AIRCRAFT SURVIVABILITY – THE EARLY YEARS (PRE-WORLD WAR I TO WORLD WAR I)
by David Legg
On 17 June 1861, Thaddeus Lowe and another observer surveyed the Confederate positions located south of Washington, DC, across the Potomac River. What made this survey unusual was that Lowe and his companion were suspended in a basket below a hot air balloon at an altitude of 500 ft above the city. The observer was relaying their observations to the White House and War Department via telegraph. Near real-time intelligence collection and communication was born. However, the Confederate soldiers under observation did not let this intelligence collection go without a response and began shooting at the balloon. Over the course of the Civil War, Mr. Lowe earned the title of “the most shot-at man of the war.” . One could also say that this event ushered in the birth of “anti-aircraft” fire.
In September of 1911, the Ottoman Empire invaded modern-day Libya, which resulted in a war with Italy. Later, on 23 October, Italian Air Force pilot CPT Carlo Piaaza flew the first wartime mission using an airplane. He flew his Bleriot XI on a reconnaissance mission near Benghazi. The first time an airplane was hit by gunfire during war followed soon after. Italian Air Force pilot CPT Ricardo Moizo’s Nieuport IVG airplane was hit by three bullets on 25 October.
One of the earliest, if not the earliest, attempts at armoring aircraft was accomplished by Louis Bleriot in 1913. Bleriot—who is better known for conducting the first flight, in a heavier-than-air aircraft, across the English Channel in 1909—built a canard pusher monoplane, designated the Bleriot XLII. Flight Magazine described the aircraft as follows: “The body of the machine, which is arranged so that the observer lies flat and looks through windows, is covered with steel plate to protect the occupants from rifle fire.”
Bleriot continued his work on armored observation airplanes, which included the Bleriot XXXVI and XXXIX tractor-configured monoplanes. The latter aircraft was armored with 3-mm chrome nickel from its nose to aft of the pilot’s seat. Testing was conducted on the armor, and it was determined that rifle bullets fired at a range of 400 yards would either glance off or dent the armor [2, 3].
Subsequent to Bleriot’s efforts, the first operational requirement for armored aircraft (i.e., survivability) was issued in September 1913 by GEN Félix Bernard, Director of French Military Aviation . Unfortunately, the instruction was never implemented, and the French would make little use of armored aircraft in the forthcoming conflict.
THE RISE OF THE COMBAT AIRPLANE
World War I saw the first large-scale use of the airplane in combat roles. Initially, operations were limited to the use of unarmed airplanes conducting reconnaissance of enemy field positions and troop movements. As opposing reconnaissance airplanes came more frequently into contact, their pilots and observers would exchange pistol or rifle shots at each other. Other pilots took more unusual measures. Russian pilot CPT Alexandr Kaskov dragged hooks suspended by cables behind his Morane monoplane and brought down an Austrian biplane. He would later become Russia’s leading ace of the war, scoring 20 aircraft kills.
With the introduction of improved cameras and radio, the airplane became so effective as an intelligence gatherer that some sort of countermeasure rapidly needed to be found. Here, the French took the early initiative in developing the fighter airplane as we know it. On 5 October 1914, the first credited air-to-air shoot-down was accomplished by French pilot SGT Joseph Franz and his gunner, CPL Louis Quenault, in their Voisin Type III biplane. Quenault fired a few dozen rounds from his Hotchkiss 8-mm machine gun, hitting the German Aviatik B.I biplane’s fuel tank, catching the plane on fire, and causing its loss.
As the war continued from 1914 into 1915, more powerful engines were fitted to reconnaissance airplanes, thereby increasing their performance, allowing for the carriage of more payload, and leading to the expansion of the airplane’s role to include infantry support (i.e., close air support) and eventually strategic bombing. The success of the airplane in these mission areas resulted in the development and employment of countermeasures, consisting of ground-based anti-aircraft artillery defenses, dedicated fighter airplanes, and, in the case of England, an integrated air defense system (IADS) for the protection of London.
GUNS VS. PLANES
The Allied and Central Powers’ tactical level anti-aircraft defenses employed machine gun and anti-aircraft artillery units (such as those shown in Figures 1 and 2). These units included personnel, equipped with binoculars, dedicated to aircraft spotting and manned range finder equipment.The British Vickers machine gun’s capabilities are representative of a typical Allied or Central Powers machine gun. The machine gun was of .303-inch caliber with a 450-rounds/min rate-of-fire, a 2,440-ft/s muzzle velocity, and a 2,000-yd effective range. The Germans also fielded a variety of medium-caliber anti-aircraft cannon. The 3.7-cm Flak M14 automatic cannon fired a 37-mm x 95R Hotchkiss round with a 250 rounds/ min rate-of-fire, a 1,620-ft/s muzzle velocity, and a 2,500-yd effective range.
Larger-caliber anti-aircraft guns were also fielded by both sides. The German KRUPP 7.7 CM K-Flak fired a 76-mm high explosive (HE) with a 20 to 25-rounds/ minute rate-of-fire and an effective range of 15,000 ft. Few aircraft were actually directly shot down by these larger-caliber anti-aircraft guns, each requiring an average of 4,000 to 4,500 shells, but these guns were often employed in aerial barrages to deny airspace to aircraft rather than to simply shoot down individually targeted aircraft. These barrages brought attacking aircraft within range of defensive machine guns.
During 1916 and 1917, as surface-to-air defenses became more prevalent and lethal, both sides sustained heavy losses of infantry support aircraft. During the Battle of Cambrai, Australian and British DH.5 aircraft were used heavily in trench strafing, bombing, and support. Australian Flying Corps No. 2 Squadron took 35% casualties. The DH.5 incorporated no vulnerability reduction (VR) features.
The Allies did little to respond to the mounting losses. The Germans, however, did. The German Inspectorate of Aviation Troops issued a 1917 spec for armored infantry support aircraft (J-type aircraft). The pilot, gunner, engine, and fuel were to be protected by 5-mm steel armor against .303-inch-caliber projectiles. The following aircraft were developed in response to the 1917 J-type specification:
- The AEG J.I used 5-mm steel armor (400 kg bolted onto the metal frame) to protect the cockpit, observer/ gunner position, and engine; double lift-bracing wires were used for structural redundancy; and double control cables were used for flight control redundancy. The AEG J.I radiator remained unarmored to save weight; however, this design made the aircraft vulnerable, resulting in the follow-on AEG J.II version adding radiator armor.
- The Albatros (Alb) J.I included the same features with the addition, in 1918, of a 20-mm anti-tank Becker cannon.
- The Junkers J.I (shown in Figures 3 and 4) used a 5-mm-thick armored “bath tub” to protect the crew, engine, fuel tanks, and wireless equipment from ground fire. The fuselage, flying, and control surfaces were made of steel tube. The fuselage, from the back of the gunner to the rear, was covered in fabric while the flying and control surfaces were covered in .015-inch-thick corrugated duraluminum sheeting. Control lines ran through steel tubing in the wings. The armor protecting the engine was hinged to provide access for maintenance (as seen in Figure 3). There is no recorded instance of a Junkers J.I being shot down. On 23 September 1917, a Junkers J.I returned to base with 85 hits.
Without a doubt, one could call these J.I series of aircraft the A-10 Warthogs of World War I.
OTHER VR EFFORTS
As the war drew to a close, additional VR features were being developed by the Germans. The Zeppelin (Dornier) D.I and Siemens-Schuckert Werke D.VI fighter aircraft designs both included a jettisonable fuel tank carried underneath the fuselage. In case of fire, the fuel tank could be jettisoned by the pilot. Note that these tanks contained the entire fuel load of the respective aircraft. Thus, once the tank was jettisoned, the pilot would have to glide back to his side of the front lines. These aircraft were to have been deployed in 1919 had the war continued.
The Germans also developed the specialized AEG DJ.I single-seat armored fighter designed to attack Allied ground attack aircraft flying at low altitude. The design features included cockpit, engine, and fuel tank protection provided via a steel frame, forming an integral part of the airframe and including the replacement of vulnerable wing and wing-to-fuselage bracing wires with substantial I-section struts. The first flight of the AEG DJ.I was in 1917, but it was too late for service in World War I.
While the French had developed the first “operational requirement” for armored aircraft, it appears that they had not made much progress beyond generating the requirement. However, the French Air Force did employ the Salmson 2 A2 reconnaissance airplane, which included self-sealing fuel tanks. And in the final year of the war, the French military defined requirements for a two-seat tactical reconnaissance aircraft with light armor to protect it from small-caliber fire at distances over 300 m. The Salmson 4 Ab2, a fairly straightforward development of the successful Salmson 2 A2 observation aircraft, was built in response to these requirements. The additional weight of the armor was compensated by giving the Salmson 4 Ab2 a three-bay biplane wing of larger span. A dozen of these aircraft were in service when the war ended; no more were built.
The Royal Flying Corp (RFC) also eventually developed an armored close air support aircraft. The Sopwith T.F.2 Salamander (shown in Figure 5) was an armor-clad version of the Sopwith Snipe fighter. The T.F.2 included 492 lb of armor to protect the pilot and included double control levers on the ailerons and corresponding double inter-aileron wires for flight controls redundancy (as seen in the figure). The double wires and the upper wing aileron actuators can be seen in the The prototype underwent its initial trials in April 1918 and was sent to France for evaluation in May 1918. By the end of the war, only 37 T.F.2’s had been accepted by the RFC, and only two of these were in France. With the end of the war, there was no need for a specialist close support aircraft, so no squadron was ever fully equipped with the T.F.2, and it disappeared from RFC service altogether by the mid-1920s .
CAMOUFLAGE AND SUSCEPTIBILITY REDUCTION
While the survivability of World War I aircraft was primarily limited to the development and employment of VR features, there were limited efforts to address susceptibility reduction.
The primary observable exploited during World War I was visual signature. Therefore, susceptibility reduction was primarily limited to camouflage painting of aircraft. The French aircraft were typically painted using a six-color camouflage consisting of ecru (which is French for raw or unbleached), light green, dark green, chestnut brown, and black on the upper surfaces with a light yellow finish on the lower surfaces.
The RFC primarily used a standard scheme of khaki-green on the upper surfaces with clear-doped fabric underneath. On RFC Home Defense Squadron night-fighting aircraft (based in England), the white portion of the national insignia was sometimes painted over in khaki-green to reduce observability by German bomber crews.
Due to the unique look-down background of the barren, shell-holed, front line positions of the Western Front, the RFC also experimented with camouflage that was specific to this environment. The camouflage was designed to hide low-flying close air support aircraft from higher flying fighter aircraft. A Sopwith T.F.2 Salamander was painted in this experimental camouflage per Confidential Information Memorandum No 733, dated 3 September 1918. Four colors were used: dark purple earth, green, light green-grey, and light earth. The outline of each area was picked out by a black line varying in width from 2 to 4 inches. The upper-wing roundels were of different diameters, presumably to confuse the aim of any attacking aircraft.
In addition to this type of camouflage, the German Air Force experimented with a more aggressive approach. A Fokker Eindecker fighter was covered with Cellon, a transparent material from acetate cellulose that was developed in 1901 as a replacement for the explosive nitrocellulose. Overall, the woven Cellon had a thickness of 0.4 mm. While existing photographs of an in-flight side-by-side comparison of a conventional fabric vs. Cellon-covered Fokker Eindecker illustrate the effectiveness of the Cellon, the fabric had a few drawbacks. Cellon strongly reflected sunlight in some conditions and resulted in an increase in detection. In addition, the cellon stretched and fluttered in rain, which resulted in fabric tears, loss of lift, and potential crashes. Thus, Cellon was not adopted for use.
THE EARLY IADS
World War I also saw the implementation of the first IADS. In response to German Zeppelin and bomber raids, the London Air Defense Area (LADA) was developed and was fully operational by September 1918. The LADA brought together units composed of coastal and inland observation posts, sound locators, searchlight and anti-aircraft artillery stations, balloon aprons, and fighter aircraft. Reports from the units were fed through to subcenters and then onward from the subcenters through to the LADA central operations room. Reports were then displayed in the LADA central operations room on a squared map. Ten plotters transferred the incoming information with different-colored symbols and discs onto maps and a vertical plotting chart.
Searchlight crews were connected with the squadron headquarters by telephone. The searchlights were placed forward of the nearest RFC Home Defense Squadron airfield to allow time for the defending aircraft to reach the required height of 5,000 ft to intercept. When news was received that enemy aircraft were approaching, the normal practice was to send up two or three aircraft from each flight to patrol the specified areas.
Each balloon apron consisted of three Caquot balloons (and two spherical types) joined together by a horizontal wire, and from this were suspended 1,500-ft-long weighted wires, set 50 yd apart. These balloon aprons formed obstacles to day and night bombing aircraft. The first apron started operating on 6 October 1917, and by the end of the war, 10 aprons were in operation. The aprons had considerable morale effect on the German pilots, and in March 1918, German General von Hoeppner made a report that “the aprons had increased enormously, and that they added greatly to the difficulties of the attack. If they were increased and improved much more, they would make a raid on London almost impossible” .
While the LADA was only declared fully operational near the end of the war, it formed the basis of the IADS that would later prove key to the success of the Royal Air Force during World War II’s historic Battle of Britain.
The efforts put forth by the Allied and Central Powers in World War I were the beginnings of what we consider aircraft combat survivability today. And many of the modern VR efforts and technologies still being developed can trace their roots back to this first global conflict. Unfortunately, for many aircrews at the beginning of World War II (as well as in many of the conflicts that have occurred since then), these early lessons learned over the bloodied skies of France during the early 1900s would have to be relearned.
ABOUT THE AUTHOR
David Legg is currently the Fixed-Wing Aircraft Branch Head of the Naval Air Warfare Center – Aircraft Division. With more than 32 years of experience in the aircraft survivability discipline, he has also served as the Survivability Team Lead for many U.S. Navy aircraft, including the P-8A Maritime Patrol and Surveillance Aircraft, and weapons programs, and he assisted in the rapid development and implementation of tactical paint schemes for in-theater U.S. Marine Corps helicopters during Operation Desert Shield/Storm. Mr. Legg was named a NAVAIR Associate Fellow in 2011 and holds bachelor’s degrees in mathematics and mechanical engineering from Saint Vincent College and the University of Pittsburgh, respectively.