Popular Science has learned, military engineers are already hard at work on the technologies needed to build such a plane. Special lights, coatings, and other technologies under investigation could not only make future fighters disappear from radar screens, but could also make them almost completely invisible to the human eye. By the early 2000s, stealth may be practical in broad daylight.

Today's experiments exploit a principle that was demonstrated half a century ago, in a secret project codename " Yehudi" . In that project, engineers mounted lights on an antisubmarine aircraft to make it harder to spot against a bright sky. Similar technology was used in the Vietnam War to shorten the distance at which the F-4 Phantom could be detected. Lighting systems were available when Lockheed's Skunk Works was awarded the contract to build Have Blue, the world's first stealth aircraft, in 1976.

The breakthrough that made Have Blue possible was the ability to reduce an aeroplane’s radar reflectivity to less than one-hundredth of what was considered normal in the 1960s, slashing the effective range of enemy radar. Reducing the radar reflectivity so dramatically meant that the designers of Have Blue also had to reduce the visual and infrared signatures of the plane, according to a rule of thumb known as "balanced observables."

This rule says that a stealth aircraft should be designed so that every detection system arrayed against it has roughly the same range. There is no point in building an aeroplane that is invisible to radar at five miles if optical sensors can see it at ten miles.

Have Blue was the prototype for an aircraft that would make its attack run at a moderate altitude of 10,000 to 15,000 feet-close enough to designate the target accurately, but high enough to elude medium-calibre gunfire. At the time, the designers' goal was an aircraft that would be as stealthy in daylight as at night. The designers realised that visual detection depends on a number of factors, including the position of the observer, his angle of view, the position of the sun, and the presence of haze or clouds.

Altitude is extremely important. A jetliner at its cruising height always appears brightly lit in the sky, because dust and moisture in the air beneath the aircraft scatter light onto its underside. There are relatively few particles of dust and water in the thin air above the aeroplane. So the higher the plane flies, the more light is scattered onto it, and the darker the sky behind it.

A dark colour that absorbs as much light as possible provides the best camouflage for a high-flying aeroplane. But even the jet-black Blackbird and U-2 spyplanes look brighter than the sky when seen from below as they cruise at 80,000 feet. At lower altitudes, there is less light-scattering atmosphere below the aircraft, so lighter colours provide the least contrast.
For Have Blue, Lockheed devised a scheme of graduated greys, lighter on the bottom and darker on top. The aircraft's designers also planned to test light apertures, which would be installed on the sides and under surfaces of the aeroplane, about two feet apart. (Seen from a distance, the individual lights would blur into a single image.) The apertures would be connected to a central light source by fibre optic lines, and their output would be controlled by light sensors on the upper side of the aircraft.

The sensors would "read" the background light and adjust the skin's luminance to mirror it. This system never flew on Have Blue, possibly because the first aircraft was lost in an accident. Work on visual stealth continued, however.
In 1980, the Air Force tested a small aircraft, probably unmanned, under a project known as IMCRS (what the acronym stands for is not known). The aircraft's lower wing skins incorporated slit-like Fresnel lenses to beam light ahead of and below the aircraft, in the direction of the most likely threats. The IMCRS experiment may have been related to a Defence Advanced Research Project Agency program known as Active Camouflage. Under that program, a small powered drone was fitted with fluorescent lamps and tested at the White Sands Missile Range with so much success that the project has since been reclassified as Top Secret Neither of these lighting systems were adopted for stealth aircraft in the 1980s. They were complex to install and design. Their effects were hard to predict and difficult to test.

Carefully designed conventional camouflage worked well enough under most circumstances to ensure that an aircraft would not be visible before a radar could detect it. So why were the first F-117s painted soot black instead of a toned grey scheme that would provide better camouflage? One Lockheed engineer recalls that the commander of Tactical Air Command "didn't believe that real fighter pilots flew pastel-coloured aeroplanes."

An Air Force source close to the program says that some senior officers doubted the F-117 could survive in daylight, and wanted to ensure that nobody would try it. Black is one of the least stealthy colours for daytime flying at medium altitudes. In fact, the British Royal Air Force is painting its trainers black to make them more visible and reduce the risk of collisions. Black isn't much good at night either, because there is nearly always some light from the moon. That's why the latest F-117s have been seen in a more sensible grey colour.

The B-2 stealth bomber's underside is a very dark grey. Many people think that it is designed to attack only at night, like the F-117. This is unlikely, because the B-2 was designed to bomb Russia, and the most direct route from the centre of the United States to central Russia lies smack across the Arctic Circle, where the sun shines 24 hours a day for a large part of the year. The B-2's underside is dark because it cruises at altitudes as high as 50,000 feet, where a dark grey blends into the sky. It does not use an "active camouflage" lighting system, but it may have an upward-facing light sensor that tells the pilot when to increase or reduce altitude slightly to match the changing luminance of the sky

It appears likely that active camouflage will make a comeback in the 2000s. Improvements in radar stealth have reached a point where visual and infrared signatures are dominant. One sign of increasing interest in the non-radar aspects of stealth is that the Air Force has commissioned a new flying laboratory called FISTA II (Flying Infrared Signature Technology Aircraft), to replace a vehicle that has been used since the early 1960s to measure the heat signatures of aeroplanes.

A modified tanker aircraft, FISTA II carries not only ultra-sensitive infrared imagers but also a visual imaging system, an indication that the Pentagon is becoming serious about visual stealth. Modern follow-ons to Yehudi are both more effective and easier to install. Instead of individual lights, the Pentagon has tested thin fluorescent panels of the type already used on military aircraft for night-time formation flying.

A civilian technician working at the isolated Tonopah Test Range airstrip in Nevada says he witnessed a test of an F-15 Eagle with a prototype system. According to the technician, the fighter virtually disappeared as it lifted off the runway "We had no problem acquiring the aircraft from about a mile away," the technician recalls, "but at distances over two miles it became harder and harder to spot. Although it was a crude system, it was pretty impressive. Trying to pick out the aircraft against a clear, blue sky was next to impossible. The only time we could easily spot the aircraft was when it produced an unexpected contrail." (Contrails form when the water vapour in aircraft exhaust freezes. On the B-2 and F-117, anti-contrail systems inject chemicals into the exhaust stream to break water into droplets too small to be seen.)

An even more experimental active-camouflage system uses thin sheets of light-emitting polymer that glow and change colour when charged. Different voltages cause the sheets to glow blue, grey, white, or whatever shade is needed to match the sky. As an added advantage, the thin sheets are easy to apply to existing aircraft. One such "electrochromic" polymer has been developed at the University of Florida, and the Air Force is studying it as a way of applying a variable tint to the cockpit canopy of a fighter aircraft. In theory, such a coating could also be used over a white-painted skin to vary its colour.
But what about concealing an aircraft from an enemy flying above it? Defence contractors have told Popular Science that an even more exotic invisibility system is being tested on two new stealth aircraft at the high-security Groom Lake air base in Nevada. The skin is derived from an electromagnetically conductive polyaniline-based radar-absorbent composite material. It is optically transparent except when electrically charged, much like the LCD panels used in laptop computers.

What makes this new material attractive is that it can change brightness and colour instantaneously. Photosensitive receptors, mounted on all sides of the plane, read the ambient light and colour of the sky and ground. An onboard computer adjusts the brightness, hue, and texture of the skin to match the sky above the plane or the terrain below it. The system is also claimed to make the aircraft even stealthier

The electrically charged skin dissipates radar waves, reducing the range at which an air defence radar can track the aircraft by as much as 50 percent. Such systems do not have to be perfect.

Engineers have also taken steps to reduce the heat signatures of military aircraft. In the 1970s, infrared sensors had a much greater range than visual imaging systems-video cameras with telephoto lenses that were mainly used to track or identify targets that had already been detected. Infrared accordingly became the stealth designers' second priority, after radar. Infrared sensors detect hot spots, such as engine exhaust or the leading edges of the wing, which are heated by air friction. At closer ranges, infrared sensors detect solar radiation glinting off curved surfaces or scattering from the skin. Designers countered infrared sensors in several ways.

The exhaust nozzles were flattened into slits, because a flat nozzle has a longer perimeter than a round plume, and the exhaust mixes more quickly with the cool air. Designers also developed paints containing compounds such as zinc sulphide, to suppress reflections from the aeroplane’s skin.

Paint cannot eliminate the heat generated by skin friction, but special coatings can change the "emissivity" of the surface-that is, the efficiency with which it transforms heat into infrared radiation. Only certain wavelengths of infrared radiation travel efficiently through the atmosphere, so the goal is to concentrate infrared radiation outside those bands and let the atmosphere soak it up. Low-infrared paints and coatings are now widely used on many aircraft. Lockheed Martin even coated a 747, reducing its infrared signature tenfold.

After years of research focused on the suppression of infrared and radar signatures, aircraft designers now appear to be giving more attention to visual stealth. There are still some basic physical problems to be solved. For example, even a very efficient lighting system requires a lot of energy to match the brightness of the sky, equivalent to several times the power absorbed by the fighter's radar.

Experts in the field of electrochromic materials caution that there are major technical hurdles that have not yet been cleared in the unclassified world, and not for lack of interest:

The building industry would love to see a practical, large-area electrochromic film, because it could greatly reduce the energy needed to heat and cool buildings. Electrochromic materials must not only be able to change colour, but also to withstand sunlight and extreme weather, and continue operating through many switching cycles.

The problems are compounded for a stealth aircraft, because the material must also be compatible with existing radar and infrared stealth technologies. This may well be the reason why, for now, visual stealth measures are confined to a few experimental aircraft-and may stay that way for some time to come.

Today's experiments with visual stealth have their roots in a 1943 U.S. Navy project codename Yehudi. The intent of the program, which was highly secret at the time and came to light only in the 1980s, was to give Navy patrol aircraft a better chance of sinking enemy submarines.

During 1942, German U-boats took a heavy toll on merchant marine shipping off the East Coast of the United States. Aircraft scrambled to attack the U-boats, but submarine captains called for crash dives whenever they spotted approaching planes. By the time an aircraft got close enough to fire upon a sub, it had disappeared.

Yehudi's inventors needed a way to make the aircraft harder to see, and they realised that camouflage paint wouldn't do the job: Regardless of its colour, the aeroplane would be a black dot against the sky. The only way to make the plane less visible was to light it up like a Christmas tree.

The engineers fitted a portly TBM-3D Avenger torpedo-bomber with 10 sealed-beam lights, installed along the wing's leading edges and the rim of the engine cowling. When the intensity of the lights was adjusted to match the sky, the Avenger blended into the background. Tests proved that the Yehudi system lowered the visual acquisition range from 12 miles to two miles, allowing the Avenger to get within striking distance of its targets before they submerged.

A B-24 Liberator bomber was also modified, with similar results. Yehudi was not put into production, because better radar had already enabled Navy aeroplanes to regain the tactical advantage, but the idea was revived after air battles over Vietnam.

Concerned that the big F-4 Phantom could be seen at a greater range than its much smaller Russian adversary, the MiG-21, the Pentagon started a program called Compass Ghost. An F-4 was modified with a blue-and-white colour scheme and nine high-intensity lamps on the wings and body, reducing the detection range by as much as 30 percent.

The electronic age in which we live is where technology is a very crucial decisive factor in the outcome of a war, the daily advancements we make on which the lives of hundreds of people depend. Stealth-tech has recently become a paramount tool in battle, invincibility means invincibility. For example, during the Gulf War, the F-117A Stealth fighters were the preferred weapons for the US. Their performance was more successful than expected and they were the only aircraft not to have suffered even a single scratch. Their virtual invincibility to radar below are due to their unique shape, design and material. The stealth fighters had nevertheless definite disadvantages. Most importantly, their sole function was to drop laser-guided smart bombs onto surface targets below, participation in air-to-air combat or in dog fights were beyond their capacity; If detected, they wouldn’t be able to confront successfully other fighters. These aircraft were only able to operate at high subsonic speeds, therefore outrunning of the enemy was not an available option. Besides that, its’ arsenal contained a maximum of two smart bombs, intercepting other vehicles in the air was also impossible (no shooting capability). Finally, their flights were limited to nighttime as they could be spotted at ease during the day, and alas the subsequent task in future stealth aircraft development…the creation of a plane invisible to the eye. The concept of day-time stealth has been researched by Lockheed Martin, such a plane would need to blend into the background sky and also carry anti-radar and infrared stealth technology. One of these systems is the "electrochromic polymer" that is in the process of being developed at the University of Florida. 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 colour change. Another similar "skin" is being tested at the top-secret Groom Lake facility at Area 51 in Nevada. It is reputed to be composed of an "electro-magnetically conductive polyaniline-based radar-absorbent composite material." The system also disposes photo-sensitive receptors all over the plane that scans the surrounding area, subsequently the data is interpreted by an onboard computer which outputs it much like a computer screen. At the Tonopah test range airstrip in Nevada, yet 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 3,2 Km away. Perhaps one day, in the very near future, one may fly in a completely invisible aircraft. A very enigmatic question poses itself, how did this dream of an invisible plane displace itself from man’s imagination to the sky horizons of our planet? A large rubric of problems that dogged research were abruptly resolved at once, it’s results are guarded within the Pentagon’s confines. Is it possible that a deal had been struck with extraterrestrial scientists who gave engineers the knowledge necessary to build these silent flying machines? The premier reports of stealth fighters corresponded very much with reports of UFO sightings. It may be that the UFO’s were in fact stealth aircraft, and they were perhaps otherworldly guides evaluating the works of man. Given the amount of secrecy enveloping this subject, the truth would not be found easily.