<< Page 49 Page 50: Page 51 >>   Visual Stealth Cammo, low-visibility grey, and... pink?   The original approach to making a combat aircraft hard to spot is still used today in most military hardware: camouflage. Even some of the very first combat aircraft, flown during the first World War, were painted in tones that mimicked the terrain under them. This usually meant blobs of black, white, and shades of brown, green, and/or blue. The underside of the fuselage, wings, and tail, however, were painted in tones that mimicked the sky: light blue, light grey, or white. The thought is that, when seen from above, the aircraft would be camouflaged into the terrain around it, and when seen from below, it would look like a part of the sky. The same idea has evolved its way onto many birds, fish, and marine mammals: Their backs/tops are dark grey, making them hard to pick out when seen from above (under them being the dark ocean), and their bellies/bottoms are white, making them hard to pick out when picked from below (above them being the sun and the bright sky). During the first World War, German designers modified some Fokker fighters and at least one larger bomber aircraft by covering them with skins made of Cellon, a mostly-transparent material. However, it was opaque when looked at from an angle, and it glinted in the sun, so these first attempts at making an “invisible” aircraft failed. During the second World War, an interesting development was made towards making aircraft even harder to pick out. This development was kept secret until the 1980s, believe it or not. Remember that the idea of camouflage is not to make the airplane invisible, but to make it look like its surroundings. During the day, low-altitude aircraft usually look darker than their surroundings, unless they are painted in very light colors. Eventually almost all low-altitude combat aircraft came to be painted in light colors, but during World War 2 this still seemed like a bad idea (you have to admit it’s counter-intuitive) although the Japanese and the British did paint some aircraft in light colors. So the US Navy tackled the question of how to make their aircraft not look dark against the sky, without painting them almost white. Particularly, torpedo-bombers going after German U-boats would often be spotted before getting close enough to drop their torpedoes, usually from about 12 miles away. The U-boats would then have time to dive out of range before they could be hit. The navy took a TBM3D Avenger and fitted lights along the engine cowling and wings. These lights could illuminate the airplane with adjustable intensity. After many experiments, the proper lighting could keep the TBM from being spotted until it was only 2 miles away! A B-24 Liberator, a large 4-engine bomber, was then similarly modified, and delivered similar results. This was called Project Yehudi. This concept was not incorporated into production aitcraft, since aircraft radar was being developed that allowed these aircraft to locate and attack their targets from beyond visual range. The idea was revived during the Vietnam war when F-4 Phantoms (among the largest fighters of the time) kept getting spotted from long distances. Under Project Compass Ghost, an F-4 painted in light blue tones and fitted with lights saw its detection range reduced by 10-30 percent – not bad, but not the 85% achieved with the TBM in World War 2. (In modern times, it has been observed that jets are often sighted first by the darkness in their shadowy jet intakes. The placement of lights in the jet intakes has been proposed, but as far as I know, painting the inside of the intake ducts white is the only solution to this problem that has actually been used). But back to the question of what colors to paint combat aircraft. As jets became common in the late 1940s and early 1950s, camouflage became of extremely limited use: these aircraft flew so high that the sky, clouds, and haze dominated their surroundings, not vegetation or ocean or desert terrain. The first answer was to not paint these jets at all, but to finish their metallic skin to as to reflect the colors around them. This may sound like a good idea until you realize that they also reflected the sun a good deal of the time. As convex mirrors and M.C. Escher lithographs reveal, a curved-out shiny surface reflects almost everything around it, no matter what direction it’s seen from. The glint of sunlight reflected off these shiny jets made them easy to pick out. Many operators of combat aircraft performed studies to try and figure out how to best paint an aircraft so that it could not easily be picked out against blue sky, clouds, or haze. The US military performed studies and actually concluded that pink made for an outstanding low-visibility color, but only the Brits actually implemented this idea, on their high-altitude recon aircraft (see http://www.spitfiresociety.demon.co.uk/whatmark.htm#MkPRX , http://www.acf.clara.net/scale/scale-pics-4/low/pink-spit/spitfire.html ) and ground vehicles (http://www.csse.monash.edu.au/~lloyd/4/LandRover/Military/Pink-Panther ). Still, a light grey color was found to be about as effective. Throughout the 60s and 70s, more and more military aircraft were painted in what is now called low-visibility grey. This color is similar to that of a cloudy sky, or of the haze that often covers distant objects. The background of an aircraft seen from below is still generally brighter than if it is seen from above, so many combat aircraft are painted mostly in light grey, with somewhat darker grey on the tops of the wings and fuselage, and/or somewhat lighter grey on the belly. These two-tone or three-tone low-visibility light-grey schemes are standard today over most of the world’s air forces and navies. (It is interesting to note that most birds that live by the shore, and most fish and marine mammals, have similar two-tone or three-tone grey camouflage). It should also be noted that higher-flying aircraft fly under a darker sky, with more haze between them and the ground, and are lit from below at least as much as from above (due to the scattering of light in the atmosphere). They are therefore painted dark grey. One way to understand this is that the “sky” is really black (space), but air and dust create a deep, misty, light-colored, hazy layer between us and the black sky. The higher up something is, the more “lightening” is caused by the air’s haze when we look up from the ground, and the more this haze (which is illuminated by the sun) shines light on the aircraft’s underside. An airliner flying very high almost always looks white, because it is being illuminated from below by the hazy atmosphere and because we have a lot of light-colored haze between us and the airplane. So for lower altitudes, a light-grey aircraft will look to be the same color as the sky (since the air does not lighten its color, and the low-altitude sky is light) but at high altitudes, a dark grey color is better (since the air lightens it to the same color as the rest of the sky as we see it from the surface – or, when seen from altitude, it is dark like the sky around it even when lit from below). If you go beyond the atmosphere into space, black would be ideal for disappearing into the background. So helicopters, A-10 Warthogs, and other aircraft meant for low-altitude operations are painted light grey. KC-10s, bombers (like the B-1, B-2, and B-52), the F-22, and other aircraft meant to do a lot of flying around 50,000 feet are painted dark grey. Extreme high-flyers like the U-2 and SR-71 are painted black. How the F-117 came to be painted black is an interesting story. Even at night, black is not as good as dark grey for blending into the background. Stars and the moon provide some light, and a shadow against them is noticeable. In areas with more light pollution, where stars are hard to see and moonlight does not add much to ambient illumination, a black shape moving through the sky is also noticeable. The air illuminated at night over cities (especially when cloudy) would also make a black aircraft stick out like a sore thumb, while a grey aircraft would blend into the clouds. For non-light-polluted areas where the stars can be seen at night, an almost-white color might actually be least visible. When the F-117 was being test-flown, Lockheed engineers researched all this, and presented a report to the US Air Force suggesting that their new stealthy fighter-bomber be painted the standard low-visibility grey with a very light grey belly. The commanders in charge of the F-117 program, however, thought that the F-117 could never survive daytime operations, and decided that painting them black would prevent mission planners from ever deploying the F-117 outside the cover of night. So it was painted black not for stealth, but so that the aircraft would not be flown through undue risk. However, in late 2003, some F-117s were finally painted in low-visibility grey, and (just as the commanders feared) were evaluated for daytime operations. (Just how bad is it to paint your airplane black? Well, try this: Military training aircraft are painted so as to be as visible as possible. This minimizes the risk of mid-air collisions and allows for the students’ flights to be most easily observed. Most of the world’s air forces and navies paints their trainers in combinations of white and red, sometimes with some orange and yellow. However, the Royal Air Force conducted a study and found out that black planes were even easier to spot from very far away than were red ones or white ones, and so they repainted their entire fleet of trainers black. Unless they are flying a few tens of thousands of feet high, black aircraft can be seen through the atmosphere’s haze for miles and miles and miles). Canada has also adopted this scheme, and some USAF T-38 trainers are also black One last important aspect to low visibility is managing contrails. Contrails form when the water-vapor-rich air coming out of a jet engine is cooled by the air around it, causing the water vapor to condense, just like your breath on a cold day. Your airplane can be absolutely invisible, but that won’t do it any good if there’s a giant white arrow pointing to it across the sky. How can these be minimized? Well, they only form when the air is too cold to absorb the water vapor coming out of the engine. So a pilot can simply fly in warm air that will not condense the water vapor (under 20,000 feet), or they can fly at very high altitudes where the air is dry enough to absorb the water vapor (above 50 or 60,000 feet). For airplanes that can only best perform their mission between 20 and 50 thousand feet, chloro-fluoro-sulfonic acid can be sprayed into the exhaust so as to reduce contrail formation – it causes the water droplets to break up into extremely small droplets, “hypernucleation”, making them harder to see and more easily absorbed by the air. This was confirmed through brief experiments with bombers like especially-modified B-52s. However, this material is toxic and corrosive, so its use has been found to be impractical. Besides, you’d have to carry a few tons of this stuff to last you through a whole mission. The B-2 was actually designed with such a system in mind: Two huge bays inside the flying wing were originally designed to carry tanks of this acid – they are located just ahead of the control surfaces and just outboard of the engine exhaust systems. When the use of chloro-fluoro-sulfonic acid was abandoned, a system was developed that fires a laser into the air behind the B-2’s engine exhaust and alerts the pilot to fly higher if the laser encounters condensing water vapor. However, an upgrade has recently been made to the B-2 fleet (consisting primarily of improvements to the electronics, radar, engines, etc) and it supposedly includes a “new contrail-management system” with details that are still classified. So maybe those acid-tank bays near the back are in use after all, who knows. What does the future hold for low-visibility? During the 90s, Boeing flew a top-secret aircraft called the Bird Of Prey to study many different kinds of stealth, including low-visibility. Experiments with the Bird Of Prey may or may not have included lights similar to those in Project Yehudi and Compass Ghost. But it did reveal new insights into how the eye picks out an airplane from its background, and on how different areas ought to be shaded to best avoid spotting. One of the ideas studied is that the contrast between the brightly lit areas and the shadowy areas makes an airplane very visible. In other words, shadows and highlights are what draw the eye, so they should be minimized. Horizontal areas on the top of the aircraft usually look very bright, while vertical areas on the side are usually darker, and the inside of the jet intakes and the bottom/belly of the aircraft are usually darkest, in shade. So these horizontal light areas on top should be painted darker, and the areas usually in the shade are painted white. However, it was also found that some variation in shade was good: If the airplane all appeared to be a single color, it would probably either be lighter or darker than the sky, and would be picked out easily. But if some of it is painted in a light grey probably a little lighter than most skies, and some of it is painted in a grey that is just a tad darker, this will make the large overall shape harder to see. Therefore the top of the wings, tails, and fuselage were painted in two subtly different shades of grey, just for the sake of “breaking up” the large shape of the overall airplane. In other words, too much contrast is bad, but too little contrast is almost as bad. These discoveries from the Bird Of Prey experiments were implemented into the F-22’s paint scheme: The top is painted in not-too-large blobs of subtly different shades of grey. The sides and bottom are painted a lighter color, and the jet intakes are painted white. These features make the F-22 stealthy to human eyes, as well as to radars and infra-red sensors. Additionally, a light-colored outline around the perimeter of the F-22 has been observed along the underside and vertical tails, and a similar dark outline around the perimeter on the top side. While the role of the outline is unclear, it has been theorized that this outline could be helpful in that it makes the rest of the airplane appear sky-colored by hiding the difference between the airplane’s color and the sky color. This is known as the Craik-O’Brien-Cornsweet illusion. It has also been theorized that this outline makes the airplane blend into the sky by making the darker colors near the middle more gradually progress to lighter colors around the perimeter, so no sharp differences in shade are seen all the way up to (and hopefully including) when the airplane ends and sky begins. In other words, if the outline is about as bright as the sky, then the airplane seems to blend into the sky, but if the outline is the wrong shade, then the rest of the airplane is about the right shade while the outline is not thick enough to be seen by itself from far away. So the airplane is always harder to pick out, whether or not it matches the brightness of the sky. (It is also possible that the outline actually makes the F-22 MORE visible, so as to hide the true effectiveness of its low-visibility scheme during non-combat situations. It is known that the F-22, B-2, and other stealth planes deploy protrusions around themselves – for example, lights at the wingtips come out and stick out of the wings, instead of shining from recessed/flush windows – so as to make the aircraft more visible to radar when they are not in combat, so the outline might be the visual equivalent of these stealth-hiding features). When trying to protect the lives of combat pilots (and the integrity of the extremely expensive jets they fly), even the question of which shades of grey to use in your paint scheme cannot be over-thought.