Wakes of war: contrails and the rise of air power, 1918-1945 Part I--early sightings and preliminary explanations, 1918-1938.

Judging by the wakes of vapor and the lines of tracers left behind in the high, cold air, the Messerschmitts are mixing it up with Number 4 Squadron.

Francisco Tarazona, Yo Fue Piloto de Carza Rojo, September 1938. (1)

Introduction

Contrail is a contraction of condensation trail, an early term applied to the thin, white clouds that appear behind aircraft when moisture in engine exhausts forms ice crystals in cold air that is already sufficiently saturated. Vapor trail was another early term applied to this phenomenon.

Although ubiquitous today, condensation trails were apparently unknown until World War I. Indeed, what may be the earliest reported observations of contrails were made in the autumn of 1918, as the Great War was drawing to a close. By the end of 1920, other sightings had been reported and several people had advanced preliminary explanations of the new phenomenon. Yet, until the opening days of the Second World War, contrails would remain an isolated phenomenon generally unknown to the public and of limited interest to military aviators.

This situation changed suddenly and dramatically during the first days of World War II. The key to this change was a revolution in aviation that took place across the decades of the twenties and thirties, as leading aviation powers, spurred on by air power enthusiasts, worked to expand the operational envelope of combat aircraft. Because of this revolution, when the Second World War opened, the world's most powerful air forces were flying warplanes with operational ceilings in excess of 25,000 and even 30,000 feet, well into the atmospheric region where conditions are often favorable to contrail formation. As a result, these aircraft routinely trailed what aviation pioneer and writer Antoine Saint-Exupery poetically called "pearly white" scarves as airmen executed their missions in the skies high over Western Europe.

Today, we associate three main types of condensation phenomena with flight. One of these is the spiraling, ribbon-like streamers that can appear in wingtip vortices under the proper atmospheric conditions. Another type is the spectacular cone-shaped Prandtl-Glauert condensation cloud that can form around the waist of high-speed aircraft. Finally, there are the long, thin, clouds spawned by aircraft engine exhausts--the common contrails that crisscross the skies over much of the world today. This last form of condensation phenomenon is the focus of this two-part paper.

Part I covers the period from the end of World War I to the eve of World War II. It begins by describing some early contrail sightings and then discusses the explanations prompted by these observations. This is followed by a review of the major developments that made high altitude flight part of routine combat operations and led to the first recorded observation of contrails in combat, this coming during the Spanish Civil War, Europe's dress rehearsal for World War II.

Part II focuses on the role of contrails in European air operations between 1939 and 1945. It also discusses British and American efforts to understand contrails so that Allied airmen could take advantage of contrails in combat operations or at least prevent enemy airmen from doing the same.

The Argonne Battle Cloud: Early Contrail Sightings

The First World War started in 1914, a little over a decade after Orville Wright coaxed his frail, primitive flying machine aloft for a twelve-second flight that covered a scant forty yards, about the length of a long pass in the National Football League. (2) Given the immaturity of aviation technology, it is not surprising that European powers opened the war with small air forces comprised of planes that were so slow that they could scarcely keep pace with today's freeway traffic. Moreover, these planes were open-cockpit machines that were generally limited to altitudes below 12,000 feet. By the end of the war, however, frontline aircraft could reach speeds of 130 mph and operate as high as 20,000 feet. (3)

While this operational ceiling is still below the band between 25,000 and 40,000 feet where atmospheric conditions are most often conducive to contrail formation, (4) planes flying at 20,000 feet and even lower can generate contrails under the proper conditions of temperature and humidity. Therefore, in the later stages of the Great War, contrail-generating flights would have become increasingly common as the operational ceilings of first-line aircraft increased. Given the number of planes flying over the Western Front and the number of men on the ground with a vital interest in watching the skies for hostile aircraft, it was virtually inevitable that substantial numbers of people would eventually notice that at least some high-flying planes were producing long thin clouds as they crossed the skies.

In early October 1918, while the American Expeditionary Force was engaged in the Meuse-Argonne offensive, several hundred AEF members noticed a number of strange clouds that seemed to emanate from high-flying aircraft and stretch across much of the sky. Three of these observers thought the phenomenon unusual enough to take special note of it. After the war the three independently brought their observations to the attention of the public.

The first of the three to have his account of the strange clouds published was Captain Ward S. Wells, Army Medical Corps, who was serving with the 60th Infantry, 5th Division, American Expeditionary Force, during the Meuse-Argonne campaign. In early October 1918, Ward and his unit were in the Bois de Hess just back of Montfaucon, about ten miles to the west and a little north of Verdun, where they were waiting to take over a portion of the front. 5

Ward noted that it had been raining for several days when at last there dawned "a wonderfully clear and beautiful morning, with not a cloud in sight." During this particular morning, according to Ward,

Our attention was first drawn to the sky by the sudden appearance of several strange and startling clouds--long, graceful, looping ribbons of white. These were tapering to a point at one end and at the other where they dissolved into nothingness 60 degrees across the sky, were about as broad as the width of a finger held arm's distance from the eye. On close observation we noticed some distance ahead of each cloud point the tiny speck of a chasse [sic] plane.... Never before had I seen a plane writing in white upon the blue slate of sky. (6)

Wells had described his observations in a letter to his brother Everett Wells. Because he considered the phenomenon described by Ward to be "quite unusual and perhaps worthy of record," Everett himself wrote to Scientific American quoting at length from Ward's letter. An editorial note at the end of the letter echoed Everett's sentiments: "The observation of clouds formed in the wake of an airplane is, so far as we know, novel. Perhaps some of our readers can bring forward other examples of this, ..." (7) Such a comment coming from the editor of a scientific journal who presumably had a broad, general knowledge of scientific affairs suggests that we are here close to the earliest report of a contrail sighting.

A little over a year later, George B. Vaughn, apparently unaware of Wells' earlier letter, asked if any reader of The American Legion Weekly might be able to explain to him "a phenomenon that occurred, I believe, on October 10, 1918, over the battle front" in the vicinity of Montfaucon. Vaughn and his comrades were passing through a small town

when we noticed three thin parallel lines of clouds or smoke stretching far across the sky. They looked as if they had been made by three planes passing, throwing out smoke and cutting stunts, for the lines were far from straight....

Hundreds of troops were watching this display and wondering what had caused it.

Vaugh noted that some of those who saw the phenomenon believed that it was some kind of mirage, while others thought it might be "something new by the Germans." (8)

A month later, Walter N. Nead responded to Vaughn's query. Formerly a captain in the 168th Infantry Regiment of the 42d Rainbow Division, Nead described what may be the same occurrence of contrails that had prompted the reports of both Wells and Vaughn:

I would relate that the Rainbow Division, on the morning of October 10, 1918, was lying in what had at one time been a wood just back of Montfaucon. The sky was clear except for a few fleecy clouds to the northwest. Three airmen came from the northwest and passed almost over our regiment, continuing on to the southeast.

Behind each machine was a trail of white, which at first sight seemed to be smoke resulting from poor engine combustion but which upon more careful observation proved too wide to have been caused by smoke. Perhaps the strangest thing of aU was the fact that when the planes reached a certain point in the sky the rainbow (sundog) colors (9) became distinctly visible.

The editor's of The American Legion Weekly titled Nead's letter: "The Argonne Battle Cloud." (10)

There are at least two other reports of contrail sightings that predate 1920. Both took place in Germany. One was written by Alfred Wegener, a German polymath who not only had a PhD in astronomy but also exhibited wide-ranging interests in geology and meteorology. It was Wegener who first advanced the theory of continental drift which he codified in his 1915 book The Origins of Continental Drift. Wegener reported his contrail sighting in a German meteorological journal at the beginning of 1920, making it likely that the observation took place near the end of 1919. According to Wegener, "during three airplane flights over Munich at a height of 9 km. a cloud 50 km. in length was formed." Wegener also presented an explanation of contrails of which more anon. (11)

The second German sighting occurred on May 9, 1919, when a pilot flying over Berlin at about 26,000 feet noticed the generation of a cloud stream that extended for about forty miles behind his plane. This stream eventually spread out to form a cloud layer that was about 3,000 feet thick. The pilot saw a similar phenomenon two days later. (12)

It may seem odd that word of the May 1919 sighting was not published until 1930, when Nature reported the episode in its section on Historic Natural Events. Given the dates of other events reported here (e.g., May 6, 1915, May 8, 1663, and May 8, 1902), it appears that this section was used by journal editors to inform readers of unusual phenomena that might have come to light only recently. (13) More will be said on this point at the end of the following section.

Early Explanations

Because of their novelty, the Argonne battle clouds demanded an explanation. The explications offered may be grouped into three broad categories. The first is an argument from analogy that assumed the long cloud trails were essentially a familiar phenomenon now being observed for the first time in a high altitude setting. The second form is analysis based on meteorological concepts. The third category is an explanation that came from wind tunnel testing. This last explanation is interesting on its own merits, since it presents what may be some of the earliest recorded observations of another form of condensation phenomenon--wing-tip vortices. Finally, an argument from analogy advanced by two aviators will be given special attention, since it presents a powerful argument against concluding that the Argonne clouds were a new atmospheric phenomenon.

Not surprisingly, the first to offer an explanation was Captain Ward Wells, who associated the new phenomenon with changes in atmospheric conditions prompted by the passage of an aircraft. In his words: "Apparently the churning of the air was all that was needed to upset the delicately balanced meteorological conditions and precipitate this strange cloud formation." (14)

Two months after the publication of Wells' view, David W. Howe, who had been a pilot in the Thirteenth Aero Squadron during the war, took issue with Wells. In a letter to Scientific American, he stated that the phenomenon Wells described was not a cloud, but rather a trail of oil smoke generated by an aircraft engine. In Howe's words:

Several times I have seen a ribbon-like trail in the sky behind an American pursuit plane. On one occasion I noticed that my motor, a Hispano-Suiza French-made, was giving out a trail of whitish-blue smoke which hung for some time in the perfectly still air. It was probably due to excessive oil feed as in the case of automobiles. I made large sweeping S's in the air and described one complete circle which I was informed hung there for some time.

Perhaps meteorologists have also written expressing doubt that the agitation of an airplane propeller in the air would be enough to create clouds. (15)

The next edition of Scientific American contained another rejoinder to Wells, this one from W. Lee Sandberg, who was also a veteran of the U.S. Air Service. Writing with the panache one might expect of a World War I aviator, Sandberg noted that while he was not exactly a "second Fonck," he still knew "a few things about an airplane in flight" and considered Wells' theory "rather far fetched." He then stated:

I have noticed this about those "clouds." They can only be seen as distinctly as Mr. Wells says on very still days. And every time a pilot gives his motor a "shot" of oil the "clouds" become more distinct. That is absolutely all there is to it, a wake of smoke formed by burning lubrication oil. I hate to spoil a Medical Corps captain's nice theory in this manner but it hurts to see these humbug stories about the aviation game go uncalled. (16)

Howe and Sandberg raised a valid point. World War I aircraft engines were smoky. This was especially true of rotary engines, which accounted for as many as eighty percent of all aircraft engines by early 1917. The high level of smoke emitted by rotary engines was due to their "total loss" lubrication system in which the oil was injected into the cylinders where it was either burned with the fuel and air mixture or thrown out of the engine. In the case of the Gnome Type N, one of the common rotary engines, oil consumption amounted to more than two gallons per hour of flight. Given the performance of these aircraft engines, how could anyone be certain that the long cloud trails over the Argonne region were clouds and not streams of engine smoke? (17)

There is insufficient evidence in the letters of Wells, Vaughn, Howe, and Sandberg to determine whether the white streamers produced by airplanes over the Argonne battlefield were vapor or smoke trails. For one thing, it seems highly unlikely that all four men had simultaneously witnessed the same phenomenon. Furthermore, while Howe and Sandberg may have seen nothing more than streamers of oil smoke any number of times, on other occasions they may have seen a contrail and mistaken it for oil smoke or simply seen some kind of interaction between oil smoke and condensation vapor. The ambiguities in this situation arise at least in part because of a lack of observational precision, which is not surprising given that these four men apparently lacked meteorological training.

On the other hand, Walter Nead's letter indicates at least some understanding of meteorological phenomena. Furthermore, his report offers convincing evidence that Nead saw condensation trails over the Argonne region and not smoke trails. Specifically, this evidence is Nead's sighting of"the rainbow (sundog) colors," a reference to the multicolored, 22° halo that may appear around the sun when a thin layer of cirrus clouds is present.

Like the common rainbow, the 22° halo is produced by the refraction of sunlight. Unlike rainbows, which are produced when sunlight is refracted by water droplets, halos are produced by ice crystal refraction. A sundog, also known as a false sun, will sometimes appear on a halo. This is a bright spot on the colored arc that may be seen on either or both sides of the sun along a line parallel to the horizon that passes through the sun. (18) Nead's report indicates that he saw the sundog in the white streamers formed by the aircraft. Therefore, the slender Argonne battle clouds were composed of ice crystals, which means they were contrails and not oil smoke. (19)

The sundog phenomenon clearly affected Nead's explanation of how contrails are formed. In his view, these unusual clouds would form at a given altitude when the

air was almost saturated with moisture at the temperature which prevailed at that altitude. With the passing of the planes, the propeller movements caused a strong air current with a lowering of the temperature where the current was noticeable. With the lowering of the temperature, the air became supersaturated with moisture, forming a small cloud which at that altitude immediately became snow. This snow would give the white appearance noted by Mr. Vaughn and would also account for the rainbow colors. (20)

Nead was not the first to recognize the relationship between the halo phenomenon and the nature of condensation trails. That honor goes to Alfred Wegener. Because "the 22° halo was observed" in conjunction with the vapor trails that appeared over Munich toward the end of 1919, Wegener concluded that these contrails were composed of ice crystals. He also decided that clouds such as these were precipitated by "nuclei furnished by the motor exhaust" of airplanes. (21)

A few months before the publication of Wegener's article, yet another explanation for contrails had been offered by Walter Winans in the pages of Scientific American. Responding to Captain Wells' earlier report, Winans argued that the white trails over the Argonne region were nothing more than the exhaust streams of aircraft engines. In support of his view, he offered the example of motorcycles he had seen trailing "white clouds ... several hundred yards long." (22)

Accompanying Winans' letter was still another explanation of the clouds reported by Wells. William H. Pond began by describing an experiment in which condensation was shown to occur in an air sample containing dust particles, but not in a particle-free sample. Based on this experiment, Pond then argued that the clouds Wells described were formed because the aircraft inserted dust particles into high altitude air from which the dust had been washed by rains of the previous day. As Pond put it:

In the case reported by Mr. Wells the dust had all been removed from the air by the rain of the day before, but a great deal of moisture was still present in an evaporated state and was constantly being increased by the heat of the sun. The exhaust of the airplane engine evidently contained enough dust to form the nuclei of the drops of moisture which constituted the cloud observed. (23)

The week after publishing Pond's letter, Scientific American carried a response to Wells that was written by Elisah Fales of the Engineering Division of the Army Air Service, which was located at McCook Field near Dayton, Ohio. In the note, Fales stated that during recent wind tunnel tests, he and two colleagues had discovered a phenomenon similar to that described by Wells. According to Fales:

When the relative humidity is proper, a small model aerofoil held in the air current induces moisture condensation, with the result that the flight vortices are clearly seen, can be analyzed and photographed.

The effects in full scale could well appear as described in Mr. Wells" note; the vortex characteristics being indistinguishable to the distant observer. It can be inferred that the ribbon of cloud, as stretched across the line of vision, had a wedge-shaped front, the wedge-angle being less than 10 degrees. The upper limb of the angle then constituted a tip-vortex rotating about an axis extending from each wing tip toward the rear. A cross-section of the cloud taken 200 feet behind the airplane would not show up as a solid rectangle, but as a wide, flat-bottomed "U." At great distances behind the airplane, the vortex energy would be dissipated and there would remain quiescent cloud. (24)

Fales was a graduate of MIT where he studied aeronautics and flew in glider competitions. He later became an assistant professor of mechanical engineering and chair of aeronautics in the College of Engineering at the University of Illinois where he taught the school's first course in aeronautical engineering. During World War I, he wrote the text book used in the ground training program for Air Service pilot candidates. Here, Fales "set forth the main principles of flying, such as the aviator must know in order to properly understand his airplane, keep it trued up, and operate it in cross country flights as well as at the flying field." (25)

Sometime around the end of World War I, Fales began working in the Air Service Engineering Division at McCook Field where he teamed up with Frank Caldwell, whom Fales had known since their student days at MIT. In 1918, these two men designed McCook's high speed wind tunnel and oversaw its construction. The first such wind tunnel in America, it could produce a velocity of 465 miles per hour through the tunnel's 14-inch diameter test section. Using this wind tunnel, Fales and Caldwell became two of the first researchers to encounter the effects of compressibility, the dramatic drop in an airfoil's lift-to-drag-coefficient at higher velocities. (26)

Fales and Caldwell also used this wind tunnel to complete the work that underpinned a more detailed discussion of the mechanics of wing vortices production. This discussion appears in a 1921 report published by the National Advisory Committee for Aeronautics and indicates that Fales, Caldwell, and co-worker C. P. Grimes were not so much interested in wing-tip vortices per se as in the fact that these vortices could be used to study airflows over wind-tunnel test objects. Still, some of their observations pertaining to condensation induced by wind-tunnel models and their discussion of wingtip vortices contributed to the broader understanding of condensation phenomenology. (27)

Two other explanations of the Argonne clouds are presented in another 1921 document, this one an article by Burton M. Varney, an instructor in Geography, Meteorology, and Climatology at the University of California at Berkley. The first of these explanations comes in a lengthy quotation from the writings of Dr. William Jackson Humphreys of the United States Weather Bureau.

The end products of complete combustion of gasoline are water vapor and carbon dioxide, and it is found that if the water vapor were condensed, there would result a little more than I gallon of water per gallon of gasoline consumed. It was found by Wells and Thuras, in studying the fogs off the Newfoundland coast (see US. Coast Guard, Bull. 5, 1916) that there were 1,200 water droplets of diameter 0.01 mm. in a cubic centimeter of air in a dense fog. If we assume that an airplane travels 3 miles on a gallon of gasoline (approximately the figure given by the Aerial Mil Service) it is possible to show that if only a small part--a fourth or fifth--of the water vapor were condensed, there would be abundant cloud to produce the effect observed at the Argonne Battle. It should be stated, however, that this water vapor would have to be discharged into air which was very cold and nearly saturated. This seems to be the correct explanation, and is substantiated by scientists at the Bureau of Standards, who say that they have actually observed this cloud behind airplanes and automobiles. The Bureau of Standards is working on a device for condensing and using this water aboard dirigibles as ballast. (28)

Varney also provided his own theory about the cause of contrails. He humbly noted that his explanation was probably less likely to be valid than that of Humphreys, for although it squared with experimental results, it depended upon the existence of atmospheric conditions that might not occur. Varney was aware that shock had been used in experiments to cause condensation in supersaturated air and believed that "it may be possible for supersaturation to occur in the atmosphere and for shock of some sort to induce condensation in air in which this unstable condition exists." Where the "Argonne Battle Cloud" was concerned, he wrote, the question is "whether supersaturation can occur in the free air, and whether atmospheric vibrations set up by the exhausts from the engines would be a sufficient cause of condensation in such air." (29)

Finally, another early explanation of contrails appears in Nature's 1930 report of the May 1919 contrails over Berlin. It seemed "probable," the report stated, "that the exhaust gases supplied condensation nuclei to the air, thus giving the necessary stimulus to cloud formation." (30)

That Nature would still be treating contrails as an unexplained novelty in 1930, suggests that vapor trails were still relatively uncommon as late as that date. Furthermore, Nature's handling of this matter suggests that awareness of the contrail phenomenon and knowledge of earlier explanations were not widespread in the scientific community. Nevertheless, by 1930, high altitude flight was becoming increasingly common. Indeed, as we are about to see, advances in aviation technology that took place across the twenties and thirties, assured that the major air forces of the world would soon be equipped with aircraft that routinely operated in the cold regions of the upper atmosphere where contrails are commonly produced.

The Interwar Years: Facilitating High Altitude Flight

The impetus behind efforts to increase the operational ceilings of aircraft came from air power enthusiasts. World War I in Western Europe had been a bloody, costly war of attrition that had lasted for more than four years. By the time it finally ended in November 1918, both the defeated Germans and the victorious French and English were morally and physically exhausted by the war's waste and slaughter. (31) National leaders were convinced that their countries could not afford another war similar to the Great War of 1914-1918. As a result, post-war military leaders faced the challenge of finding a way was to restore decisiveness to warfare.

During the First World War, the airplane had shown its potential for future warfare, including its ability to carry the war directly to an enemy's homeland. Based on their experiences in the war, air power advocates concluded that future wars could be won quickly at relatively low costs by air forces that could over-fly ground defenses and strike directly at an enemy's industrial base and population centers. These attacks would not only destroy the enemy's ability to produce the materials of war, but would also terrorize civilians and end their willingness to continue the war effort.

These views were most famously pronounced by airmen like Giulio Douhet and William "Billy" Mitchell. Douhet had developed a preliminary concept of strategic bombardment by the middle of 1917 and would advance these views more fully in his 1921 treatise, Command of the Air. In Douhet's words:

The idea that a war could be decided by the collapse of the nation's morale is considered paradoxical, and this in spite of the fact that the World War was decided by the collapse of the moral resistance of the defeated peoples.

The armies involved in that war were only the means by which the nations of each side tried to undermine the resistance of the other; so much so that, though the defeated side was the one whose armies won the most and greatest battles, when the morale of the civilian population began to weaken, these very armies either disbanded or surrendered, and an entire fleet was turned over intact to the enemy. This disintegration of nations in the last war was indirectly brought about by the actions of the armies in the field. In the future it will be accomplished directly by the actions of aerial forces. In that lies the difference between past and future wars. (32)

Mitchell expressed similar views in his 1925 Winged Defense.

No longer will the tedious and expensive processes of wearing down the enemy's land forces by continuous attacks be resorted to. The air forces will strike immediately at the enemy's manufacturing and food centers, railways, bridges, canals and harbors. The saving of lives, man power and expenditures will be tremendous to the winning side. (33)

In this vision of aerial warfare, there would be no limits on the savagery of attacks. For example, Douhet advocated brutal assaults that would entail using the most powerful poison gases and biological agents against civilian population centers. Moreover, Douhet believed that there was no way to stop these terrifying attacks. While a defender must protect all potential targets, an attacker would be free to concentrate his forces at a time and place of his own choosing and would, therefore, have overwhelming superiority at the point of attack. Offensive air power would bring about a "swift, crushing decision on the battlefield," which was now the entire territories of the warring nations. (34)

The vehicle for delivering the decisive air attacks in Douhet's scheme was "the battleplane." In addition to its load of bombs that were to be dropped on an enemy's homeland, the battleplane would be so armed and armored as to allow a formation of these aircraft to fight its way through enemy air defenses and bomb its target. Where operational ceiling was concerned, Douhet noted that "the higher the altitude, the less a warplane's vulnerability to antiaircraft fire." Since bombing attacks would characteristically result in the dispersal of bomb loads, altitude would not detract from the effectiveness of bombing raids, which could "be carried out effectively even at very high altitudes." Still, as a practical matter, the "normal ceiling" for a battleplane would be "between 3,000 and 4,000 meters." In Italy's case, the operating altitude would have to be raised to "between 6,000 and 7,000 meters" so that Italy's air forces would be able to cross the Alps to attack Italy's prospective European enemies. Battleplanes, Douhet believed, should constitute the bulk of a modern, independent air force. (35)

For most of the two decades separating the last century's two world wars, developing a Douhetian battleplane with an operational ceiling that would facilitate penetration of enemy air defenses did not appear to present insurmountable challenges. World War I air defenses were severely limited by the defender's inability to detect approaching aircraft at a distance that would allow defending aircraft sufficient time to take off and reach the altitude of the attackers before they had delivered their bombs and escaped. The detection of attacking aircraft at this time was based on the senses of sight and sound; and even though these senses were enhanced by binoculars and sound sensing devices, air defenders of the First World War could effectively spot aircraft only when they were within a range of about five miles. Even at the relatively slow approach velocity of 120 miles per hour for World War I aircraft, this meant that the defenders had only about two and a half minutes notice of an impending attack. Of this difficulty, Douhet, himself a World War I aviator, said that even with "the most elaborate system of signals, if our pursuit squadrons were not already in the air when the enemy reached its objective--and obviously they could not remain in the air continuously--they could seldom take off in time to prevent the enemy from dropping his load of bombs on his chosen targets." (36)

As late as the mid-1930s, the situation for defenders seemed hopeless. Aircraft with speeds in excess of 300 miles per hour were becoming operational. On the other hand, while the speed of operational aircraft had more than doubled since the end of the First World War, detection sensors remained virtually unchanged. In the early 1920s, the British had installed sound locators on their southern coast, even though they had a range of only about eight miles and were only intermittently operational. The British also experimented with large sound detectors that were known as "acoustical mirrors," large concrete rectangles that would have been as long as 200 feet. However, all of these suffered from the same difficulties: limited range, intermittent operation, and being subject to interference from everyday sound sources such as passing automobiles, cackling sea birds, and the pounding of surf. (37)

The first anti-aircraft guns had already been developed before World War I started, and their use expanded considerably over the course of the war. While it is true that ground-based anti-aircraft guns could take the attackers under fire more quickly than could defending aircraft, these guns were limited throughout much of the war by a lack of accurate tracking information for attacking aircraft. Moreover, anti-aircraft gunners had to contend with shell fuzes that often did not operate properly at higher altitudes. Nevertheless, anti-aircraft fire improved steadily and by the end of the war was capable of reaching as high as 20,000 feet. (38)

While there was considerable experimentation with integrating ground observation posts and air defenders through the use of radio and cloth signaling panels, these efforts were still in their infancy when World War I ended in 1918. (39) Effective early warning of aircraft attack would have to await the development of radar and more effective radio communications, both of which would become hallmarks of air defenses during World War II. In the meantime, aircraft developers focused on producing faster aircraft capable of operating at greater altitudes in the hopes of developing an invincible attack plane.

Critical to the development of aircraft that could fly high enough to escape detection and avoid antiaircraft fire were experimental flights in which new technologies were tested. Included among these technologies were improved propellers that operated more effectively at high alitudes, superchargers to ensure adequate oxygen for engine combustion in the thin air of the upper atmosphere, improved gasoline, heated flying clothes to protect pilots against temperatures that could drop below-60° F, and effective systems for delivering oxygen to airmen. A by-product of these flight tests was a steady increase in the world's record for aircraft altitude. (40)

Two of the principal test pilots in the high altitude flight program of the Army Air Service were First Lieutenant John A. Macready and Major Rudolph W. "Shorty" Schroeder. The latter began his work before the end of World War I, setting a world altitude record of 28,899 feet on September 18, 1918. In 1919, he set three more altitude records before his final record flight of February 27, 1920. (41)

Schroeder's February 1920 flight illustrates well the dangers faced by aviation pioneers, who made their flights with only primitive oxygen systems and inadequate protective clothing. During this particular flight, Schroeder's automatic oxygen system failed at 18,000 feet as he was ascending. While switching to the manual backup system, he noted that the temperature was -67° F. As he continued his climb, at times he encountered winds so strong that he was flying backwards relative to the ground. At the peak of his flight, 33,143 feet, his manual system ran out of oxygen. When he pulled off his mask and goggles in an attempt to breathe, his eyelids froze and he was almost immediately overcome by carbon monoxide fumes from his engine. However, before passing out, he managed to switch off the engine and put his plane into a steep dive. Miraculously, after his plane had fallen five miles in a matter of minutes, Schroeder regained consciousness, stopped the dive of his aircraft, and found his home field in spite of being virtually blind. (42)

Schroeder's harrowing experience resulted in criticism of the high altitude flight program, one newspaper referring to this episode as a "'suicidal altitude flight." The Air Service countered by noting that in future wars, deadly anti-aircraft fire would force military aircraft to fly higher and higher. Therefore, it was essential to gain as much knowledge as possible about high altitude flying. Furthermore, any knowledge gained from the Air Service program would also benefit commercial aviation. The high altitude flight program would continue even though Schroeder left military service near the end of 1920. (43)

During the twenties and thirties, aviators set new altitude records sixteen times. By the end of 1930, efforts to increase the operational altitude of aircraft had pushed the world's altitude record to above 43,000 feet. Then, in October 1938, on the eve of World War II, Colonel Mario Pezzi, an Italian pilot, extended this record to 56,046. (44)

Contrails were probably produced during a number of the record high altitude flights. At least one of these flights resulted in a specific account of a contrail's appearance. This report is associated with a flight made by 1st Lt. John A. Macready, who replaced Major Schroeder when the latter left military service. Macready made several attempts to break Schroeder's last altitude record before finally succeeding on September 28, 1921, when he climbed to 34,508 feet. (45)

On one of Macready's earlier flights in 1921, his aircraft produced a contrail that was described in the US. Air Service Newsletter for July 1921.

An altitude flight was made in the morning at McCook Field recently by Lieut. J. A. Macready in a La Pere with supercharged Liberty [engine]. When the airplane reached a height of 26,000-27,000 feet at 11:50 a.m., a long feathery white streamer was observed forming behind a rapidly moving dark speck. The cloud was of the cirrus variety, well defined at the edges and apparently 10 to 15 times the width of the plane. The sky behind the first portion was clear blue with no clouds in the near neighborhood. The first streamer seemed perhaps 2 miles long. Then a gap of one-quarter mile. The second streamer formed with a background of light cirrus cloud and after 2 or 3 miles the plane seemed to go into the cirrus background, for the streamer formation ceased while an apparent path of blue continued beyond for a way in the cirrus cloud. The whole streamer may have been 3 miles long. After 20 minutes the streamer had drifted and spread until it merged indistinguishably with the other cirrus clouds visible. (46)

While the generation of contrails in temperate climes is normally associated with high altitude flights such as Macready's, aircraft flying in frigid regions of the world can produce contrails right down to the earth's surface. Low-level contrails were observed as early as 1930 by A. M. Campbell, a Canadian engineer, who reported that he had seen aircraft generate contrails from the ground up to 10,000 feet in temperatures that ranged from--25° F to -60° F. Regarding the source of these contrails--whether they were wing-tip vortices or engine-exhaust condensation--Campbell wrote the following:

At no time while cloud trails were evidenced could any of this phenomena [sic] be credited to wing tip vortices. These trails were observed from the aircraft by skidding the machine in order to obtain a view directly behind the aircraft. There was no doubt but that the trailing cloud originated from the exhaust pipe. These trails would hold their formation anywhere from a few hundred yards to a mile behind the aircraft depending no doubt on the humidity.

Included in Campbell's report of his observations was an account of an aircraft contrail that described a complete circle. In Campbell's words:

On a bright moonlight night a flight was undertaken in the Wollaston Lake area in northern Saskatchewan at which time the ground temperature was approximately -50 degrees F. The air was clear with the exception of a very slight frost precipitation which was noticeable but of no great hindrance to visibility. Possibly this would be a clue as to humid conditions at the time. Flying was done at approximately 1,000 to 1,500 ft. On circling the bay at which the aircraft was to land, a dark streak was noticeable across the surface of the bay. This was being more closely examined by the pilot, in the event of it being an ice-heave, when it was noticed that the dark streak was drifting across the bay. On looking back it was found that a dense trailing cloud had formed and the dark streak observed was a shadow of this cloud. The turning of the aircraft was continued until the tail end of this cloud was met, at which time it was still showing no sign of disintegration. It was estimated that this trailing cloud was over two miles long. (47)

Advent of High Altitude Operations: The Thirties

Across the decade of the 1930s, America's military aviators regularly adjusted their thinking and training in response to the improved performance of aircraft that were entering the operational inventory. At the very end of 1929, the Army Air Corps received its first Boeing P-12 pursuit planes. Although the cockpit of this biplane was still open, exposing aviators to the extremely low temperatures of high altitudes, Air Corps pilots promptly used this plane to increase the operational ceiling for fighter aircraft. (48)

In early 1930, the 95th Pursuit Squadron, which was equipped with P-12s and stationed at Rockwell Field at San Diego, conducted unit operations at altitudes as high as 30,000 feet. However, since the P-12 performed more efficiently at lower altitudes, the standard training altitude for the 95th was 26,000 feet. One especially notable training flight took place high over Los Angeles on June 7, 1930. Flying so high that their planes could not be seen from the ground, the pilots of the 95th completed an exercise in which they flew top cover for Curtiss B-2 bombers that simulated bombing the city from an altitude of 15,000 feet. The P-12s also carried out mock attacks on imaginary antiaircraft positions on the ground. At this point, the 95th was claiming to be the only unit in the world capable of formation flying at such high altitudes. (49)

Other Air Corps units followed the lead of the 95th as they received P-12s, making high altitude squadron operations relative routine. Also coming into use at this time was an improved oxygen system that automatically regulated oxygen flow to pilots, who now breathed through masks that were strapped to their faces. This improved oxygen system facilitated training at altitudes in the range of 20,000 to 25,000 feet. Nevertheless pilots continued to be plagued by the extreme cold of open cockpits in which the temperature routinely fell as low as--40° F. This extreme cold forced pilots to wear as much as fifty pounds of clothing, adding to the challenge of high altitude operations. (50)

In the mid-1930s, the P-26 became the standard Air Corps pursuit aircraft. Although it was America's first monoplane pursuit aircraft and its first all-metal fighter, the P-26 cockpit was still open, leaving its pilots vulnerable to the deleterious effects of the frigid air at its service ceiling of over 27,000 feet. This situation was soon ameliorated, as the Air Corps began introducing its first closed cockpit pursuit planes, the Seversky P-35 and the Curtiss P-36, near the end of the decade. Both of these were also metal monoplanes, and they came with the added feature of retractable landing gears. With the entry into service of the P-35 and P-36, the air arm of the U.S. Army had fighters that could operate above 30,000 feet. (51)

By the time the Air Corps had received its first enclosed cockpit fighters, the American air service was also acquiring the Boeing B-17 "Flying Fortress." Here was the"battleplane" the air service would soon use to implement its doctrine of daylight precision strategic bombing against Nazi Germany's industrial base. Capable of carrying a bomb load of 4,800 pounds, the first model of this plane could operate at over 30,000 feet, with later versions having service ceilings that exceeded 36,000 feet. (52) With its four powerful motors and high operational ceiling, it would soon become a cloud-making machine in the skies over Europe.

While America was working to expand the operational envelopes of her warplanes, Europeans were doing the same. One of the most impressive products of these European efforts was Germany's Messerschmitt 109, also known as the Me-109 or Bf 109. First flown in May 1935, its early models had operational ceilings just under 28,000 feet. However, within three years, more advanced models such as the Bf 109E could operate above 34,000 feet. Early models of the 109 were field tested during the Spanish Civil War (1936-1939), Europe's segue from peace to general war. (53)

Wakes of War. The Spanish Civil War

The Spanish Civil was a clash between long-standing traditionalism and pent up pressures for change. Historically, Spain had been dominated by nationalism, a long-standing monarchy, and a conservative Army and Church. While Spain remained neutral during the First World War, demands for materials from Europe's warring nations led to an expansion of industry in Spain, strengthening the forces of labor organization, socialism, and anarchism. The clash between these forces and those of traditionalism came to a head in July 1936 when the Nationalists, who were led by General Francisco Franco, attempted a coup against Spain's Republican government. When the coup failed to topple the government and the government failed to promptly suppress the rebellion, a bloody civil war ensued; it did not end until 1939, on the eve of the outbreak of World War II in Europe. (54)

Almost immediately after the outbreak of the Civil War, the major powers of Europe began supporting the side in the war that seemed most sympathetic to its own political philosophy and promised the most advantages from the standpoint of its national security concerns. Italy and Germany favored the Nationalists, given their fascist, anti-communist leanings. Furthermore, a fascist Spain on France's southern border would weaken France strategically. The Soviet Union supported the Republicans, as part of its goal of spreading communist governments around the world. France also supported the Republicans at least in part to protect her strategic rear in case of hostilities with Germany.

The image of this war as a struggle between socialism and communism on the one side and fascism on the other was instrumental in drawing idealistic young socialists and communists into the service of the Republicans. One of these was Francisco Tarazona, a Mexican national whose parents were both from Spain.

In December 1936, the Republicans sent Tarazona to the Soviet Union for flight training. After returning to Spain in July 1937, he flew the Soviet Polikarpov I-16 fighter, which had been introduced into the war near the end of 1936. A stubby, low-wing monoplane, the I-16 was one of the first aircraft equipped with a retractable (although hand-cranked) landing gear. The Spanish nickname for this highly maneuverable plane was Mosca (Fly). (55)

When Tarazona began flying the Mosca, it was faster than any aircraft in the Nationalist inventory, including the German Heinkel 51 and the Italian Fiat C.R. 32, both of which were highly maneuverable biplanes. The introduction of the I-16 and its biplane cousin, the Soviet I-15 Chato (Snub Nose), into the air battles over Spain in November 1936 had shifted the balance decidedly in favor of the Republican air force. Recognizing the insufficiencies of its He-51, the Germans decided in December 1936 to send to Spain their most advanced fighter, the Bf 109. (56)

The first major clash between the 109s and Moscas came in July 1937 and took place over the battlefields of Brunete to the west of Madrid; it ushered in a new era of air combat. Up until this time, battles between highly maneuverable, relatively slow biplanes had "occupied fairly limited space, from a distance olden resembling a swarm of gnats." The new air combat between less maneuverable, high-speed monoplanes "spread over hundreds of square miles." Such would be the nature of air combat during the coming world war. (57)

At first the I-16 held its own with the Bf 109s. However, as 1937 was coming to a close, German pilots worked out tactics that took advantage of the Messerschmitt's higher operational ceiling. The performance of the I-16 dropped off increasingly as it climbed above 3,000 meters until at 5,000 meters the plane was extremely sluggish. On the other hand, the 109 achieved its optimum performance at 5,000 to 6,000 meters. To take advantage of this difference in performance, the 109s would routinely fly about 1000 meters above and behind opposing I-16s. The threat posed by 109s in this position allowed them to effectively neutralize larger formations of I-16s, with the 109s choosing the terms of an engagement. In the words of Gerald Howson, historian of the air war over Spain: "With the Bf 109s above, the [I-16] formation leaders became afraid to turn, because the formation would lose its cohesion as the novices began to straggle. The Messerschmitts could pounce as soon as this occurred, and make their escape by continuing in a dive which the I-16s could not match." (58)

These tactics were described by Tarazona in his diary. During a combat patrol on March 9, 1938, he and his comrades engaged a number of Fiat fighters, probably C. R. 32 biplanes. While this fighting was in progress, Tarazona recorded in his diary, "high, high above, the 109s flew as if waiting for some prey to bolt from the pack." (59)

An earlier entry in Tarazona's diary provides a graphic description of what it was like to be bounced by the high-flying Messerschmitts. The enemy planes were first spotted as specks in the distance about 2,000 meters above Tarazona's own altitude. He and his comrades had no sooner taken up positions to protect each other's rear than two 109s made a diving attack and passed through the Republican formation, while other 109s remained above. Then Tarazona himself was attacked. He suddenly felt his aircraft being struck by bullets and looked back to see the yellow nose of a 109 a mere seventy meters from his tail. Fortunately, he managed to escape the attacker by doing a half barrel roll and diving away from the attacker. (60)

The 109s continued their dominance until the fall of 1938 when the Republicans introduced the I-16, Type-10. Nicknamed Super Mosca, the Type-10 was fitted with a more powerful engine that allowed it to operate effectively at altitudes as high 8,000 meters. In August 1938, twelve of these aircraft were integrated into the 4th Squadron of the Republican air force. Moreover, for the first time, squadron pilots were equipped with oxygen masks, making it possible for them to operate for extended periods at high altitudes. Now, Bf 109 pilots cruising complacently above inferior Republican aircraft would have the unpleasant experience of themselves being attacked from above by the Super Moscas. (61)

It was a high-altitude engagement between the Super Moscas and the 109s on September 21, 1938, that prompted what may be the first recorded observation of combat-related contrails. On that particular day, Tarazona and his unit were patrolling at a lower altitude, while the 4th Squadron with its Type-10s was trying to surprise the Messerschmitts at the higher altitudes where the Germans had previously lurked impervious to Republican attacks. As Tarazona kept his eye on the higher altitudes from which a German attack might come, he noticed in the distance an air battle that he latter described in his diary as follows: "Judging by the wakes of vapor and the lines of tracers left behind in the high, cold air, the Messerschmitts are mixing it up with 4th Squadron." (62)

The air combat of September 21, 1938 brings us to the threshold of World War II. Within a year, state-of-the-art Nazi aircraft would fill the skies over Poland as Germany plunged Europe into a general war. During this war, vapor trails would routinely streak the skyscape over Europe, transforming contrails from an object of mere curiosity to an element of air combat with life-and-death consequences. The impact of contrails on World War II air operations is the focus of part two of this article.

NOTES

(1.) Francisco Tarazona, Yo Fue Piloto de Carza Rojo (Madrid: Liberia Editorial Ban Martin, 1974), p. 217. This book is the diary Tarazona kept during the Spanish Civil War. The Spanish for the passage quoted above is: "Los Messerchmitt se las estan viendo con la 4.[sup.a], a juzgar por las estelas de vapor y las lineas que dejan las trazadoras en la frialdad de la altura." A translation of parts of Tarazona's diary may be found at http://www.aire.org/gce/ english/history/1938.htm#tara. I have used this translation as the basis for the English version given above. I have restructured the sentence so that it is more readable and have changed the translator's rendering of "estelas de vapor" from "contrails" to the more literal "wakes of vapor." Other expressions in Tarazona's diary that could refer to contrails include "estela de humo blanco" ("wake of white smoke") and "estelas blancas" ("white wakes") both of which occur on p. 76 of Tarzaona's book. The more literal translation of "estelas de vapor" seems more appropriate, since the term contrail does not appear to have come into common usage until World War II.

(2.) The American Heritage History of Flight (N.P.: American Heritage Publishing Co., Inc., 1962), p. 87.

(3.) Ezra Bowen, et. al., Knights of the Air (Alexandria, VA: Time-Life Books, 1980), pp. 24, 147.

(4.) For the atmospheric band where contrails most commonly form, see "NASA Site for Contrail Education: Frequently Asked Questions," at http://asd-www.larc. nasa.gov/GLOBE/faq.html. This site states that contrails normally form in "the upper portion of the troposphere and in the lower stratosphere where jet aircraft normally fly." This is generally between "~26,000 to 39,000 feet."

(5.) Everett D. Wells, "Clouds Formed by Airplanes," Letter to the Editor, Scientific American, June 7, 1919, p. 601. For the location of the Bois de Hess, a rather obscure reference, see Letter, Second Lieutenant David S. Lamb, March 18, 1919. This letter was found at the on-line memorial for the Funkhouser brothers, both of whom died in World War I: http://www.usgennet.org/usa/in/ county/vanderburgh/pf_chap_iv_a_1.html#CO/. I am indebted to former Colorado College librarian Ms. Julie Jones-Eddy for her assistance in securing copies of the letters that appeared in Scientific American.

(6.) Wells, "Clouds Formed by Airplanes," p. 601.

(7.) The Editor, Note, Scientific American, June 7, 1919, p. 601. All but the first paragraph of Everett's letter is a quotation of the earlier letter from Ward. Of the date of the Ward Wells letter, Everett states only that the letter from Ward was recent. Also, portions of Captain Wells' letter were later quoted in B[urton] M. Varney, "The Argonne Battle Cloud," Monthly Weather Review, June 1921, p. 348. Varney incorrectly identifies Captain Wells as "W. F. Wells."

(8.) George B. Vaughn, "Who Can Tell Him?" Letter to the Editor, The American Legion Weekly, September 24, 1920, p. 28. Ms. Patricia L. Marschand of The American Legion Magazine kindly provided the author with copies of letters from this periodical. Vaughn gave the position of the small town as being on "the highway between Jouy-en-Argonne to [sic] Nettancourt." Jouy-en-Argonne is about ten miles south-southeast of Montfaucon and about five miles west-southwest of Verdun, while Nettancourt lies about twenty-five miles south-southwest of Montfaucon. An assumption that Vaughn was near the Jouy-en-Argonne end of the highway would place him in the same vicinity as Wells when he observed the cloud trails.

(9.) The sun dog is a bright spot caused by the refraction of light by ice crystals in the atmosphere. It will be discussed in some detail shortly. See NASA Goddard Space flight Center, "Science Question of the Week: What is a Sundog?"at www.gsfc.nasa.gov/scienceques2005/20060 210.htm.

(10.) Walter H. Nead, "The Argonne Battle Cloud," The American Legion Weekly, October 22, 1920, p. 12. Nead's description of his location, in a wood back of Montfaucon, would put him south of Montfaucon, possibly in the same Bois de Hess that Captain Wells gave as his position.

(11.) Alfred Wegener, "Frost Supersaturation (Frostubersattigung) and Cirrus," Meteorologische Zeitschrift, January-February 1920, pp. 8-12, as abstracted in Monthly Weather Review, June 1921, p. 349. Given the delay between article submission and its appearance in a journal, I conclude that this contrail sighting probably occurred before January 1920, given the date of the edition in which the article appeared. For a biography of Wegener, see "Alfred Wegener (1880-1930)" at www.ucmp. berkely.edu/history/wegener.html.

(12.) I found this report at "Chris Chatfield's Cabinet of Curiosities," s.v. "1919 May 9," www.phenomena.org.uk/ curiosities.htm. In response to an email query, Mr. Christopher Chatfield informed me that the source of this entry was a brief item in Nature Magazine, No. 3147, 1930, p. 125. (E-mail, Chris Chatfield to Donald R. Baucom, 5:04 pm, January 11, 2006.) I was, indeed, able to locate a copy of this report.

(13.) "Historic Natural Events: May 9, 1919--Cloud Formed by Aeroplane," Nature: A Weekly Journal of Science, May 3, 1930, p. 693.

(14.) Wells, "Clouds Formed by Airplanes," p. 601.

(15.) David W. Howe, "Clouds Formed by Airplanes," Letter to the Editor, Scientific American, August 9, 1919. Frederick Libby, Horses Don't Fly (New York: Arcade Publishing, 2000), p. 185, describes what may be an example of the kind of smoking engine that inspired Howe's comments. Here Libby wrote: "When a trail of smoke from the rotary hits the sky and the machine gun goes to work, it is a sight to thrill anyone."

(16.) W. Lee Sandberg, "Clouds Formed by Airplanes," Letter to the Editor, Scientific American, August 16, 1919, p. 157. Rene Fonck was France's leading World War I ace with seventy-five kills.

(17.) Kimble D. McCutcheon, "Gnome Monosoupape Type N Rotary," p. 3. Rotaries took their name from the fact that the engine's crankshaft is fixed while the rest of the engine rotates about the crankshaft. The propeller is attached to and rotates with the engine. The lubricant used in rotary engines was castor oil. As a result, McCutcheon notes, pilots flying aircraft with rotary engines often suffered from the laxative effects of castor oil because of their breathing castor oil fumes and otherwise ingesting oil thrown from engines. McCutcheon's article is part of the website of the Aircraft Engine Historical Society and may be found at www.enginehistory.org/Gnome%20Monosoupape.pdf.

(18.) NASA Goddard Space flight Center, "Science Question of the Week: What is a Sundog?" at www.gsfc.nasa.gov/scienceques2005/20060210.htm.

(19.) Nead, "Argonne Battle Cloud," p. 12. Nead states that the only clouds in the sky were fleecy clouds to the northwest, suggesting that the sky over his regiment's position was clear except for the cloud trails generated by the three planes. Furthermore, he established a causative relationship between the position of the clouds generated by the planes and the appearance of the sundog when he stated that the sundog appeared "when the planes reached a certain point in the sky." In other words, once the planes and their contrails reached a point that was approximately twenty-two degrees of arc from the sun, the refraction of sunlight by the ice crystals of the contrails produced a halo/sundog.

(20.) Nead, "Argonne Battle Cloud," p. 12.

(21.) Wegener, "Frost Supersaturation (Frostubersattigung) and Cirrus," p. 349.

(22.) Walter Winans, "Clouds Formed by Airplanes," Letter to the Editor, Scientific American, August 16, 1919, p. 157.

(23.) W[illiam] H. Pond, "Clouds Formed by Airplanes," Letter to the Editor, Scientific American, August 16, 1919, p. 157.

(24.) Elisha N. Fales, "Clouds Formed by Airplanes," Letter to the Editor, Scientific American, August 23, 1919, p. 185.

(25.) F. N. Fales, Learning to Fly in the US. Army: A Manual of Aviation Practice (New York: McGraw-Hill Book Company, Inc., 1917), pp. vii-viii. For a discussion of the establishment of Air Service ground training schools at American universities, see Rebecca Hancock Cameron, Training to Fly: Military Flight Training, 1907-1945 (Washington, D.C.: Air Force History and Museums Program, 1999), pp. 112-14, 582 (note 13). For a brief, roughly written, biography of Fales see "Elisha N. Fales (1887-1970)" at www.earlyaviators.com/efales.htm.

(26.) John D. Anderson, Jr., "Research in Supersonic Flight and the Breaking of the Sound Barrier," Chapter 3 in Pamela E. Mack, ed., From Engineering Science to Big Science: The NACA and NASA Collier Trophy Research Project Winners, NASA SP-4219, The NASA History Series (Washington, DC: NASA History Office, 1998), pp. 66-69; and Steven T. Corneliussen, "The Transonic Wind Tunnel and the NACA Technical Culture," Chapter 4 in Mack, Engineering Science to Big Science, pp. 106-107. Compressibility degrades a propeller's ability to produce "propulsive thrust."

(27.) F.W. Caldwell and E. N. Fales, "Wind Tunnel Studies Aerodynamic Phenomena at High Speed," NACA Report No. 83, [1921], pp. 85-86. On p. 87, the authors noted: "An interesting variation of the flight vortices is burnished by replacing the aerofoil by a flat disk normal to the wind. There the phenomenon can be seen as a 'streamline' fog surface, converging toward a point half a dozen diameters downstream.".

(28.) Professor [William Jackson] Humphreys, quoted in Varney, "The Argonne Battle Cloud," p. 349. Varney gives nothing more than the title and last name--Professor Humphreys. This is almost certainly William Jackson Humphreys whose byline appeared frequently in the pages of the Monthly Weather Review. In 1921, Humphreys, who held a PhD in physics from Johns Hopkins University, was a senior official at the headquarters of the U.S. Weather Bureau in Washington, DC, where he served from 1908 until his retirement in 1935.

(29.) Varney, "The Argonne Battle Cloud," p. 349.

(30.) "Historic Natural Events: May 9, 1919--Cloud Formed by Aeroplane," Nature: A Weekly Journal of Science, May 3, 1930, p. 693.

(31.) While the United States was allied with France and Britain in World War I and contributed significantly to their victory in the war, the United States did not enter the conflict until April 1917. As a result, the U.S. did not experience a proportionate share of the casualties and financial cost.

(32.) Alfred F. Hurley, Billy Mitchell: Crusader for Air Power (New York: Franklin Watts, Inc., 1964), p. 31; Giulio Douhet, The Command of the Air, trans, by Dino Ferrari (Washington, D.C.: Office of Air Force History, 1983; new imprint of edition published by Coward-McCann, Inc., 1942), USAF Warrior Studies, eds. Richard H. Kohn and Joseph P. Harahan, pp. 139-40.

(33.) William Mitchell, Winged Defense: The Development and Possibilities of Modern Air Power--Economic and Military (New York: Dover Publications, Inc., 1988; originally published in 1925 by G. P. Putnam's Sons, New York), pp. xv-xvi.

(34.) Douhet, Command of the Air, pp. 6, 9-10, 50-60. Where the use of biological weapons were concerned, a nation's scientists would first develop vaccines for the agents to be used on enemy populations; the weapons would be used only after one's own population was inoculated. Mitchell also envisioned the use of poison gas. See Winged Defense, p. 47, and Hurley, Billy Mitchell, p. 62.

(35.) Douhet, Command of the Air, pp. 39, 114-19, 121.

(36.) John F. Kreis, Air Warfare and Air Base Air Defense, 1914-1973 (Washington, D.C.: United States Air Force Office of Air Force History, 1988), p. 21; Douhet, Command of the Air, pp. 17-18.

(37.) Richard Hough and Denis Richards, The Battle of Britain: The Greatest Air Battle of World War II (New York: W. W. Norton & Company, 1989), pp. 48-49.

(38.) Kreis, Air Defense, pp. 7-11.

(39.) Kreis, Air Defense, p. 12.

(40.) Mauer Mauer, Aviation in the US. Army, 1919-1939 (Washington, DC: United States Air Force, Office of Air Force History, 1987), pp. 165-67.

(41.) Mauer, Aviation in the US. Army, pp. 165-167; Eugene M. Emme, Aeronautics and Astronautics: An American Chronology in the Exploration of Space, 1915-1960 (Washington, D.C.: National Aeronautics and Space Administration, 1961), p. 9. For an account of Schroeder's flights, along with several interesting photographs, see "Schroeder's Altitude Flights, 1918-1920," at www.wpafb. af.mil/musemn/history/postwwi/saf.htm.

(42.) Mauer, Aviation in the US. Army, pp. 165-67.

(43.) Ibid., p. 167.

(44.) American Heritage Editors, The American Heritage History of Flight (NP: American Heritage Publishing Company, Inc., 1962), p. 254; Emme, Aeronautics and Astronautics, p. 160. In 1930, U.S. Navy pilot Apollo Soucek reached an altitude of 43,166 feet in a Wright Apache, regaining the world altitude record he had held in 1929. (Emme, p. 26.)

(45.) Mauer, Aviation in the US. Army, pp. 167-68; Emme, Aeronatutics and Astronautics, pp. 14, 159. While Mauer and Emme agree on the altitude that Macready reached, they give different dates for the flight. I have taken Mauer's date of September 28 as opposed to the September 18 date given by Emme, as a number of Air Force sources favor Mauer's date..

(46.) "Cloud Formation by Supercharged Plane," US. Air Service Newsletter, July 1921, p. 13, as reprinted in Monthly Weather Service Review, July 1921, p. 412.

(47.) A. M. Cambell to Air Transport Association of Canada, Letter, nd, pp. 4-5 in United Kingdom, Meteorology Subcommittee of the Aeronautical Research Committee, "Formation of Vapour Trails behind Aircraft: Copy of Pilots' Replies to Questionnaire Issued by the National Research Council of Canada," nd [early 1942], United Kingdom National Archives document DSIR 23/11406.

(48.) Mauer, Aviation in the US. Army, p. 229. For information on the P-12, see Gordon Swanborough and Peter M. Bowers, United States Military Aircraft since 1909 (Washington, D.C.: Smithsonian Institution Press, 1989), pp. 94-98.

(49.) Mauer, Aviation in the US. Army, pp. 229-230. The 94th Pursuit Squadron at Selfridge Field obtained P-12s soon after the 95th and also began operational training, including formation flying, at altitudes of between 27,000 and 28,000 feet. The 94th was led by First Lieutenant Harry A. Johnson who had been a test pilot at Wright Field. Under Johnson's supervision, his squadron began operational trials of a new oxygen system that automatically regulated the pilot's oxygen flow, which was delivered through a mask the pilot strapped to his face. (Mauer, Aviation in the U.S. Army, p. 230) In April 1930, Air Corps pilots had set a world record for formation flying at high altitude. In this feat, nineteen planes reached an altitude of 30,000 feet. The record to this point had been 17,000 feet. (Emme, Aeronatucis and Astronautics, p. 26.)

(50.) Mauer, Aviation in the U.S. Army, pp. 230-31.

(51.) Swanborough and Bowers, Military Aircraft, pp. 99, 227-30, 547-48; Mauer, Aviation in the U.S. Army, p. 365.

(52.) Mauer, Aviation in the U.S. Army, pp. 354-55; Swanborough and Bowers, Military Aircraft, pp. 104, 112.

(53.) For details about the capabilities of the Bf 109 and its operations during the Spanish Civil War, see Gerald Howson, Aircraft of the Spanish Civil War: 1936-1939 (Washington, DC: Smithsonian Institution Press, 1990), pp. 231-235. Although the 109 is widely known as the Me-109 (Messerschmitt 109), Bf 109 (where Bf signifies Bayerische Flugzeugerwerke) was the official German designation of this aircraft.

(54.) For information on the war, see Antony Beevor, The Battle for Spain: The Spanish Civil War, 1936-1939 (New York: Penguin Books, 1982, 2006).

(55.) Tarazona, Piloto de Carza Rojo, pp. 28, 31, 39-30; Howson, Aircraft of the Spanish Civil War, pp. 197-198.

(56.) Beevor, Battle for Spain, p. 208; Howson, Aircraft of the Spanish Civil War, pp. 135-37, 174-75, 193, 232. The I-15 and I-16 were designed at the same time with the expectation that their performances would complement each other. The faster, less maneuverable I-16 would attack enemy bombers, while the highly maneuverable I-15 biplane would take on enemy escort fighters. The Fiat pilots soon learned to match their better maneuverability against the superior speed of the I-16--if the later chose to engage the Fiats in close combat, the superior maneuverability of the biplane could prove decisive. The Fiat continued to be an effective fighter throughout the war when properly flown.

(57.) Howson, Aircraft of the Spanish Civil War, p. 233.

(58.) Ibid.

(59.) Tarazona, Piloto de Carza Rojo, p. 112. Tarazona's Spanish reads: "... arriba, muy arriba, como si esperaran alguna presa espantada por la jauria, vuelan los Me 109." See also p. 206 where Tarazona reports that the Messerschmitts "prowl" high above all other aircraft.

(60.) Tarazona, Piloto de Carza Rojo, pp. 75-76.

(61.) Howson, Aircraft of the Spanish Civil War, p. 200.

(62.) Tarazona, Piloto de Carza Rojo, p. 217.

A nation which once losses the command of the air and finds itself subjected to incessant aerial attacks aimed directly at its most vital centers and without the possibility of effective retaliation, this nation, whatever its surface forces may be able to do, must arrive at the conviction that all is useless, that all hope is dead. This conviction spells defeat.

Giulio Douhet, Command of the Air, p. 140.

Whatever the performance of an airplane, it seldom satisfied the airmen. They wanted to fly higher, faster, farther, and longer, this being as true of U.S. Army flyers as others. Those in the Army needed altitude to reduce the chance of detection and to lessen vulnerability to ground fire, speed to engage and defeat an enemy in aerial combat, distance and duration to spy on the enemy from the air and to attack him far within his own territory.

Mauer Mauer, Aviation in the US. Army, p. 165.

Donald R. Baucom is a 1962 USAF Academy graduate, who earned his Ph.D. in the History of Science from the University of Oklahoma in 1976. During twenty-eight years in the Air Force, he served tours as a communications-electronics officer in Spain and Thailand, taught history at the Air Force Academy, and established the official history program for President Reagan's Strategic Defense Initiative. Upon retirement from the Air Force in 1990, he returned to federal service as the civilian historian for DOD's missile defense program. His book, The Origins of SDI, was awarded the 1992 Leopold Prize by the Organization of American Historians. Dr. Baucom retired from DOD in 2003 and now lives in El Prado, NM.

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Author:	Baucom, Donald R.
Publication:	Air Power History
Geographic Code:	1USA
Date:	Jun 22, 2007
Words:	11596
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Topics:	Air forces History Military aspects Services Condensation trails Forecasts and trends Usage