier. But American Vipers have 1m2 or less RCS thanks to Have Glass v5 and our F-16s have terribly huge RCS of 5m2!!!

So this test doesn't prove much at all. For example it doesn't say much about their chances against Greek F-16s which may also get better coatings and radars. They should at least, test them against Qatar's Rafales, Eurofighters and F-15QAs.

https://twitter.com/i/web/status/1741753012790383034

 

[TheInsider]

Theoretical RCS value of Kizilelma was around 0.24 m2 without Ram paint according to software calculations.

https://twitter.com/i/web/status/1596576470306263040

Now it's relatively lower RCS was field tested against Turkish F-16s and it indeed detects them earlier. But American Vipers have 1m2 or less RCS thanks to Have Glass v5 and our F-16s have terribly huge RCS of 5m2!!!

So this test doesn't prove much at all. For example it doesn't say much about their chances against Greek F-16s which may also get better coatings and radars. They should at least, test them against Qatar's Rafales, Eurofighters and F-15QAs.

https://twitter.com/i/web/status/1741753012790383034

Now it's relatively lower RCS was field tested against Turkish F-16s and it indeed detects them earlier. But American Vipers have 1m2 or less RCS thanks to Have Glass v5 and our F-16s have terribly huge RCS of 5m2!!!

We have our own paint.

 

[DBdev]

I said Hürjet SHOULDN'T be anything more than a trainer unlike Kizilelma series and Kaan.

And I am saying F-16 is one of the worst stealth fighters out there in current day batlefields of "First Look, First Shot, First Kill" for the same reason.

Stealth is like %80 percent or so, SHAPE, SIZE, ANGLES, INTAKES, EXHAUSTS, IR, ETC, that's visible to anyone with eyes,
The secret, live or die part is countless types of stealth paints, tapes, honeycomb, multi-spectrum matrixes, ceramics, etc, etc.

F-16 and Hürjet can't win any modern day battle. Period. If we can we should try to sell and replace our F-16s.
Because they lose most basic %80 structural stealth part, from the start. Let alone different secret RAM materials.

Americans tried for 5 generations of RAM to lower 5m2 RCS of F-16 to 1m2. Which isn't still any good at all.
Our iron ball paint is like primitive Level 2 or so, not 5. Paint of any kind won't cut it in RAM tech.

HAVE GLASS: MAKING THE F-16 LESS OBSERVABLE​

Stealth is widely hyped, but rarely well understood. Stealth, or low observability (LO), does not make an aircraft invisible to radar. However, if an aircraft’s radar cross section (RCS) can be reduced, it will appear ‘smaller’ on radar and may be detected later, at shorter range.
Today’s fifth-generation types incorporate a degree of low observability that gives them a much reduced RCS compared to conventional aircraft. But the low RCS is a function of optimized design, purpose-built structure and advanced specialized materials. Radar cross section can be measured in dBm² (decibels per square meter) or more simply in square meters. However, in the latter case, the radar cross section of an aircraft is NOT the same as its actual cross-sectional area.

This Have Glass V 480th Fighter Squadron F-16CM Fighting Falcon looks almost black – it has just taken on fuel from a KC-135 Stratotanker assigned to the 50th Expeditionary Aircraft Refueling Squadron over Southwest Asia. The mission was on December 22, 2020. USAF/Staff Sgt Trevor T McBride

Instead, the RCS is quoted in terms of the equivalent area of a metal sphere that would reflect back the same radar energy as the aircraft, when viewed at the same altitude, from straight ahead. For example, the Lockheed Martin F-22 Raptor has a frontal RCS of 0.0001-0.0002m², equivalent to a marble-sized sphere, while the F-35A has an RCS of about 0.0015m² – equivalent to a golf ball-sized metal sphere.
This kind of RCS makes an immediate tactical difference – an F-22 can be detected by a fighter radar, but only at ranges of 5-10km (APG-68 and APG-80), far too late for defensive systems to react. Of course, the LO fifth-generation fighters are designed for something close to ‘all aspect’ stealth – having a low radar cross section when viewed from any angle. This is difficult to achieve, and some aircraft that are thought of as being LO aircraft are not stealthy from all angles. Even those that are, will often have to carefully calculate where they can turn and what bank angles they can use in order to remain as inconspicuous as possible.
But this kind of full-on stealth, or true LO, can only be achieved by an aircraft designed to be stealthy from the very start, with appropriate shaping, including the carefully tailored orientation and curvature of surfaces, alignment of edges and the shielding of any ducts or cavities. The LO aircraft will use carefully-selected materials, including advanced composites and radar-absorbing materials (RAM), metamaterials as well as other artificial types, and will benefit from active and passive cancellation of a threat radar’s ‘skin return’.

A Block 42 F-16C Fighting Falcon from the 112th Fighter Squadron, part of the 180th Fighter Wing, based at Toledo, Ohio. The aircraft has colored fin markings superimposed on the Have Glass V scheme, and is seen off the wingtip of a KC-135 Stratotanker from the 121st Air Refueling Wing in Columbus, Ohio. US ANG/Senior Airman Christi A Richter

Conventional, non-LO combat aircraft have much larger radar cross sections – that of the B-52 has been calculated at 100m², the F-15 at 25m², the F-16 and MiG-29 at 5m², and the Super Hornet and Rafale at 1m², with the Eurofighter Typhoon variously reported at the same level, or at 0.5m² (GlobalSecurity.Org).
So, while these aircraft are visible to radar at tactically useful ranges, reducing their RCS will delay detection, and may make EW tactics and techniques more effective. If an F-16’s RCS was to be reduced to something equivalent to that of a Super Hornet, for example, radar detection range would be shortened by about 30-45% – something that could be critical to mission success, or even survival!

Legacy boost
Since the USAF has long since abandoned its once-planned ‘all stealth’ combat force, and with F-35 procurement proceeding more slowly than had originally been expected, boosting the effectiveness of legacy platforms has become a priority.

A USAF Block 40 F-16C, assigned to the 'Lobos' (the 175th Fighter Squadron), part of the South Dakota ANG's 114th Operations Group, kicks up smoke as it touches down. The aircraft is painted in the latest Have Glass V color scheme. US ANG/Tech Sgt Luke Olsen

Air National Guard wings in Alabama, Vermont and Wisconsin have started to receive F-35As, but about 300 Block 30 F-16C/Ds still fly with ANG and Air Force Reserve units, some of the oldest fighters in the Air Force inventory. It has been estimated that it could take more than a decade to replace them all. Nor is there any immediate plan to retire the later Block F-16Cs operated by three regular USAF units.
These are the 20th Fighter Wing at Shaw Air Force Base, South Carolina (with the 55th, 77th and 79th Fighter Squadrons); the 52nd Fighter Wing at Spangdahlem Air Base, Germany (480th Fighter Squadron) and the 35th Fighter Wing at Misawa AB, Japan (13th and 14th Fighter Squadrons). These aircraft are also operated by the 148th Fighter Wing at Duluth ANG base, Minnesota (179th Fighter Squadron ‘Bulldogs’) and the 169th Fighter Wing at Joint National Guard Base McEntire, South Carolina (157th Fighter Squadron ‘Swamp Foxes’). As a result, the Air Force now plans to conduct a service-life extension program on more than 800 of its roughly 900 F-16s, with only the oldest Block 25 models scheduled to retire imminently.
The USAF will thus continue to rely on the F-16 for decades to come, and if those aircraft are to be as operationally useful and as survivable as possible, then they need to be as stealthy as they can reasonably be.

Two F-16s from the 148th Fighter Wing, Minnesota ANG, flying from Duluth on October 30, 2019. Both wear Have Glass V paint. USAF


Surface treatments
It would be possible to drastically reduce an F-16’s RCS by making major structural changes – reshaping the engine intake and inlet ducts and revising the engine afterburner nozzle. This has actually been undertaken on various F-16 research aircraft, mainly in support of the F-35 program. But this kind of major structural modification is neither practical nor affordable on a larger, wider scale, and the incorporation of a diverterless supersonic intake into a production variant of the F-16 is not feasible, despite the RCS reductions that would result.
But surface treatments are possible and these can make a real difference. The importance of surface finish to an aircraft’s radar cross section can be gauged by the Royal Navy’s experience during the Falklands War. As the Task Force exercised while it sailed south, it became clear that there was a significant disparity between the RCS of different individual Sea Harrier airframes. Investigation revealed that this disparity was down to the amount of WD-40 (a spray oil used to repel moisture and inhibit corrosion) applied to the aircraft. Paint contaminated by WD-40 was found to be more reflective of radar energy.

This Block 42G F-16C, assigned to the Ohio Air National Guard's 180th Fighter Wing (112th Fighter Squadron), shows the advantage of the metallic sheen of the Have Glass V paint. US ANG/Senior Master Sgt Beth Holliker

The designers of the Lockheed F-117 tackled the problem by completely coating the aircraft with 2,000lbs of RAM, while the F-35 uses a range of special techniques and technologies, including radar-absorbing structures for the fuselage and leading edges, plus paint-type RAM and an Infrared (IR) top coat. RAM is embedded into the aircraft’s skin, making the entire airframe a radio frequency grid, that behaves like a single resistance circuit pathway, cancelling the radar energy that hits it.
But treatments, and especially surface coatings and edge treatments, can be applied to existing designs in order to reduce their RCS. The F-16 has benefited from a long-running program of improvements to minimize its RCS, under the name Have Glass.

F-35s have a range of LO technologies, including a stealthy RAM coating, which can appear shiny and metallic in some lighting conditions. This VMFA-211 F-35B deployed to RAF Marham in Suffolk, UK, in March 2020, from its home base at Yuma, Arizona. MoD Crown Copyright


Have Glass
The first phase, Have Glass I, covered the addition of an indium-tin-oxide layer to the gold-tinted cockpit canopy. This was a vapor-deposited coating, which was applied in a similar way to the application of coatings to sunglasses. The gold tinting formed a very thin and very delicate film, which was reflective to radar frequencies, and tended to hide the ejection seat and pilot’s head from radar.
When applied to Dutch F-16As, the canopy film was known as the Pacer Bond modification. The first to be modified (J- 358) was delivered on September 5, 1986. The Have Glass II program saw some 1,700 F-16s receiving further changes. Have Glass II encompassed two separate modifications, known as Pacer Mud and Pacer Gem I/II.

Capt Matthew Feeman of the 55th Fighter Squadron, approaches a South Carolina Civil Air Patrol Cessna during an exercise. Feeman's F-16 wears the Have Glass II scheme, which is very well-worn. USAF/Senior Airman Michael Cowley

Pacer Mud was a modification that reduced RCS, adding FMS-3049 RAM to several areas of the airframe, including the air intake, with RAM foam installed behind the radar antenna. The RAM coating contained ferromagnetic particles embedded in a high-dielectric-constant polymer base. The dielectric material slowed down the incoming radar wave and the ferromagnetic particles absorbed the energy. The small reflection from the front face of the absorber was ‘cancelled’ by the residual reflection from the structure beneath it.
Overall, the RAM covered about 60% of the F-16's structure (mainly forward and side facing areas) in 10-12mm thickness, adding 100kg to the empty aircraft weight. Pacer Gem entailed the application of an FMS-2026 top coat that used fiberglass particles to reduce the infrared signature. Relatively few aircraft received the Pacer Gem upgrade, because the costs were too high and the cure times were much longer.

This 149th Fighter Wing, Texas ANG, F-16, with a new, darker, single-color Have Glass V paint scheme, was one of the first to have its markings applied in black, rather than gray. This particular aircraft shot down a MiG-23 with an AMRAAM during a patrol of the Iraqi no-fly zone on January 17, 1993. This was the second kill using the AMRAAM missile. US ANG/Senior Master Sgt Vincent De Groot

Have Glass II was primarily applied to ‘Wild Weasel’ F-16CJ aircraft (now designated as Block 50/52 F-16CM/DMs) tasked with the SEAD role – attacking deadly surface-to-air missile sites. These were mainly assigned to Shaw AFB, Eglin AFB and Spangdahlem AFB. The new paint had a distinct metallic sheen and was applied using the same CASPER (Computer Aided Spray Paint Expelling Robot) system used for the F-22. The use of paint-spraying robots allowed the operator to reach confined areas, such as the inlet ducts and to work without stepping on the aircraft.

Aging disgracefully
The paint looks great on first coming out of the paint barn, but does become metallic and dirty looking in a short period of time. It was also not very resilient, and Have Glass aircraft often had a grubby, faded appearance with a flaking finish. It remains unclear as to whether that was due to the paint itself, or to new ‘environmentally friendly’ primers used on the aircraft.

Have Glass was initially aimed squarely at the USAF's Wild Weasels - those F-16CMs assigned to the SEAD role. Here one of the flagship aircraft from the 20th Fighter Wing tucks in close off his wingman's starboard wingtip. USAF/Tech Sgt Gregory Brook

This original Have Glass I/II phase was estimated to reduce the RCS of an F-16 by about 15%, but more was to come. Since 2012 (and possibly a little before), USAF F-16s have started to receive a new, single-tone, dark-gray color scheme, similar to that applied to the F-35 Lightning II Joint Strike Fighter. The new, single-color paint scheme marks a departure from the two-tone gray scheme long associated with the F-16, and the new ferromagnetic paint, which can absorb radar energy, may be more robust and less prone to deterioration than the previous Have Glass I/II paint.
Some Have Glass V aircraft may appear to have a different colored nose, because radar-absorbing paint cannot be applied to the dielectric radome (nor indeed can most conventional aircraft paints), as it may disrupt the signals from the F-16’s own radar.

Probably the stealthiest F-16 was this Lockheed-operated testbed, which was fitted with a diverterless supersonic inlet in support of the F-35's development. Lockheed Martin

But while there are clear similarities with the F-35 color scheme, Have Glass V is not an attempt to match the F-35’s low-observability characteristics – which would be impossible without a ground-up redesign. It will, however, deliver a significant boost to the F-16’s survivability and operational capability, and it promises to deliver a performance advantage over competing fighter aircraft, with several reports suggesting that a Have Glass V F-16 will, on average, have a 1.2m² radar cross section, compared with about 5m² for an ‘untreated’ F-16, straight off the line.
This new Have Glass V, or ‘Have Glass 5th-generation’ paint, was initially applied to the SEAD-assigned Block 50 F-16CM (formerly F-16CJ) aircraft, but, in December 2019, the first Block 30 F-16 to receive the Have Glass V paint was rolled out from the Iowa Air National Guard Paint Facility in Sioux City. The aircraft was assigned to the 149th Fighter Wing, Texas ANG, flying out of Joint Base San Antonio-Lackland. Interestingly, the two-letter tail code, serial number and squadron markings were all applied in black instead of light gray.

Two views of the same F-22 maneuvering over the US Central Command area of responsibility, after being refueled by a KC-135 Stratotanker from the 340th Expeditionary Air Refueling Squadron, based at Al Udeid Air Base, Qatar. The lighting makes the same colour scheme look very different. Both images taken by USAF/Staff Sgt Vernon Young Jr

Less detectable signature
In Europe, the new Have Glass V paint scheme has been applied by SABCA at its Gosselies (Charleroi) facility, in Belgium. SABCA is responsible for depot-level MRO work on USAF F-16s operated in Europe, and for the maintenance and upgrade of the type from nine different operators.
Apart from surface treatments, one of the most crucial things that can be done to make a fourth-generation fighter stealthier is to better manage its electronic signature, making it less detectable through the emissions that it puts out. This means using secure, frequency-agile communications systems and datalinks, and sensors which incorporate LPI (low probability of intercept) technologies.

The shiny metallic finish of the Have Glass II paint is immediately apparent in this view of F-16CM Fighting Falcons from the 480th Fighter Squadron at Spangdahlem Air Base in Germany. USAF/Airman 1st Class Dillon Davis

Fighting Falcons at Joint Base Andrews, Maryland received the first AN/APG-83 radars in January 2020 and in July, the Operational Flight Program Combined Test Force, 40th Flight Test Squadron and the 85th Test and Evaluation Squadron at Eglin AFB, tested the APG-83 in a four-ship formation for the first time. This mission included F-16s and F-15s, with 12 fighter pilots participating, including Active Duty, Reservist, Guard, civilian and contractor aircrew.
None of these improvements will turn the F-16 into a fifth-generation fighter, and none will render it invisible to enemy radar, but they will make it less visible and they promise to make it a whole lot more effective operationally.

 

[DBdev]

FRONTAL RCS OF 4TH GENERATION FIGHTER JETS​

Radar cross-section (RCS) is a measure of how detectable an object is by radar.




The factors that influence RCS are:
• the material of target
• the size of target
• incident angle & reflect we angle of target
• shape of the target
• polarization

List of 4th generation fighter jets (in active service)

A. US fighter jets :

• F-14 Tomcat:- RCS of 25 sq.m


• F-15 Eagle/F-15 E:- RCS of 25 sq.m


F-15 EX:- RCS of 1 sq.m (prototype, on order)


• F-16:- RCS of 5 sq.m


F-16 Block 60:- RCS of 1.2 sq.m


• F/A-18 C/D:- RCS of 2 sq.m


F/A-18 E/F Super Hornet:- RCS of 0.5- 1 sq.m


F/A-18 E/F Block III:- RCS of 0.1 sq.m (Prototype)
It is the Stealthiest 4.5th generation aircraft.



B. Russian Fighter Jets:

• Sukhoi Su-27:- RCS of 15 sq.m



• Su-30 MKI:- RCS is 4 sq.m
(Indian variant)


Su-30 MKK:- RCS is 5 sq.m
(Chinese variant)


• Su-33:- RCS is 5 sq.m


• Su-34:- RCS is 1- 3 sq.m


• Su-35:- RCS is 1 sq.m


• MiG-29/Mig-29 K:- RCS is 5 sq.m


• MiG-31:- RCS is 15 sq.m



• MiG-35:- RCS is 0.5- 2 sq.m


• Yak-130:- RCS is 1sq.m


C. Chinese fighter jets:

• Xi’an JH-7:- RCS is 10 sq.m


• Chengdu J-10:- RCS is 1.5 sq.m
It is developed from Israeli IAI Lavi program.


• Shenyang J-11:- RCS is 15 sq.m
It is the licensed copy of Su-27.


• Shenyang J-15:- RCS is 5 sq.m
It is the unlicensed copy of Su-33 (stolen)


• Shenyang J-16:- RCS is 3 sq.m


• Hongdu JL-10:- RCS is 0.5- 1 sq.m



• CAC/PAC JF-17 Thunder:- RCS is 3 sq.m
It is rejected Russian LCA design


JF-17 Block III:- RCS is about 1~1.5 sq.m



C. European Fighter Jets:

• Panavia Toronto:- RCS is 5- 10 sq.m


• Eurofighter Typhoon:- RCS is 0.5- 1.2 sq.m



* British fighter jets:

• British Aerospace Hawk 200:- RCS is 1 sq.m


* French fighter jets:

• Dassualt Mirage 2000:- RCS is 1- 2.5 sq.m


• Dassualt Rafale:- RCS is 0.5- 1.25 sq.m



* Swedish fighter jets:

• JAS-39 A Gripen:- RCS is 1.5 sq.m


Gripen Block E/F:- RCS is 0.5 sq.m



D. Indian Fighter Jet:

• HAL Tejas:- RCS is 0.3- 0.5 sq.m
It is the stealthiest 4.5th generation fighter jet which is operational.



E. Asian Fighter Jets:

* Japanese fighter jet:

• Mitsubishi F-2:- RCS is 1.5 sq.m
It is the licenseed copy of F-16


* Taiwan (ROC) fighter jet:

• AIDC F-CK-1 Ching-koo:- RCS is 1.5 sq.m
Developed with the help of USA


* South Korean fighter jet:

• KAI T-50 Golden Eagle:- RCS is 0.9 sq.m
Developed with Lockheed Martin

 

[DBdev]

RF-IR Stealth (Techniques)​

Brief :​

The advantage of detecting, identifying and engaging a target while stay invisible is undeniable, thus for years designers has been attempting to minimize the ability of radar, RWR and Infrared system to detect aircraft. Aircraft with significant low observability characteristics embodied in are called stealth aircraft. This article will discuss some common techniques used by stealth aircraft, their benefits and clear out some common misconceptions.

​

Low Observability in Electronic Spectrum.​

Radar is the main sensor systems for most aircraft and air defense systems so it is not a surprise that most of the detection reduction efforts are concentrated in the electronic spectrum.
Recalling the basic radar range equation discussed before:

It easy to see that the radar detection range is proportional to σ∧¼ where σ is the radar cross-section (the RCS ,the measure of a target’s ability to reflect radar signals in the direction of the radar receiver, often measured in dBsm or m2). Reducing the cross-sectional area, therefore, affects radar range, although only according to the fourth root. However, by carefully designing an aircraft, the value of σ may be reduced by many hundreds of times thus present an effective approach.
It is important to note that stealth aircraft are not invisible, the goal is to delay enemy detection as much as possible.

Contributors to high RCS: ​

To start with, the total radar reflection of a complex body such as aircraft made from several different kinds of reflections:

They can be grouped into these following types:
Specular return: this is the most significant form of reflection, the surface acts like a mirror for the incident radar pulse. Most of the incident radar energy is reflected according to the law of specular reflection ( the angle of reflection is equal to the angle of incidence).

Traveling/Surface wave return: an incident radar wave strike on the aircraft body can generate a traveling current on surface that propagates along a path to surface boundaries such as leading edge, surface discontinuous …etc, such surface boundaries can either cause a backward traveling wave or make the wave scattered in many directions.

Diffraction: waves striking a very sharp surface,and edges are scattered instead of following the law of specular reflection.

Creeping wave return: this is a form of a traveling wave that doesn’t face surface discontinuous and not reflected by obstacle when traveling along object surface, thus it is able to travel around the object and come back at the radar. Unlike normal traveling wave, creeping wave traveled along surface shadowed from incidence wave (because it has to go around the object). As a result, the amplitude of creeping wave will reduce the further it has to travel because it can’t feed energy from the incident wave in the shadow region. In principle, creeping waves are the result of waves hitting the tangent of a tubular or circular body then get diffracted into the shadow region. It important to note that the creeping wave return effect appears when the wavelength is at least 1/10 of the object’s circumference and the effect is most powerful when wavelength equal to the circumference of the object.

Basic RCS reduction approaches:
1) Shaping

• Orientation and curvature of surfaces
• Alignment of edges
• Shielding of cavities and ducts

2) Materials selection

• Composites and RAM
• Metamaterials and other artificial materials

RCS reduction techniques:​

Surface orientation (to deal with specular return):

• Make sure surface normals do not point in high priority threat directions
• Specular component is frequency independent, but scattering lobe widths decrease with increasing frequency
• Principal planes (i.e., cuts with the highest sidelobes) are perpendicular to edges
  Example above: square versus diamond with the same surface area

Retro-directive Reflectors (to deal with specular and surface wave return):

• Avoid corner reflectors (dihedral and trihedral reflectors)
  No vertical/horizontal tail surfaces on aircraft
• Surface orientation is the most important feature of a stealth aircraft, even without radar absorbing materials, a stealth airframe can achieve much lower RCS compare to conventional aircraft

Discontinuities treatment (to deal with traveling-wave return, diffraction, creeping wave return):
Gaps in conductivity lead to edge diffraction. A surface can look continuous, but there may not be good conduction between the two sides. Once traveling waves hit a discontinuity, they diffract in all directions and once traveling wave travel to the end of a surface, it can come back from the other side.
Example 1: RCS Comparision between continuous wire and wire with a break

Example 2: gaps between wing and aileron

Solution:

• Minimize the number of edges and gaps on the surface.
• The maximum intensity of the diffracted lobe from an edge (in the Keller cone direction) increases with edge length. Serrations are used on unavoidable gaps and discontinuities to break up edges length to reduce lobe intensities

• Trailing edge and serrated panel are designed so that reflection lobes and creeping wave return lobes point to low priority directions.

• Surface wave absorber is used in panel gaps and trailing edges (notice the distinct color)

Leading and trailing edges treatment
When radio waves hit a sharp conductive wedge such as the leading edge or inlet lips of aircraft, they scattered strongly in all directions.

• To reduce this effect, one common method is to make the leading edge of aircraft a soft electromagnetic surface, this is done by stick a high resistive strip on the edge. The strip is designed in such a way that the resistivity will reduce from the maximum electrical resistance at the edge front to near zero at the rear. This allows the surface current to transition slowly rather than abruptly as well as be absorbed. To improve further the edge can be made from radar absorbing material

Furthermore, surface wave can travel along surface of aircraft wing and return back from the other side

• A solution for this is to apply a thin resistance strip on the surface of the wing trailing edge to absorb the energy of the surface wave, thus reduce the magnitude of creeping wave return reflection

The trailing edge and leading edge treatment is most noticeable by the distinct color in the leading edge and trailing edges of stealth aircraft.

Passive Cancellation:

• Approach: add a secondary scatterer and adjust it so its scattered field cancels that of the bare target
• Only effective over a narrow range of angles and frequency bandwidth
• Only practical for canceling low RCS levels
• Examples: parasitic elements and lumped loads

• Passive cancellation structure ( or RAS )

Intake RCS reduction:

• Intake cavity and engine fan blades are a great source of radar reflection. Stealth aircraft intakes are often more sophisticated, for example F-35, F-22, B-2 inlet being a serpentine duct rather than a direct, more conventional intake they use complex techniques to reduce reflection over a range of frequencies. The intake is designed to counter radar threats at three wavelengths loosely termed long ( 30 cm), medium ( 10–20 cm) and short ( 3 cm), equating to 1 GHz (long-range surveillance radar), 1.5–3 GHz (AWACS radar) and 10 GHz (fighter radar) respectively.

• At long wavelengths (30 cm ) the stealth fighter inlet ducts behave as follow:

• At medium wavelengths (10-20 cm ) the stealth fighter inlet ducts behave as follow:

• At medium-short wavelength, stealth fighter inlet ducts behave as follow:

An alternative way to reduce radar reflection from the inlet cavity and the engine fan blades is by using an engine fan blocker, such solution is being used on F-18E/F, F-117, Su-57 and X-32.

The main advantage of inlet blocker over serpentine duct is lighter weight. However, the structure of serpentine duct can support thicker layer of radar absorbing material thus giving better absorbing capability. Moreover, serpentine ducts will always make radio wave bounce multiple times inside before reflected back, thus even if the RAM coating of the duct has modest absorbing capability of -5 dB to -15 dB, the total accumulated absorbing power can reach 60 dB

Radome RCS reduction:

• Antenna mode reflections. The antenna mode reflections mimic the antenna main beam and sidelobes.

• Random scattering. This is caused if the antenna characteristics are not uniform across the antenna.

• Radar antenna edge diffraction. Mismatches of impedances at the perimeter of the antenna can cause reflections called edge diffraction. In effect the outer perimeter of the antenna acts as a loop and reflections tend to be abeam of the antenna rather than fore and aft.

Canopy RCS reduction:

• Radar waves can go through the canopy and reflected off objects inside the cockpit, thus increase RCS significantly. Solution: coat the inner of the canopy with a thin layer of gold to prevent radar waves from entering the cockpit, the outer cockpit is coated with transparent radar absorbing materials.
• Example :

Weapons RCS reduction:
Missiles, bombs are all great contributors to radar reflection due to the perpendicular angle of their wings, fins, Pylons is another great contributors because of the corner they make with aircraft wing. As a result, stealth aircraft often carry weapons internally, the added benefits is the reduction in drag. However, due to the limitation in size when using internal configuration, a stealth fighter cannot carry as many weapons as a normal fighter. In some case, external pylons of stealth aircraft are designed to have a unique shape so as to reduce their signature.
Example: radar scattering characteristic of short range heat seeking missiles

Example: F-35 weapons bay

Example: Low observable pylon vs legacy pylon

 

[DBdev]

...
Skin RCS reduction:
No matter what shape they have, airframe will always reflect radar waves.The only different that shaping will make is the directions that the airframe will reflect radar.While this may be enough in most situation.The adversary may consist of very complex radar network that can illuminate stealth platform from different angles, so along with unique shaping to redirect radar wave from the original source, stealth aircraft often have radar absorbing paint or use radar absorbing material (RAM ).One might be very tempted to construct stealth aircraft skin from such “radio transparent” materials, but radar would then reflect off objects beneath the surface such as sensors, fuel, metallic airframe and engine parts and the pilot.The result may be a RCS value even higher than if the skin was radar reflective. As a result, in practice, the bottom layer of a stealth skin is a highly conductive material, such as metal, which strongly reflects radar waves before they reach the complex reflecting environment below.
Electromagnetic wave consist of two components, the magnetic field and electric field which perpendicular to each other and perpendicular to the direction of travel of the wave

The ability of a substance to absorb electromagnetic (EM) waves depends on two material properties called permittivity (using the symbol ε ) and permeability (using the symbol μ). Permittivity represent the material ability to store electrical energy while permeability represent the material ability to store magnetic energy

The source of both is the existence of electric or magnetic dipoles at the atomic, molecular or crystal lattice level. When an EM wave passes through the material, these dipoles orient opposite to the field’s direction. In some materials, the dipoles effortlessly return to neutral after the EM field returns to zero. In other materials, the dipoles are “sticky” and require energy to orient them or return them to neutral. That additional energy is lost and the material’s permittivity or permeability is said to have a loss component. As its permittivity, permeability (loss components) increase, a material can absorb more EM energy because EM wavelengths shrink as these values rise. Normally the permittivity ε and permeability of a material is compared with the free space permittivity (using the symbol ε0 )and free space permeability (using the symbol μ0) to get the relative permittivity (using the symbol εr) and relative permeability (using the symbol μr ).

Common sense would let us to believe that calculating the relative permeability and permittivity of material is redundant when the exact value is already known, but that way of thinking is incorrect. The reason is that, for radio wave to be absorbed by the radar absorbing material, it must entered the material first. When waves reach a boundary between two mediums (from free space to material), energy can be reflected rather than admitted. The propagation of radio wave through a material depends on two quality the refractive index n and the characteristic impendence Z .

Whereas the amount of specular reflection and transmission of electromagnetic wave at a boundary is expressed in the form of Reflection coefficient (using the symbol R or Γ) and Transmission coefficient (using the symbol T).
When the electric field polarization is perpendicular to the plane of incident the reflection coefficient at the boundary will be:

When the electric field polarization is parallel to the plane of incident the reflection coefficient at the boundary will be:

Z as mentioned earlier represent the medium intrinsic impendence (in most case the first medium is the free space while the second medium is the material) and k represent the wave number which is given by:

From the equation we can see that reflection coefficient varied with incident angle, medium impendence and polarization, it is important to note that, for parallel polarization, the reflection coefficient will become near 0 at about 65 degrees, this also known as the Brewster angle

When the incident angle is perpendicular to the material boundary, the reflection coefficient of horizontal polarization and vertical polarization will be the same and will equal to:

If the second medium is a very good conductor (such as metal) so that the impendence is very low => Z2=0, then R = -1, that mean the wave get reflected entirely and get a 180 degrees phase change. On the other hand, if the second medium has very high impendence then R = +1, that mean all the wave energy still get reflected but without a phase change. However, if the second medium has the same impendence as the first medium Z1 = Z2 then R =0, that mean all the radio wave transmitted from the first medium to the second medium and no energy is reflected. To sum up, the greater the impedance change, the more energy is reflected before it can be absorbed. So RAM design must balance absorptivity with surface reflectivity to maximize absorption. The equation tell us that, if the goal is to get small reflection, the wave should never see a big change in impendence, instead a small gradual change in impendence is desired.
In general, RAMs are composites made up of a matrix material and a filler. The matrix is a low-loss dielectric material with appreciable permittivity and negligible permeability. They are effectively “transparent” to EM waves and are usually chosen for their physical properties. Typically, they are insulating polymers like plastic, glass, resin, polyurethane and rubber. Ceramics have higher permeabilities and heat tolerance. Foams and honeycombs have especially low permittivity—electrical energy storage—because they contain a lot of air. The RAM filler, on the other hand, is typically particles composed of or coated with a lossy material. Carbon is the material of choice for dielectric absorption because electrical lossiness is proportional to conductivity and carbon’s conductivity is below metals but above insulators. Magnetic absorbers, which have some permittivity but far greater permeability—magnetic energy storage—are typically carbonyl iron (a pure powdered form of the metal) or iron oxides, also called ferrites. These materials can be impregnated into rubber or dissolved into a paint and ferrites are often sintered into tiles.
A material’s EM properties also varied significantly with frequency. At higher radar bands, no magnetic materials have permittivity and permeability in a ratio close to that of air, so high surface reflection is inevitable. However, if the material is a quarter-wavelength deep, reflection from the metal backing partially cancels the surface reflection. Radar absorbing materials operate via phase cancellation like this is often called magnetic absorber. Because of the high permeability of magnetic RAM, the depth required is small. Absorption performance of 20 dB (99%) is achieved by commercially available “resonant absorbers” with resonant frequencies of 1-18 GHz and thicknesses of 0.04-0.2 in. The main disadvantages of such absorber is very narrow absorbing bandwidth, however, with significant absorption extending perhaps 15% from the resonance frequency.

Given that magnetic absorbing material has limited bandwidth, as well as high weight and cost, dielectric absorbers are often preferred for wideband absorption at high frequencies. Since dielectrics have no magnetic properties, their impedances never match air, but by using layers of materials—each with an increasing concentration of carbon particles—permittivity, conductivity and dielectric losses all gradually increase while impedance gradually decreases. Layers can also be adjusted to maximize cancellations. These graded dielectric absorbers can reduce reflection by 20 dB, and their bandwidth easily covers higher frequencies. High levels of reflection loss, in many cases better than 20dB, can be achieved in materials which are <1/3 wavelength thick. One of the most common type of graded dielectric absorber is the reticulated foam absorbing material.

Another approach is to use a physical gradient. These “geometric transition” absorbers use pointed objects of homogeneous material oriented perpendicular to waves. At high frequencies, waves bounce among these structures, losing energy with each strike. If the wavelength is large relative to the structure, the waves act as though encountering a gradual change in material properties rather than a geometric shape. Absorbers of this type can reduce reflection by up to 60 dB, but require structures very big structures and high weight so their only application is the pyramidal absorbers that line anechoic chambers used for RCS testing.

It is a common misconception that radar absorbing material is only effective around X band (8-12 Ghz) or that RAM work at low frequencies will always need to be thick or heavy. In fact, some magnetic materials actually become more effective at lower frequencies because their energy storage (permeability) increases. At frequencies of 30-1,000 MHz, certain ferrites exhibit extreme wave compression and impedance close to air. Commercial ferrite tiles can achieve over 20 dB reduction in VHF band and 10 dB reduction through UHF, with a thickness of only 0.25 in. and a weight of 7 lb./ft.2.
So far, what has been discussed is reducing specular reflections—those that bounce off an object like light off a mirror—but RAM is also particularly effective at reducing surface waves. These are the waves emitted by currents induced in a conductive surface when struck by radar. As they move along the surface they emit traveling waves, usually at angles close to grazing, and when they encounter discontinuities—an airframe edge, a gap or step in the surface or a change in material—they emit edge waves, concentrated closer to the specular reflection. Surface currents travel along a material’s length rather than through its thickness, and the RAM acts as a waveguide, trapping the currents and absorbing them. Magnetic RAM can suppress surface currents well in a thickness of only 0.03 in. There are ways to combine techniques. Layered magnetic materials can reduce RCS by 10 dB from 2-20 GHz with 0.3 in. of depth. Hybrid RAMs can be created with a front layer of graded dielectric and a back layer of magnetic material to attenuate radar reflections from VHF to Ku-band (30 MHz-18 Ghz).

One disadvantage that all radar absorbing materials mentioned above share is that they add weight and volume without adding structural integrity. The radar absorbing materials developed for F-117, B-2 F-22 can kept their RCS small, but their maintenance burdens proved too heavy. Their durability disappointed, necessitating frequent replacements that ballooned support costs and time while restricting aircraft availability. RAM fillers tend to be spherical, a few to tens of micrometers in size and densely packed, which is good for absorptive qualities but bad for durability. Bonding them to aircraft surfaces also proved troublesome. As a result, from the beginning of the F-35 program, Lockheed’s goal was to achieve acceptable stealth while reducing maintenance needs. In May of 2010, Tom Burbage, then executive vice president for the F-35 program, disclosed the incorporation of “fiber mat” technology, describing it as the biggest technical breakthrough they’ve had on F-35 program. The fiber mat would replace many RAM appliques by being cured into the composite skin, making it durable. Burbage further specified the mat featured a “non-directional weave” which would ensure EM properties do not vary with angle. Baked into the skin, this layer could vary in thickness as necessary. Lockheed declined to provide further details, citing classification. Without further evidence, fiber mat would imply the use of fibers, rather than particles, which would make for stronger surfaces and the word “conductive” points to carbon-based RAM. That wasn’t the first time it is hinted that F-35 has a unique kind of RAM. One month before Burbage’s disclosure, Lockheed filed a patent claiming the first method of producing a durable RAM panel. The patent details a method for growing carbon nanotubes (CNT) on any kind of fiber—glass, carbon, ceramic or metal with unprecedented precision in control of length, density, a number of walls, connectivity, and even orientation. The CNT-infused fibers can absorb or reflect radar, and connectivity among the CNTs provides pathways for induced currents. Moreover, the CNTs can be impregnated with iron or ferrite nanoparticles. Fibers can have differing CNT densities along their lengths and homogenous fibers can be layered or mixed. The embodiments described include front layers with impedance matching air, use of quarter-wavelength depths for cancellation, stepped or continuous CNT-density gradients and continuously varying densities at specific depths for broadband absorption. The fibers can be disposed with “random orientation” in materials including “a woven fabric, a non-woven fiber mat and a fiber ply.”.The patent claims composites with CNT-infused fibers are capable of absorbing EM waves from 0.1 MHz to 60 GHz with particular effectiveness in L- through K-band.That is a bandwidth unheard of commercial radar absorbing material before. The patent does not quantify the absorptivity, but does say the panels would be “nearly a black body across . . . various radar bands.” Also, interestingly, a layer can be composed so an attached computer can read the induced currents in the fibers, making the layer a radar receiver.While the patent mentions stealth aircraft, it does not mention the F-35 specifically, and the manufacturing readiness level of the material at the time it was granted is not known. But the proximity in timing and technology of the filing to the “fiber mat” disclosure is hard to ignore.

​

Results:​

With careful designs stealth aircraft can have RCS equal a fraction of conventional aircraft.

Radar scattering chart of stealth F-35 and some conventional aircraft
Example 1: Simulated radar scattering chart of F-35

Example 2: Simulated radar scattering characteristic of AV-8B Harrier (conventional aircraft)

Example 3: Simulated radar scattering characteristic of AH-64 Apache

Example 4 Simulated radar scattering characteristic of F-15 at 1 Ghz

Example 5: radar scattering chart of Mig-21

When looking at radar scattering graph of aircraft, one common mistake is to assume that aircraft will be detected by radar at a significantly longer distance from the side aspects or tail aspects, because radar scattering charts often show much higher radar cross section values for beam aspect and tail aspect compared to the frontal aspect of aircraft. That misconception raised from the fact that most enthusiasts treat radar return as equally valuable regardless of aspect angle. That is not the case, however, and here is why:
Most radar energy is transmitted and received via a main lobe aligned with the antenna’s boresight, but smaller amounts enter through sidelobes that point in almost all directions.

Radar performance degrades at viewing angles where a target must be distinguished from background clutter. Clutter can enter the receiver via the sidelobes, and the processor has no way of knowing the return did not come from the main lobe. Such returns can mask that of the target. Modern radars mitigate this phenomenon with Doppler processing. A pulse-Doppler radar records the time of arrival of a return and also compares its phase with that of the transmitted wave. The difference between the two reveals the target’s radial velocity. The computer creates a 2D range/velocity matrix of all returns, which puts approaching targets in cells with no stationary ground clutter. This is why airborne radars exhibit their best detection ranges against approaching targets. But if the target is being chased, its radial velocity will match some of the ground clutter, and it will be harder to detect.

For example, the Sukhoi Su-35’s Irbis-E radar in high-power, narrow-beam search can detect a 3-m2 (32-ft.2) target at 400 km (250 mi.) from the front but only 150 km from behind, and these ranges drop by half in normal search mode. The hardest airborne targets to see are those moving perpendicular to the radar, because their Doppler profile matches the ground directly below the aircraft. For ground-based radars, the same principles apply, but the antenna is stationary. Fleeing targets stand out as much as approaching aircraft. But ground-based radars are especially challenged in detecting targets moving perpendicularly, because their Doppler profile matches the stationary clutter all around. A tactic used by fighter pilots against ground radars, called “notching,” is to turn perpendicular to the radar, placing the aircraft in the “Doppler notch” in which the radar suffers significantly reduced range.

Benefit of low RCS​

• Reduce radar detection range:

One easy to see benefit of RCS reduction is the deduce in enemy detection range ,thus giving pilots more times to react to the threat or getting into weapon engagement zone
Example : radar detection range between conventional and stealth aircraft.

• Improve jamming effectiveness:

It is a common misconception that stealth technology is short live and as radar get more powerful, soon, they will be able to out range weapon engagement envelop , thus renders all money spend on RCS reduction a waste. This impression is inaccurate because any technology that can increase a radar peak power or gain will also benefit a jammers in the same ways. And stealth has a synergy relationship with jamming .
Another common opinion is that the gap in RCS can easily be close by using a more powerful jammer .This is also inaccurate because RCS directly proportional to the power required to jam a radar at a certain distance.Which mean when RCS is reduced to 1/100th the original value, the required jamming power is also reduced to 1/100th the original value to achieve the same effect.In others words, if a stealth aircraft need a 10 kW jammer , a conventional asset will need jammer with power of 10Mw or more
If the jamming power is keeping the same then burn-through range is reduced by 10 times, which mean stealth assets( RCS =0.001m2 ) can get 10 times closer the threat compared to conventional aircraft ( RCS=0.1m2).In other words ,even if adversary radar can see through jamming of conventional assets from 400 km aways, a stealth asset can still get within 40 km of such radar using exactly same jamming system

• Example : burn-through distance of F-35 compared to Rafale with same jamming assets, same threat radar ( image not to scale )

• Burn-through Range is the radar to target range where the target return signal can first be distinguished from the Jamming signal ( rendering jamming ineffective).

​

 

[DBdev]

Low Observability In Infrared Spectrum.​

All objects with a temperature above absolute zero emit heat energy in the form of radiation. Usually this radiation is invisible to the human eye because it radiates at infrared wavelengths, but it can be detected by electronic devices designed for such a purpose , these devices are called infrared (IR )sensor. With the improvement in optics and processing power of CPU , nowadays infrared sensors can see much further than human eyes .As a result, all stealth aircraft use one way or another to suppress their infrared signature.
Infrared signature suppression has two objectives:

• Reduce the range at which an IR missile or sensor can detect and track the
  aircraft.
• Increase the effectiveness of countermeasure systems and devices.

It is important to note that infrared sensor detecting assets by comparing the contrast of such assets infrared signature with background radiation , thus the effectiveness of infrared suppression is affected significantly by the temperature of background. In general clear sky is the worst background due to their low temperature while cloud and/or hot land surface make the best backgrounds for stealth aircraft to hide from adversary infrared sensor.( for the same reason, aircraft fly higher are much easier to detect by IRST )
Example: infrared photo of C-130 in 2 different background

Like all electromagnetic radiation, IR interacts with matter in a variety of ways:

• Reflects—A wave is reflected from a surface. The angle of reflection equals the
  angle of incidence.
• Refracts—The direction of a wave bends when passing between two transparent
  media with different propagation speeds (Snell’s law).
• Scatters—Scattering occurs upon interaction with particles whose size
  approaches the length of the wave (why the sky is blue).
• Diffracts—This interaction occurs around the edges of an obstruction.
• Interferes–This interaction occurs in both a constructive and destructive manner.
• Absorbs—When absorbed by matter, radiation is converted into another form of
  energy. The most common conversion is to heat.
• Emits—Radiation is emitted from matter by conversion from another form of
  energy.
• Transmits—IR propagates through a transparent medium (or vacuum).
• Polarizes—An electric field is partially polarized by reflection from dielectric

Infrared wavelength range from 0.7-14 µm , divided to short ( 0.7-1.5 µm ) medium (1.5-6 µm ) and long infrared wave (7-14 µm ), with different characteristics they all have different military application.

Infrared signature of aircraft:
An aircraft’s infrared signature is a complex mixture of emissions and reflections from different materials with different emissivity and different areas. Signature is complex in its spectral distribution, in its contrast against background, and in its dependence on conditions. Aspect angle, altitude, airspeed, ambient air temperature, power setting, and
sun angle are only a partial list of conditions affecting signature values.

High infrared signature component of fighter aircraft:

• Engine “hot parts,” which usually consist of the aft turbine face, engine center
  body, and interior nozzle sidewalls.
• Engine exhaust plumes, which are emissions from the combustion constituents
  of CO2 and water vapor.
• Airframe, which includes all of the external surfaces of the wings, fuselage,
  canopy, etc. Airframe signature includes solar and terrestrial reflections, Mach shock wave in addition to direct emissions.

Similar to radar cross section, IR signature of an aircraft is very aspect angle dependence thus lead to very different detection range, For example : OLS-35 ( IRST system on Su-35 ) can easily detect an aircraft from 90 km aways from tail aspect, however in head on aspect the detection range reduce significantly down to 30 km

Airframe Aerodynamic Heating:
The temperature of the airframe is warmer than ambient by the amount of aerodynamic heating. A good estimate of airframe temperature is given by the formula for the recovery temperature given below. Note that the temperature units are Kelvin.The temperature of the skin of an aircraft stabilizes at the ambient air temperature plus aerodynamic heating. Aero heating increases as the square of Mach number. The formula below gives a good approximation:

Aircraft moving at supersonic speed also produces compressed air ( Mach cone ) which not only increase the airframe temperature significantly but also increase frontal area present to the infrared sensor.

Aircraft moving at Mach 1 can be detected by IR sensor at twice the distance compare to aircraft moving at Mach 0.8

• As shown in table above flying supersonic can increase aircraft infrared signature significantly, so the most simple solution is to stay subsonic, the trade off of such decision is smaller weapon engagement envelope, longer reaction time for adversary , this solution works well in design that high speed is not a requirement such as F-117 , B-2 .
• Another solution is to use fuel as heat sink , most modern stealth aircraft have internal fuel tanks distributed evenly through out the airframe, the fuel being use all the time thus they can transfer the built up heat away from the aircraft. Fuel can also be used to reduce heat generated by electronic equipment, avionics systems like radar and jammer can generate very high amount of heat. Example : fuel tank contribution of conventional aircraft ( on the left) vs stealth aircraft ( on the right)

• Modern stealth aircraft such as F-22 and F-35 are coated with IR-suppressive skin coatings, the exact composition of the coatings is unknow but in general, they have very low emissivity. Emissivity is of the surface of a material is its effectiveness in emitting energy as infrared radiation, coating with a lower emissivity mean the aircraft emit a lower level of IR radiation at any given temperature.

• For example it is reported that the Top coat on F-22 , F-35 can reduce their skin infrared signature in long and infrared wavelength (8–12 microns) by more than half.

• Example of IR suppression coating with low emissivity

• Some type of IR suppression coating also has the ability to shift the the infrared radiation to non-atmostpheric window region, making the IR radiation of aircraft better absorbed by the atmosphere. The main advantage of this IR surpression coating is that it can help lower the inner temperature of the coated object, since the broad-band coating acts as a thermal insulator, the temperature of the underlying object is increased to a much greater extent.

Atmospheric window:

​

Heating of engine fan blades:​

Because all jet engines have very significant rotational speed, even the first stage of jet engine will be heated up to temperature much higher than the airframe skin temperature. As can be seen from photos below, when aircraft use a straight inlet such as on Mig-29/35, Mig-35, Su-27/30/35, F-15, F-14 or exposed nacelle such as on B-52, C-17 the amplitude of infrared radiation from engine fan stage is significantly higher than from fuselage.

The simple solution to this issue is to either use an S-duct inlet such as on F-35, F-22, Rafale or a physical blocker such as on F-18E/F to block the direct line of sight to the engine fan stage. As can be seen from the photo below, the head on infrared signature is significantly lower on fighter with an S-duct​

Internal Equipment Heating:​

Electronics equipment such as radars and jammers generate significant amount of heat, the more sophisticated and powerful the equipment is the more heat they will create , cooling are required not only to reduce enemy’s infrared sensor detection range but also to prevent the equipment from being overheat and shutdown
aircraft that lack significant cooling features for electronics often have higher body temperature , thus easier to detect. Lacking cooling also limit the output of their equipment such as radar
For example:
picture of F-16 in mid-infrared wavelength

• Solution : As mentioned earlier , avionics can be cooled using fuels , furthermore aircraft can use open vents, thus atmosphere air can act as heat exchanger with the fuel which got heated by avionics

For example: F-35 has a scoop located on the top of the right wing-glove to provide air to the fuel/air heat exchanger. A deployable scoop is located on the left-aft fuselage to provide air to the IPP and to the avionics

Some stealth fighter also use engines bypass air as heat exchanger

Airframe heating due to engines:​

With the core temperature of several hundreds degrees of modern jet engines , without appropriate measuares, they can increase aircraft body temperature dramatically
For example : picture of Typhoon in mid-infrared wavelength

​

Solution :​

• Airframe heating due to jet engines can be reduced by extensive use of cooling vents, the cold air at high altitude provides an isolation layer between the engine and the airframe

For example: the F-35 has two scoops located in the wing/fuselage to provide nacelle bay ventilation

​

• Moreover, the view to the high pressure turbine stage can be blocked by cooled blocker, thus further reduce the Infrared signature from the rear

​

Fun fact:​

For some fighter aircraft, as their operating envelope include very high-speed flight (above Mach 2) so their engine nacelle can be heated up quickly to very high temperature. Thus, to prevent structure failure, their engine bay is made from titanium which has much higher melting temperature than aluminum. However, most paints have bad adhesion with titanium, especially in high-temperature condition. Furthermore, titanium resists corrosion very well, so no corrosion protection, primer, and paint is required. Thus, on some fighters, the engines area are often left bare without paint and instead what we often see is a silver looking area.

Temperature from the exhaust fumes:​

The biggest contributors of signature in mid-infrared wavelength on a jet aircraft is their exhaust fumes, reduction of exhaust temperature as little as 100 degrees can reduce aircraft infrared emission by more than haft

One very common misconception about jet engine and infrared signature is : an engine with higher thrust will always have a higher infrared signature, however that is an inaccurate assumption. It is entirely possible to have higher IR signature with lower thrust value.

To understand why lets have a look at the design of jet engines: below is a diagram of a normal turbofan engine commonly used in all aircraft flying nowadays:
The 2 main components that responsible for thrust are the Fan and engine core.

The compressor , turbine and combustor ( also know as the engine core ) move air at very high speed hence, they are less dependence on air density and aircraft velocity. On the other hand, the fan stage moves air at much slower rate ,which is much more fuel efficient and also mix the exhaust plume with cold air, thus reduce the temperature of the plume. Not all air suck in by the first stage fan will go through the engine’s core (compressor , turbine and combustor ) some will pass through the outer duct. The air that passed through the outer duct is called bypass air. To get to a certain thrust level, an engine can either have very big fan and small core ( good for combat radius and thermal signature ) or very big core and relatively small fan (good for speed and high altitude performance )

• Due to reasons stated above one of the solution for exhaust temperature reduction is to use engines with higher bypass ratio , the trade off of such design choice is the aircraft will not be able to fly very high or very fast. ( For example F-135 have much higher bypass ratio compared to F-119 , EJ200 , Snecma M88 , R-15 ,F404 )

• A common misconception is that engine turbine inlet temperature is also proportional to exhaust temperature, that is wrong, however, in reality, turbine inlet temperature does not reflect the engine case temperature or even the exhaust plume temperature. It simply means that the gasses entering the turbine is at a higher energy state and the engine will yield more gross energy per drop of fuel or air entering the combustor(s). That energy however, is extracted to do work first by the high-pressure turbine, then by the low-pressure turbine before going out the tail pipe at a given velocity. The final temperature depends on how much energy is extracted to drive the compressors and the fan, and how much bypass air is mixed into the exhaust. The F-35 has twice as many low pressure turbine stages which in theory will extract more energy. It also has a bigger and higher pressure ratio fan which adds energy to the exhaust as well as introduce relatively cold air into the mix. The exhaust plume temperature and engine case temperature can never be derived from the turbine inlet temperature alone.
• It is also important to remember that unlike a rocket, jet engines are air-breathing engine, which means their performance depending a lot on air density, the thinner the air the less thrust they will be able to generate, so aircraft thrust will reduce as they go higher. For example: a jet engine that can generate more than 190kN at sea level can be struggled to push out 10kN at 40-50K feet. On the other hand, a rocket engine can generate the same amount of thrust regardless of altitude

Using high bypass engine is not the only method of reducing exhaust temperature, however. Modern stealth aircraft also use exotic engines nozzles that either long and flat or with the serrated pattern so that exhaust fumes become unstable and mixed quicker with cool ambient air .As a result, the heat will be dissipated rapidly. In some aircraft, there are spacing between nozzle plates linked to cooling vents to help reduce the temperature of outer nozzle surface
Example :

1. F-117 and B-2 have flat exhaust nozzles

2.F-35 with serrated nozzle and cooling vents on nozzles

3. Thermal image of exhaust fumes. First nozzle uses conventional circular design. Second nozzle uses serrated design. It is estimated that fumes length is reduced by around 40%

4. Test data comparison between a conventional exhaust nozzle and a serrated nozzle:

• Stealth aircraft are also designed so that from frontal, the view of their engines nozzles will be blocked by their vertical and horizontal stabilizer

Example: Aircraft ( on the left ) have exposed engine nozzles while aircraft ( on the right ) have masked nozzles

Source: https://basicsaboutaerodynamicsandavionics.wordpress.com/2016/03/04/stealth-techniques-and-benefits/

 

[DBdev]

F-22 Raptor Stealth​

A quick look at the F-22 reveals an adherence to fundamental shaping principles of a stealthy design. 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 canopy seam, bay doors, and other surface interfaces are sawtoothed. The vertical tails are canted. The engine face is deeply hidden by a serpentine inlet duct and weapons are carried internally.
Advances in low-observable technologies provide significantly improved survivability and lethality against air-to-air and surface-to-air threats. The F-22's combination of reduced observability and supercruise accentuates the advantage of surprise in a tactical environment. The most publicized and most revolutionary technology for aircraft is stealth. Stealth makes an object become very difficult to detect by sensors such as radar, heat seekers (infrared), sound detectors and even the human eye. While not invisible, the F-22's radar cross section is comparable to the radar cross sections of birds and bees. Compared to other current fighters, the F-22 is much more difficult to detect.
To make a stealthy aircraft, designers had to consider five key ingredients: reducing the imprint on radar screens, muffling noise, turning down the heat of its infrared picture, stifling radio transmissions and making the plane less visible. 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 canopy seam, bay doors and other surface interfaces are saw-toothed. The vertical tails are canted. The engine face is deeply hidden by a serpentine inlet duct and weapons are carried internally.
The F-22 represents a significant design evolution beyond the highly successful F-117A Nighthawk stealth fighter, with performance not achievable by today's front-line fighters. Low observable, or stealth, technology has advanced to the point where conventional aerodynamic configurations can be made incorporating low observability without compromising aerodynamic performance or increasing costs significantly. Design development risk was greatly reduced by the performance demonstrated in the dem/val program where angle of attack attitudes up to 60 degrees were flown. The validity of the low observability features of the F-22's design were confirmed by full-scale pole model testing.
Low observability is achieved by a range of measures. The F-22 employs planform shaping and faceting with blended facet boundaries, the latter a necessary concession to high performance aerodynamics. This is apparent in the shape of the nose, the fuselage sides about the inlets and engines, and the upper forward fuselage. Lockheed/B/GD used serrated edges extensively, as with the F-117A, to control the returns from panel boundaries, this is very visible on the undercarriage and weapon bay doors. The planform results in a multiple lobe design, as the boundaries of the major surfaces are not parallel with respect to each other. Planform return lobe structure is defined by the radiation pattern lobes resulting from surface wave reflections which occur at the leading and trailing edges of the airframe's major surfaces. The objective of lobing is to concentrate this unavoidable radar return into specific directions so as to minimise frontal/aft/beam aspect return and maximise scintillation in the direction of the lobe. Scintillation is a measure of how rapidly the size of the return varies with 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.
Radar absorbant materials, or RAM is applied sparingly on the F-22 airframe as opposed to the entire airframe on the F-117. This is because designers have incorporated curves on crucial surfaces and edges, which lessens the need for RAM. For example, new ceramic-matrix RAM is utilized on the engine exhaust nozzles to reduce radar and IR signatures, and a greater amount of wide-band structural RAM is used on the wing edges. The interesting shape of the radome on the F-22 reflects radar signals at all frequencies except the precise wavelengths emitted from the F-22. This can be attributed to the radome's low bandpass type.
To apply the complex system of paints and coatings necessary to meet the F-22's stringent radar cross section (RCS) requirements takes not only state-of-the-art equipment and hands-on technicians, but also a wide-ranging support system. A new type of paint, or topcoat, increases the F-22 Raptor's stealthiness by reducing its vulnerability to infrared threats. To meet F-22 requirements, Boeing developed the topcoat to protect the aircraft against a broad range of wavelengths. The new paint replaces conventional topcoats, performing all the required environmentally protective functions while also reducing the aircraft's vulnerability to detection. The topcoat does not add to the F-22's weight, and provides performance enhancement at a very modest cost. It is applied in a two-tone camouflage design, patterned after the F-15 "Mod-Eagle" paint scheme. Development of the new topcoat began during the early stages of the F-22 program. Since that time, a small team at Boeing in Seattle has worked to refine the paint and improve its application characteristics in a production-level environment. Technicians at Lockheed Martin painted the first few aircraft by hand, however, robotic application is planned for future Raptors, including Raptor 04, which is scheduled to fly this summer. The topcoat application for each Raptor is expected to take one to two days.
Another important feature of the F-22's stealth characteristics is the new low-RCS air data system. This system uses four ports distributed along the forward fuselage to reduce emission control (EMCON). In addition, the F-22 is the first fighter aircraft to include a completely frameless canopy. This eliminates RCS reflections from the windshield arc without compromising structural integrity.

Fundamentals of Stealth Design​

Design for low observability, and specifically for low radar cross section (RCS), began almost as soon as radar was invented. The predominantly wooden deHavilland Mosquito was one of the first aircraft to be designed with this capability in mind.
Against World War II radar systems, that approach was fairly successful, but it would not be appropriate today. First, wood and, by extension, composite materials, are not transparent to radar, although they may be less reflective than metal; and second, the degree to which they are transparent merely amplifies the components that are normally hidden by the outer skin. These include engines, fuel, avionics packages, electrical and hydraulic circuits, and people.
In the late 1950s, radar absorbing materials were incorporated into the design of otherwise conventionally designed aircraft. These materials had two purposes: to reduce the aircraft cross section against specific threats, and to isolate multiple antennas on aircraft to prevent cross talk. The Lockheed U-2 reconnaissance airplane is an example in this category.
By the 1960s, sufficient analytical knowledge had disseminated into the design community that the gross effects of different shapes and components could be assessed. It was quickly realized that 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 tail on viewing directions, and the vertical tail dominates the side on signature.
Airplanes could now be designed with appropriate shaping and materials to reduce their radar cross sections, but as good numerical design procedures were not available, it was unlikely that a completely balanced design would result In other words, there was always likely to be a component that dominated the return in a particular direction. This was the era of the Lockheed SR-71 'Blackbird'.
Ten years later, numerical methods were developed that allowed a quantitative assessment of contributions from different parts of a body. It was thus possible to design an aircraft with a balanced radar cross section and to minimize the return from dominant scatterers. This approach led to the design of the Lockheed F-117A and Northrop B-2 stealth aircraft.
Since then there has been continuous improvement in both analytical and experimental methods, particularly with respect to integration of shaping and materials. At the same time, the counter stealth faction is developing an increasing understanding of its requirements, forcing the stealth community into another round of improvements. The message is, that with all the dramatic improvements of the last two decades, there is little evidence of leveling off in capability.

Radar Cross Section Fundamentals​

There are two basic approaches to passive radar cross section reduction: shaping to minimize backscatter, and coating for energy absorption and cancellation. Both of these approaches have to be used coherently in aircraft design to achieve the required low observable levels over the appropriate frequency range in the electromagnetic spectrum.

Shaping​

There is a tremendous advantage to positioning surfaces so that the radar wave strikes them at close to tangential angles and far from right angles to edges.
To a first approximation, when the diameter of a sphere is significantly larger than the radar wavelength, its radar cross section is equal to its geometric frontal area.
The return of a one-square-meter sphere is compared to that from a one-meter-square plate at different look angles. One case to consider is a rotation of the plate from normal incidence to a shallow angle, with the radar beam at right angles to a pair of edges. The other is with the radar beam at 45 degrees to the edges. The frequency is selected so that the wavelength is about 1/10 of the length of the plate, in this case very typical of acquisition radars on surface to air missile systems.
At normal incidence, the flat plate acts like a mirror, and its return is 30 decibels (dB) above (or 1,000 times) the return from the sphere. If we now rotate the plate about one edge so that the edge is always normal to the incoming wave, we find that the cross section drops by a factor of 1,000, equal to that of the sphere, when the look angle reaches 30 degrees off normal to the plate.
As the angle is increased, the locus of maxima falls by about another factor Of 50, for a total change of 50,000 from the normal look angle.
Now if you go back to the normal incidence case and rotate the plate about a diagonal relative to the incoming wave, there is a remarkable difference. In this case, the cross section drops by 30 dB when the plate is only eight degrees off normal, and drops another 40 dB by the time the plate is at a shallow angle to the incoming radar beam. This is a total change in radar cross section of 10,000,000!
From this, it would seem that it is fairly easy to decrease the radar cross section substantially by merely avoiding obviously high-return shapes and attitude angles.
However, multiple-reflection cases have not yet been looked at, which change the situation considerably. It is fairly obvious that energy aimed into a long, narrow, closed cavity, which is a perfect reflector internally, would bounce back in the general direction of its source. Furthermore, the shape of the cavity downstream of the entrance clearly does not influence this conclusion.
However, the energy reflected from a straight duct would be reflected in one or two bounces, while that from a curved duct would require four or five bounces. It can be imagined that with a little skill, the number of bounces can be increased significantly without sacrificing aerodynamic performance. For example, a cavity might be designed with a high-cross-sectional aspect ratio to maximize the length-to-height ratio. If we can attenuate the signal to some extent with each bounce, then clearly there is a significant advantage to a multi-bounce design. The SR-71 inlet follows these design practices.
However, there is a little more to the story than just the so called ray tracing approach.
When energy strikes a plate that is smooth compared to wavelength, it does not reflect totally in the optical approximation sense, i.e., the energy is not confined to a reflected wave at a complementary angle to the incoming wave.
The radiated energy, in fact, takes a pattern like a typical reflected wave structure. The width of the main forward scattered spike is proportional to the ratio of the wavelength to the dimension of the reradiating surface, as are the magnitudes of the secondary and tertiary spikes. The classical optical approximation applies when this ratio approaches zero. Thus, the backscatter - the energy radiated directly back to the transmitter increases as the wavelength goes up, or the frequency decreases.
When designing a cavity for minimum return, it is important to balance the forward scatter associated with ray tracing with the backscatter from interactions with the first surfaces. Clearly, an accurate calculation of the total energy returned to the transmitter is very complicated, and generally has to be done on a supercomputer.

Coatings and Absorbers​

It is fairly clear that although surface alignment is very important for external surfaces and inlet and exhaust edges, the return from the inside of a cavity is heavily dependent on attenuating materials. It is noted that the radar-frequency range of interest covers between two and three orders of magnitude. Permeability and dielectric constant are two properties that are closely associated with the effectivity of an attenuating material. They both vary considerably with frequency in different ways for different materials. Also, for a coating to be effective, it should have a thickness that is close to a quarter wavelength at the frequency of interest.

High Temperature Coatings​

Reduction of radar cross section of engine nozzles is also very important, and is complicated by high material temperatures. The electromagnetic design requirements for coatings are not different from those for low temperatures, but structural integrity is a much bigger issue.

Jet Wakes​

The driver determining radar return from a jet wake is the ionization present. Return from resistive particles, such as carbon, is seldom a significant factor. It Is important in calculating the return from an ionized wake to use nonequilibrium mathematics, particularly for medium and high altitude cases.
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 afterburning wake could be dominant.

Component Design​

When the basic aircraft signature is reduced to a very low level, detail design becomes very important. Access panel and door edges, for example, have the potential to be major contributors to radar cross section unless measures are taken to suppress them.
Based on the discussion of simple flat plates, it is clear that it is generally unsatisfactory to have a door edge at right angles to the direction of flight. This would result in a noticeable signal in a nose on aspect. Thus, conventional rectangular doors and access panels are unacceptable.
The solution is not only to sweep the panel edges, but to align those edges with other major edges on the aircraft.
The pilot's head, complete with helmet, is a major source of radar return. It is augmented by the bounce path returns associated with 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, a pilot would prefer not to have noticeable instrument-panel reflection during night flying.
On an unstable, fly by wire aircraft, it is extremely important to have redundant sources of aerodynamic data. These must be very accurate with respect to flow direction, and they must operate ice free at all times. Static and total pressure probes have been used, but they clearly represent compromises with stealth requirements. Several quite different techniques are in various stages of development.
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 cannot be described here.

Infrared Radiation​

There are two significant sources of infrared radiation from air breathing propulsion systems: hot parts and jet wakes. The fundamental variables available for reducing radiation are temperature and emissivity, and the basic tool available is line of sight masking.
Emissivity can be a double edged sword, particularly inside a duct.
While a low emissivity surface would reduce the emitted energy, it would also enhance reflected energy that may be coming from a hotter internal region. Thus, a careful optimization must be made to determine the preferred emissivity pattern inside a jet engine exhaust pipe.
This pattern must be played against the frequency range available to detectors, which typically covers a band from one to 12 microns.
The short wavelengths are particularly effective at high temperatures, while the long wavelengths are most effective at typical ambient atmospheric temperatures. The required emissivity pattern as a function of both frequency and spatial dispersion having been determined, the next issue is how to make materials that fit the bill.
The first inclination of the infrared coating designer is to throw some metal flakes into a transparent binder. Coming up with a transparent binder over the frequency range of interest is not easy, and the radar coating man probably won't like the effects of the metal particles on his favorite observable.
The next move is usually to come up with a multi layer material, where the same cancellation approach that was discussed earlier regarding radar suppressant coatings is used. The dimensions now are in angstroms rather than millimeters.
The big push at present is in moving from metal layers in the films to metal oxides for radar cross section compatibility. Getting the required performance as a function of frequency is not easy, and it is a significant feat to get down to an emissivity of 0.1, particularly over a sustained frequency range. Thus, the biggest practical ratio of emissivities is liable to be one order of magnitude.
Everyone can recognize that all of this discussion is meaningless if engines continue to deposit carbon (one of the highest emissivity materials known) on duct walls. For the infrared coating to be effective, it is not sufficient to have a very low particulate ratio in the engine exhaust, but to have one that is essentially zero.
Carbon buildup on hot engine parts is a cumulative situation, and there are very few bright, shiny parts inside exhaust nozzles after a number of hours of operation. For this reason alone, it is likely that emissivity control would predominantly be employed on surfaces other than those exposed to engine exhaust gases, i.e., inlets and aircraft external parts.
The other available variable is temperature. This, in principle, gives a great deal more opportunity for radiation reduction than emissivity, because of the large exponential dependence. The general equation for emitted radiation is that it varies with the product of emissivity and temperature to the fourth power.
However, this is a great simplification, because it does not account for the frequency shift of radiation with temperature. In the frequency range at which most simple detectors work (one to five microns), and at typical hot-metal temperatures, the exponential dependency will be typically near eight rather than four, and so at a particular frequency corresponding to a specific detector, the radiation would be proportional to the product of the emissivity and temperature to the eighth power. It is fairly clear that a small reduction in temperature can have a much greater effect than any reasonably anticipated reduction in emissivity.
The third approach is masking. This is clearly much easier to do when the majority of the power is taken off by the turbine, as in a propjet or helicopter application, than when the jet provides the basic propulsive force.
The former community has been using this approach to infrared suppression for many years, but it is only recently that the jet-propulsion crowd has tackled this problem. The Lockheed F 117A and the Northrop B 2 both use a similar approach of masking to prevent any hot parts being visible in the lower hemisphere.
In summary, infrared radiation should be tackled 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. Strong cooperation between engine and airframe manufacturers in the early stages of design is extremely important. The choice of engine bypass ratio, for example, should not be made solely on the basis of performance, but on a combination of that and survivability for maximum system effectiveness.
The jet-wake radiation follows the same laws as the engine hot parts, a very strong dependency on temperature and a multiplicative factor of emissivity. 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 are favorable for using water-vapor bands for detection.

Summary​

The low signatures achieved by modern special-purpose aircraft are due to a combination of shaping, material, material selection, and careful attention to detail design. Budgeting of component signatures across a wide range of frequencies and attitude angles is mandatory. just as in a blackout, the game can be given away by one chink of light.