Weekend Wings #34 - Stealth Aircraft: The Old Becomes New Again

I'm sorry it's taken me so long to get out another article in the 'Weekend Wings' series. Unfortunately, they take a lot of time to research and prepare, and pressure of other work in recent months has meant that I didn't have the time to do a proper job. I don't want to publish a half-baked article that isn't comprehensive enough to satisfy me, let alone my readers! I have a couple more in preparation, and I hope the next one won't take so long.

Last year I published an article about 'invisible aircraft', where experts postulated that new technology might make airliners invisible to those nearby. That was an isolated reference . . . but a wave of new information coming out in dribs and drabs from various sources suggests that this is an idea whose time has come. The experts are calling it 'visual stealth', and claim it's the next wave in stealth or low-observable technology for aircraft, following the 'radar stealth' already achieved.

The field of 'visual stealth' encompasses every arm of the military, and everything from the lowest technology to the highest. In its simplest form, it can be a change of color, or a change of physical camouflage pattern to hide something against its background. For example, deep-diving nuclear submarines have traditionally been painted black, because in the deep waters where they habitually patrol, there's little or no light, and a black hull is thus well hidden. However, in littoral operations (i.e. near the shore - something that's becoming more common in a post-Cold-War world), black is a very poor choice, as it stands out against the color of the sea in shallow water.

Here, for example, is the USS Chicago (SSN-721), a US Los Angeles-class nuclear attack submarine, photographed at periscope depth from an aircraft flying overhead. The black paint on her hull, including the gray areas on top where it's faded through exposure to sea air and foot traffic, and the green of underwater plant growth, can be clearly seen against (and through) the blue of the sea.




The Royal Navy is taking steps to render its nuclear submarines less visible in shallower waters, and the US Navy is almost certainly doing likewise. It was reported in 2006 that the Royal Navy had developed a new shade of blue paint for its submarines, which is almost invisible against typical sea conditions in their operating areas. Below we see HMS Torbay, a Trafalgar-class nuclear attack submarine, first in her old black color and then in her new blue paint.




Of course, if HMS Torbay happens to patrol in an area where the sea is green, rather than blue, this might be a handicap: but even then, the blue should be less visible than a pitch black color.

The next step in visual stealth is to avoid solid colors altogether, and move to a pattern of colors and textures that will blend in better with a given background. This has been a common practice in most of the wars of the 20th century, but the advent of digital technology has meant a quantum leap in the effectiveness of camouflage. From uniforms to vehicles to weapons, the new digital camouflage is revolutionizing what can - and can't - be easily seen and identified.

As an example, let's take the work of a Canadian company, Hyperstealth Biotechnology Corp., which has become a world leader in digital camouflage techniques. They developed the KA-2 digital camouflage pattern that was adopted by the Jordanian armed forces, first as a uniform:




then on their vehicles:




and they're now considering it for their warplanes. Hyperstealth took this photograph of a USAF F-16 Fighting Falcon aircraft (top image, painted in 'Compass Ghost' gray, of which more later) and digitally edited it to apply their KA-2 digital camouflage pattern to it (bottom image). Note how much less visible the aircraft is, particularly from far away. It would be very hard to visually locate or identify it from above at a range of more than a mile or two.




The non-camouflaged fuel tanks beneath the wings are a stark reminder of how visible the 'standard' USAF paint job is to other aircraft against such a background!

Hyperstealth have also sold their camouflage patterns to other armed forces. Here's a Slovakian Air Force Mikoyan MiG-29 in their CloudCam pattern.




Hyperstealth is also looking into digital camouflage patterns for warships. Here they took an existing photograph of a US warship, the Arleigh Burke-class destroyer USS Winston S. Churchill (DDG-81), digitally altered it to apply their Razzcam digital camouflage scheme, and modified the background to represent a typical littoral operating environment. The difference is staggering!




Clearly, from anything further than close range, an enemy observer would have a hard time distinguishing the ship from her background, and even then might not be able to readily identify her, due to the visual confusion caused by her camouflage.

The principle is also being applied to hide buildings in surroundings where they appear unsightly, or detract from natural beauty. For example, Hyperstealth worked with the Bureau of Land Management of the US Department of the Interior to camouflage a generating station, to make it stand out less from its scenic background. These before-and-after pictures illustrate their success.




Hyperstealth is far from alone in developing digital camouflage patterns. The US armed forces have adopted them in their respective uniforms. The US Marines developed MARPAT, the US Army developed ACUPAT (also known as ARPAT), the US Air Force has its Airman Battle Uniform, and the US Navy has a new Working Uniform. They all use the same design principles. As an example, here are some Marines wearing snow MARPAT camouflage on an exercise (it also comes in desert and woodland patterns).




Such digital camouflage principles may also be applied to vehicles of the US armed forces: indeed, the US Marines have already applied an experimental digital camouflage scheme to one of their F/A-18 Hornet strike fighter aircraft.




Nevertheless, despite the very real advance that digital camouflage represents, it retains one fundamental disadvantage: it works only against an appropriate background. If one took soldiers wearing that Jordanian KA-2 camouflage uniform and put them in a field of snow, they'd stand out like sore thumbs! Ditto taking US Marines in snow MARPAT and placing them in a field of grass. The Slovakian Air Force MiG-29 shown earlier, or the US Marine F/A-18 shown above, will be well camouflaged against the typically cloudy, hazy skies of central Europe: but put either one low over a field of bright green grass, and it'll be visible for many miles to any aircraft flying at a higher level.

Attempts to make aircraft less visible to other aircraft, or from the ground, date back to World War I. In 1916 the German Linke-Hoffman R.I bomber was built using a type of celluloid covering over the aft fuselage and tail, in an attempt to render those parts transparent.




The experiment was not successful, largely due to the poor quality of materials available at the time, and the aircraft never reached production status or saw operational service. Other aircraft were painted in 'earth tones' to make them blend in better when flying low over the ground, but these efforts were seldom centrally co-ordinated, being typically the product of individual initiative.

During World War II the US Navy undertook Project Yehudi in an attempt to render its anti-submarine aircraft less visible during their approach to a target. A Grumman TBF Avenger was fitted with a series of lights around the engine cowling, along the leading edges of the wings and on the tailplane, as shown in the photographs below.






In tests during 1943, it was shown that the Avenger without lights could be seen silhouetted against the sky from up to twelve miles away. This meant that a submarine could see the aircraft and crash-dive before the aircraft could see the submarine. However, when the lights were switched on and adjusted to match the intensity of the light in the sky, the aircraft could approach to within two miles before being seen, giving its crew ample time to detect the submarine and set up an attack approach.

A B-24 Liberator heavy bomber was also equipped with Yehudi lights, with equally good results.




However, the Yehudi Project never progressed to production status, because improvements in radar technology meant that aircraft could detect submarines at much greater ranges, and no longer had to rely on getting close to them in order to detect them visually.

During the Vietnam War the idea was resurrected in the Compass Ghost program. USAF F-4 Phantom fighters were visible from a great distance, due to their size and the smoke trails left by their engines. Enemy fighters could easily turn away in time to avoid combat, or circle around outside visual range to come in behind the US fighters and ambush them. To make the Phantom harder to see, it was fitted with lights, and painted in a combination of light blue and gray to match the hue of the sky.




The exercise proved successful, and the distance at which the Phantom could be visually detected was said to have been reduced by 30%: but the lights were not adopted for use in service. However, the color scheme would be further refined, and become known as 'Compass Ghost gray'. It has since been applied to many different types of aircraft in USAF service. It's shown below in an early form on F-4 Phantom fighters of the USAF's 456th Fighter Interceptor Squadron.




The story of the development of the first 'stealth' aircraft, the F-117 Nighthawk, is well known, and I won't repeat it here. Those who'd like to read about it will find an excellent overview here. The F-117 was followed by the B-2 Spirit stealth bomber, and the F-22 Raptor stealth fighter. The F-35 Lightning II, the product of the Joint Strike Fighter program, is currently nearing entry into service. It's less stealthy than the F-22, being optimized for the strike role rather than interception and air-to-air combat, but is nevertheless classified as a stealth aircraft.

All these aircraft are shaped in such a way as to deflect and/or absorb radar impulses. They're not actually 'invisible' to radar, but the range at which they can be detected is drastically reduced. In terms of relative radar cross-section, the F-4 Phantom might be described as the equivalent of a whole flock of birds; the F-117 Nighthawk as the equivalent of a sparrow; and the B-2 Spirit or F-22 Raptor as the equivalent of a large winged insect such as a wasp.

All of these aircraft are well-known by now, and I don't need to speak of them in detail. However, it should be noted that the F-117 and B-2 both share one critical limitation. They were and are designed to operate within range of enemy defenses at night only. They may be invisible to radar at all except close range, but they can be seen, and they're not fast enough to run away from fighters or missiles. They must therefore rely on darkness to hide them. (The F-22 is fast enough to be able to approach - and, if necessary, outrun - enemy defenses during daylight, and it's presumed the F-35 will be too.)

It can thus be seen that stealthiness against radar is not enough, in and of itself, to render an aircraft immune to enemy defenses. It should be as stealthy as possible against any possible means of locating and identifying it, whether electronically, by radar; in the infra-red spectrum, through the heat signature of its engines or their exhaust, or its airframe, heated by friction as it passes through the air; or visually. Radar and infra-red stealth is well advanced, but the latter field, visual stealth, began to receive increased attention in the 1990's, as it was the field where 'stealthy' aircraft were - and remain - most vulnerable.

During the 1990's increasing attention was paid to the aspect of all-round stealthiness. One of the most important projects of that decade - one that is largely forgotten today, which I find strange - was Boeing's Bird Of Prey technology demonstrator aircraft.




Its existence was revealed only at the end of its test program in 2002, and it was mothballed almost immediately thereafter. However, it brought together several aspects of stealthiness in a combination never before achieved by another manufacturer. An article published at the time it was unveiled analyzed them well:

Technologically, Bird of Prey represents a level of stealth that is more advanced than anything seen on any other known aircraft.

. . .

Everything about the Bird of Prey - high sweep angles, a low-profile cockpit that masks the inlet from the front view, stabilizing surfaces blended smoothly into the wings, flexible control-surface hinge covers and razor-sharp edges - speaks of an aircraft designed for an unprecedented low RCS [radar cross-section] level. Most sources quote the RCS of current stealth aircraft - the F-117, B-2 or F-22 - at around -30 to -40 dBsm (decibels relative to a square metre target). In everyday terms, this is equivalent to a small-to-medium bird at the top end and a large insect at the lower end. But a paper co-authored by an employee of Overholser's company, Denmar Inc, refers to aircraft with an RCS of -70 dBsm, or rather smaller than a mosquito.

This would suggest that the Bird of Prey was built to demonstrate ultra-low RCS levels in flight, as Lockheed's Have Blue had done in the -30 to -40 dBsm level two decades earlier. The problem, however, is that it is very difficult to obtain any measurements of such a stealthy aircraft without flying dangerously close to it.

The clue to the puzzle is the Bird of Prey's colour scheme, which is clearly designed for low visibility in daylight, even including a pattern of counter-shading around the inlet. The shape of the aircraft, too, is rather interestingly formulated to avoid shadows.




Now, recall a number of stories in various comers of the media in 1996-97, suggesting that the USAF was evaluating different forms of active counter-illumination, using light sources to match the luminance of the target to a backlit sky.

In its basic form, this technology dates back to the US Navy's Project Yehudi of the 1940s. Counter-illumination was also evaluated on an F-4 in the early 1970s, under the Compass Ghost programme, and a system developed by Scipar Inc of Buffalo, NY, would have been tested on Have Blue had both prototypes not been lost during tests. However, visual stealth becomes more important as RCS is driven down, because the visual signature becomes dominant - that is, the signature that can be detected at greatest range.

It's also worth noting that the Bird of Prey was fitted with a Pratt & Whitney JT15D5 engine. This engine's 3:1 bypass ratio is certainly higher than optimal for a stealth aircraft, requiring a larger inlet and exhaust system than would be needed for a pure jet engine. However, it is better from the viewpoint of both infra-red and acoustic signatures.

Disclosing the fact that Boeing has demonstrated the ability to achieve stealth in daylight is very important.

There's more at the link. Bold print is my emphasis. Here's a video clip of the Bird Of Prey demonstrator at the USAF Museum in Dayton, Ohio, along with some footage of the aircraft in flight.

Remember that Bird Of Prey was publicly revealed only at the end of its test program, in 2002. In the seven years since then, I'm sure progress has been rapid. The technology demonstrated on Bird Of Prey has been incorporated into subsequent programs by many manufacturers, and the fact that the aspects identified in the article above haven't been talked about much since then speaks volumes in itself. If they're not being talked about, it's because they're highly classified, and people want to keep them that way. What we see flying on the F-35 today is early- to mid-1990's technology, which is finally reaching production status. What's incorporated into new prototypes like Northrop Grumman's X-47B (described in Weekend Wings #30, and shown below) is post-Bird Of Prey technology, and even that's already being superseded in the laboratories.

Rollout of Northrop Grumman X-47B in December 2008 (image courtesy of Northrop Grumman)

Artist's impression of X-47B in flight (image courtesy of Wikipedia)

Radar cross-section and heat signature reduction are well-known disciplines, and progress there is a matter of incremental improvements. However, in the field of 'visual stealth', we're on the verge of a quantum leap in technological capability, with the advent of 'active camouflage'. This works on the principle of taking an image of what's on the far side of an object, and projecting or displaying that image on the side nearest the observer, so that from any sort of distance, the object disappears against its background. Because the projection or display is constantly updated to reflect what's on its far side, the object will remain relatively invisible even if it's moving.

The picture below illustrates the principle. The camera on the left is photographing the scenery behind the television screen. The screen is displaying that image in a 1:1 reproduction, true to scale. As a result, one doesn't notice the TV set itself: one instinctively looks at the scene it's displaying, ignoring the screen.

(Image courtesy of Wikipedia)

Scientists and engineers are working to develop films that can be applied to the surface of vehicles and buildings, to display such images in real time. If used on an aircraft that also employed anti-radar stealth technology and had a suppressed infra-red signature, the aircraft would become as close to undetectable - i.e. invisible - as makes no difference.

This is far from a pipe-dream. A report from the University of Southern California notes:

Researchers at the University of Florida are in the process of developing an ‘electrochromic polymer’. These thin sheets cover the aircraft’s white skin and sense the hue, color and brightness of the surrounding sky and ground. The image received is then projected onto the aircraft’s opposite side. When charged to a certain voltage, these panels undergo color change.

At the Tonopah test range airstrip in Nevada, another system was tested; as claimed by a technician working at the base, an F-15 equipped with this technology took off from the runway only to disappear from sight 3 Km away.

Yet another similar “skin” is being tested at the top-secret Groom Lake facility at Area 51 in Nevada. It is composed of an electromagnetically conductive polyaniline-based radar-absorbent composite material. The system also disposes photo-sensitive receptors all over the plane that scan the surrounding area; subsequently the data is interpreted by an onboard computer which outputs it much like a computer screen.

Perhaps one day, in the very near future, one may fly in a completely invisible aircraft.

There's more at the link. The USC report appears to be confirmed by a recent article in Flight International, which states:

The secrets of next-generation stealth concepts remain well-kept within the aerospace industry, but a clear trend is beginning to emerge. The next wave of progress in stealth technology will be to reduce an aircraft's signature simultaneously across all bands of the electromagnetic spectrum, from microwave waves to foil radars to visible light to deceive even the naked eye.

Lockheed is understood to have launched the pursuit of visual stealth technology for next-generation combat aircraft, to include a future version of the F-35.

Asked to clarify its studies, the company replies: "Lockheed Martin Aeronautics and our advanced development programmes are continuously working to mature technologies and capabilities to improve all aspects of stealth. All [Lockheed] stealth technologies and research in this area are sensitive and/or classified and are co-ordinated with the appropriate US government agencies."

Just as wing-mounted lamps were used in the Second World War to obscure Grumman TBM-3D Avenger dive-bombers from ground observation, the next generation of US combat aircraft are likely to feature techniques to make them invisible to a naked human eye - or electro-optical sensor.

. . .

Sensors embedded in an aircraft skin can now precisely measure the brightness of the air as an aircraft moves through the sky.

In a modern update of Project Yehudi, Kevin Dowling, an engineer for Philips Electronics North America, has patented a concept to embed sensors and light emitting diode-based lighting into the skin of a stealthy aircraft.

Dowling says the technology exists in the lighting industry to sense the contrast between the colour of the sky as the aircraft passes through it, then adjust the colour radiated by LED panels to match it. From a certain distance, an observer would see only a shimmery, blurry object moving through the air, he says.

In recent weeks, a major West Coast-based US defence contractor has contacted Philips to develop the concept for naval applications, he says. Philips is not in a position to develop and sell a visual stealth system, Dowling adds, but would supply lighting components to a systems integrator.

More recently, a joint team of researchers at Duke University and the Southeast University of Nanjing unveiled a composite material whose properties are invisible to certain microwave bands, which is the first step in developing a structure capable of cloaking.

There's more at the link.

As the above article mentions, we're seeing renewed attention being paid to the principle of electroluminescence - the idea behind Project Yehudi in World War II. Last year a small unmanned aircraft with a wingspan of less than 7 feet demonstrated how the use of electroluminescence made it disappear against the sky from an altitude of only 1,000 feet. A report and images may be found at the link.

I'm sure the people who thought up Project Yehudi, almost 70 years ago, and their successors in Project Compass Ghost, more than 40 years ago, would never have dreamed that their idea would represent cutting-edge technology in the 'stealthy' world of today's aircraft! I wonder whether any of them are still alive? If so, I hope they have the opportunity to visit some of the plants where modern applications of their techniques are being studied. I think it would be a fitting and appropriate gesture, just as Jack Northrop was allowed to see the design of the B-2 Spirit before he died, to acknowledge that his idea for a 'flying wing' had at last come to fruition.

Peter