Airplane went into World War I, actually, as a civil model with wooden structure and skin of wings and fuselage made of canvas. The air speed was around 100 km/h and endurance around an hour or two. Load capacity was just enough to take into the air a pilot and an observer. Already in the first year of the war, with slight modifications, the airplane went through a transformation from effective scout and aircraft for fire correction to successful airplane for aerial support, aerial superiority and bomber.
Until 1918, fighter airplanes could reach the speed of 200 km/h, at the same time armed with up to two synchronized fixed forward firing machine guns with the rate of fire of over 1,000 bullets per minute. At the same time reconnaissance airplanes could reach altitudes of more than 6,000 meters.
By the end of the war, typical design of fighter airplane was biplane with pulling propeller, open cockpit, while the landing gear was still fixed without the possibility to be retracted. This kind of standard fighter airplane arrangement remained until 1930s.
From fragile, poorly maneuverable aircraft made of wooden structure and canvas, fighter airplanes developed into highly maneuverable aircraft, able to perform tight turns and withstand significant damage earned in battle. To be able to tolerate significant dynamic loads during air battles and tight turns the airplane had to become more structurally strong. So the structure started to be designed from steel tubes and front skin from aluminum and its alloys. However, Germany designed already in 1915 the first airplane of all-metal structure. It was Junkers J.1 with braced load bearing structure made of steel tubes, covered by skin made of duraluminium sheets.
The tactics of fighter airplanes use was adapted to its virtues and flaws. As a rule, faster airplanes were less maneuverable and vice versa. SPAD XIII was for sure one of the fastest fighters in World War I. If such an airplane had an air duel with a more maneuverable airplane like Fokker Dr.I, it would certainly use its advantage in air speed and try to use tactics of fast attacks and escapes. Using the tactics of sharp maneuvering with tight turns would significantly reduce its chances of survival against Dr.I.
How to achieve optimal maneuverability, the most important property of the fighter airplane, kept even today considering the closest range aerial battle hidden behind very well-known term “dogfight”?
Maneuverability of the airplane manifests itself in maximum achievable angular velocity around its longitudinal and perpendicular axis, in the ability to swiftly change direction of rotation around these two axes, and in maximum achievable and sustainable turn rate.
Experience in designing World War I fighter airplanes showed that airplanes with more than one wing (biplanes and triplanes) were more maneuverable. Firstly, multi-winged airplanes have larger wing/lift surface (and smaller wing loading) which will result in the ability to pull tighter turns. Secondly, considering that during tight turns wing loading is most significant, two or three wings braced with struts and wires gives more compact unit with much higher strength and stiffness. Thirdly, with multi-winged airplanes with tendency of bigger lifting surface, single wing can be designed with smaller span and chord, thus making the airplane able to faster change the direction of rotation around longitudinal axis and have higher angular velocity around longitudinal axis. The most maneuverable World War I airplanes, considering their aerodynamic properties, were Sopwith Triplane and Fokker Dr. I, both triplanes. Their flaw was, of course, lower flight speed. Maneuverability and controllability of the airplanes can be also largely influenced by the direction of rotation of rotary engines, but this subject will be covered later.
From fragile aircraft from the early beginning of 20th century, with main intention to realize more or less safe flight with the ability to control it, till the end of World War I, in only 15 years or so, the airplanes evolved into combat machines with the efficiency crucial to bring advantage on and over the battlefield. During only four years of world conflict, military aviation progressed to the level that would have taken twenty years in peacetime.
With the development of airplanes and arms race, the aviation industry was also developing very fast. More than 225,000 aircraft of all kinds were produced during the war. British Royal Air Corp had only 140 aircraft at the beginning of the war; at the end it had an astonishing 22,000. At the beginning of war the airplanes were manufactured manually in very small batches. Until the mid of the war, the production was reorganized into massive serial production, driven by sophisticated design systems. At the same time systems of radio communication, hindsight, oxygen supply, heating, and, of course, weapon systems were also developing very intensively.
THE REAL FIGHTER
As the war months were passing by, governments of the confronted sides started to pay the greatest attention to the development of fighter aviation and combat formations. From 1916 until 1918 some of the deadliest war machines of World War I were to appear in the sky. Faster, more agile and with higher fire power, these fighters retired airplanes like Fokker E. III or FE-2.
One of the favorite British pilot’s airplanes during World War I was a small fighter produced by Sopwith Aviation, called by the pilots “Pup”. This airplane was introduced in operational units of Royal Air and Navy forces at the end of 1916. It had installed one Vickers machine gun with Sopwith-Kauper system of synchronization. Although powered, in first variants, with engine rated at only 80 hp, this less than 6 meters long airplane was probably the easiest fighter to fly among fighters of World War I. Extremely maneuverable, it had a turn rate even twice as good as some other opponent airplanes. It was also constructionally simple; at the ends of the trailing edges of each wing it had ailerons and at the middle section of the top wing there was a cutout under which the pilot was seated. The empty airplane had a weight of only 388 kilograms. Later, after the installation of more powerful Gnome Monosoupape engine rated at 100 hp, the airplane became even more maneuverable.
British used “Pup” also as a test plane for future airplanes and system development.
The idea of launching and landing fighter airplanes on the deck of ships existed long before World War I. First airplane carriers were actually battle-cruisers with installed wooden launch ramps on the top of their cannon barrels. First take-offs from the battle-cruiser were performed in November 1910, from the cruiser Birmingham. First landings were performed in January 1911, on the battleship Pennsylvania. These trials were conducted by the American Eugene Ely, in his Curtiss biplane. Although these efforts paved the way to the development of naval fighter aviation, still quite a lot of time was to pass until reliable and safe usage of airplanes on the decks of the ships was possible. The advantages of this kind of airplane usage were significant. In the time of World War I the Allies made a lot of efforts to develop a concept of fighter airplanes use from the decks of the ships. Although at the beginning in a very impractical way, this is where the story of airplane carriers began. Taking off from adapted deck of the battleships was in some way acceptable, but the real challenge for the pilot would start after returning from the mission. In fact, it was impossible to land and airplane on the mother ship, so the landing was carried out by landing on the sea in the proximity of the ship. This was, of course, extremely dangerous for the pilot.
In August 1917, the first successful landing on the deck of a rearranged battleship H. M. S Furious was performed. It was actually the first historical successful landing of an airplane on a ship in motion; performed by pilot E. H. Dunning in his Sopwith “Pup”. On the basis of Sopwith “Pup”, one more excellent British fighter airplane emerged – Sopwith Camel. This airplane, credited with 1,294 downed enemy aircraft, was the most successful fighter of World War I. It was in operational units from May 1917.
Camel was the conventional biplane of its time. Wooden fuselage and wing structure reinforced with wires, with skin made of canvas, as a standard embodied 9-cylinder air-cooled rotary engine rated at 130 hp. Compared to its predecessor, Camel was extremely demanding for flying and it asked for a skilled pilot at commands to become a deadly weapon. Reactive momentum and gyroscopic effect of precession, as consequence of propeller and complete engine rotation, were especially pronounced in Camel.
Because of the reactive momentum to rotating engine, roll rate around longitudinal axis was so high to the right that some pilots actually preferred rolling the airplane 270° to the right instead of 90° to the left. The consequence of pronounced gyroscopic effect of precession was that the airplane, during sharp left turn, had the tendency to raise the nose; in this situation the pilot should have counteracted using the rudder. This kind of airplane behavior in the air, for inexperienced and unskilled pilot, could cause airplane to stall. Pilots used to say that once a pilot has learned how to fly Camel, they will never be able to fly “normal” airplane again. Also, the fact is that this fighter with two Vickers machine guns and in hands of a skilled pilot represented real terror in the sky. Some of the most successful aces of World War I flew Camel. Some of them with more than 50 aerial victories and some with 6 victories in a single day. Many technical innovations are also contributed to Camel. Some of those innovations were used later in aviation, especially in airplanes of World War II. A version with illuminated instrumental panel for night operation was developed; a version with strengthened landing gear for airplane carrier operation was developed; a version for concept study of dive bomber was developed. Naval variant of Camel was powered by a more powerful engine, rated at 150 hp. This variant also had an option to be broken in half over hinges in position behind the pilot’s cockpit, to occupy less space on the ship deck.
Overall 5,490 Camels were produced and used in air forces of Britain, Canada, USA, Belgium, Greece. In naval version Camel was deployed on ten capital ships and 17 battle-cruisers of the British Royal Navy.
The most famous fighter designed by the French during World War I was SPAD S. XIII, developed by Société Pour L’Aviation et ses Dérivés (SPAD), from its predecessor, SPAD S. VII. Great delight of the French pilots who tested SPAD S. XIII between April and September 1917, induced the French government to order initially 2,000 of these aircraft. Until the end of the war, a total of 8,472 pieces were ordered for the French and Allied squadrons. Compared to its predecessors XIII had inline water-cooled engine rated at 235 hp, which gave it higher top horizontal speed of around 220 km/h, but also smaller maneuverability compared to its contemporaries. Because of its characteristics, pilots of S. XIII relied in aerial battle mostly on the hit and run tactics; rather than trying to outmaneuver the opponent in close combat. One more important characteristic of S. XIII was its excellent structural strength, which enabled it to reach the speed of 400 km/h in dive. Many allied aces flew SPAD S. XIII, including Rene Fonck, the top allied ace with 75 aerial victories. SPAD S. XIII coped excellently with best German fighters, until the summer of 1918, when Fokker D. VII appeared in the sky, the best fighter airplane produced during World War I.
When it made its first appearance in May 1918, Fokker D. VII showed very soon its real combat potential. With excellent control and maneuvering characteristics and operational ceiling of more than 6,000 meters, this fighter could cope with any Allied fighter. In August 1918 only, German pilots managed to down 565 enemy airplanes. The structure of D. VII was designed so that it could accept Mercedes D-IIIa, inline water-cooled engine rated at 175 hp, or more powerful, BMW IIIa rated at 185 hp, also inline water-cooled. A powerful engine and small frontal aerodynamic drag gave D. VII the rate of climb unseen before on any other fighter in war. Box-like fuselage structure was designed in the form of braced girder, made of welded steel tubes and covered by metal sheets at the front part of the aircraft, and canvas on the rear part. Two wings were braced by rigid struts, but without use of additional wires to get a more rigid wing structure. As a matter of fact, even struts were not necessary because wings themselves had enough strength; the installation of struts was merely a consequence of tradition and general disbelief in new wing concept. Among the best pilots who flew D. VII were Ernest Udet, with 62 aerial victories and Erich Löwenhardt, with 52. Although D. VII squadrons were giving big punches to Allied formations, until the end of the war their number was limited and their effect decreased by the huge number of newly built Allied airplanes, with the greatest number of Sopwith Camel and SPAD S. XIII among them. Formidable reputation of Fokker D. VII was proven by the fact that this was the only type of German Airplane that was specifically mentioned by the Allies in the Armistice ending the war, requiring Germany to surrender all D. VII to Allies. The production and use of this airplane continued for several years after the war. The American Navy and Marine Corp used it until the year 1924.
Airco DH 10
With World War I bomber airplane started its affirmation on the war scene. Until the end of the war strategic bombers could carry up to three tons of bombs at a distance of a few hundred kilometers. Although in the first months of the war, bombing was limited in manual dropping of smaller bombs over the enemy, Italy and Russia were the first initiators of strategic bombers aviation concept. Already in 1913 Russia possessed the first big 4-engine airplane, designed by famous Igor Sikorsky. With the beginning of hostilities, further 80 huge bombers “Ilya Mourometz” (named after a hero from a Russian legend) were ordered.
This bomber possessed top technology of the time; it had a closed cockpit, heating over channeling of the hot engine exhaust gasses, electrical energy supplied by a generator driven by air stream during flight. Especially useful was the option to approach the engines from inside the airplane during flight.
The formation which used Ilya Mourometz bombers, performed in the period from 1915 to 1917, above the territory of Germany and Lithuania, 400 missions with the loss of only one bomber. Successful use of strategic bomber force attracted attention of all sides in the conflict and gave crucial input to heavy bomber development.
Other famous heavy bombers of World War I were Italian Caproni Ca. 3 and German airplanes of Gothas and Fridrichshafen class, with a range of 800 kilometers with 500 kilos of deadly payload. The later developed German Giant could carry a payload of two tons of bombs, flying at the altitudes unreachable for fighter airplanes of the time.
With the development of airplanes, airplane engines were also being developed during World War I. As the race in airplane performances was waged, the engines were becoming more powerful, lighter, more reliable and with better power-to-weight ratio. It is interesting how the engine was developing through the war; from practically adapted engines that followed the logic of automobile water-cooled engine, over rotary air-cooled engines, to inline water-cooled airplane engines.
The type of engines that was used by the Wright brothers, from the beginning of the aviation story, was too complex and too heavy for airplane application. Although the mentioned engine was used successfully, with the development of fighter aviation in World War I, more reliable, simpler and more powerful engines were needed. The right engines that could cope with the challenges facing airplane designers were rotary air-cooled airplane engines. Although they appeared at the end of the first decade of 20th century, the real blossom and full application emerged during World War I. The first engine that introduced the concept of rotational air-cooled engine was French Gnome Monosoupape, from 1909.
The main idea of airplane designers of the era was to get rid of engine water cooling system, considering that it asked for installations such as ducts, chiller, and water pump. All these installations made the engine heavier and were also subjected to failures which decreased the reliability of the engine and airplane. On the other side, every engine needs cooling, especially as the engines were becoming more powerful and generated higher heat emission. The main advantage of the air-cooled engines is that they do not need water installations. But to take away the heat, the engine had to become rotational to ensure adequate airflow around the engine cylinders. Rotational air-cooled engines totally changed the logic of piston engines with internal combustion. As we all know, standard airplane internal combustion piston engine turns linear motion of the pistons inside cylinders into rotational motion of the crankshaft, which is then directly or over gearbox transferred to drive the propeller. This kind of arrangement was standard for the airplane’s drive, until the advent of the rotary engines with radial arrangement of cylinders. With the rotary engines the logic of work was totally different; here the crankshaft remained motionless, while the cylinders and complete motor were rotating around the crankshaft. So, the propeller was attached to the rotary engine and was rotating with it. This principle of engine run was preventing the engine from overheating, despite the non-existence of water cooling, considering that the engine cylinders were rotating together with the engine in the surrounding air.
Because of its principle of operation where complete, quite massive engine rotates around a motionless crankshaft, rotary engines had some nasty habits that could be characterized as flaws, but once mastered they could be turned into advantage. Strong reactive momentum and gyroscopic effect of precession were the consequence of relatively substantial mass of the engine that rotates around longitudinal axis of the airplane. Reactive momentum to the airplane is direct consequence of rotary engine rotation and has tendency to rotate the airframe in the direction opposite to the direction of engine rotation. That is why the fighter airplanes of World War I could almost instantaneously rotate to the right around their longitudinal axis, but were quite sluggish in rotation to the left. This phenomenon was so expressed that airplanes could rotate faster to the right for 270°, than to the left for 90°.
The outcome of gyroscopic effect of precession, as consequence of engine rotation, was that during, for example, right turn, the airplane had the tendency to lower the nose toward the ground. The intensity of this behavior depended on the rate of turn and the number of engine revolutions. To keep the turn coordinated in horizontal plane the pilot had to counteract with the tail rudder. For an inexperienced pilot, this flight characteristic could be deadly, especially when flying at low altitude. Sopwith Camel had an especially nasty habit when we talk about gyroscopic effect of precession. It was said for the Camel that it killed more Allied pilots than German fighters did. However, once the pilot mastered flying of one of these airplanes, he would become the most dangerous adversary in very maneuverable fighter airplane.
Rotary engines were very popular through most of World War I. They were simple and more reliable than previous water-cooled engines. Nevertheless, they used to waste great amount of fuel and castor oil for lubrication. Unburned castor oil would be thrown away from the engine, greatly in the face of the pilot who was sitting in the open cockpit, only a meter or so behind the engine. The scarf the pilots used to wear around their necks was actually a towel used to wipe off castor oil from their goggles.
Limitation of rotary engines was in their power that was practically limited to around 280 hp. So, before the end of the war, the designers of airplane engines turned again toward the inline water-cooled engines, despite their complexity. In 1917 the American company Packard Motor designed the most powerful airplane engine of the time; it was Liberty, 12-cylinder inline water-cooled engine, rated at 410 hp.
In favor of the decline of popularity of rotation engines before the end of World War I, speak the fighter planes such as SPAD S. XIII and Fokker D. VII, which used inline water-cooled engines. BMW IIIa, which was installed in the best fighter plane of World War I, Fokker D. VII, was a 6-cylinder inline water-cooled engine. When optimally balanced, it had very little vibration. Rated at 185 hp, for the time it had very high compression ratio of 6.4. The installed carburetor was adjusting fuel-air mixture depending on the flight altitude, so that constant power output could be maintained until the altitude of 2,000 meters. It was a decisive advantage over competitive engines.
An important step forward came with the inline Hispano-Suiza, V-8 engine that introduced the engine concept with aluminum block, with only inner wall of cylinders made of steel sheet; this way power-to-weight ratio was significantly raised. From the initial 140 hp, the engine power was raised to approximately 300 hp.