The Sikorsky Ilya Mourometz series of aircraft were the world's first four-engine bombers in operational service. Igor Sikorsky, his name synonymous in the modern world for the company's line of American helicopters, was already making a name for himself in Russia as chief designer at the Russo-Baltic Railroad Factory. There, he developed the world's first four-engine aircraft - the 1913 Russky Vitaz (S-21). This spawned the basic design for the Bolshoi Bal'tisky ("Great Baltic") Type B aircraft which flew for ten minutes on May 13th, 1913. The Bolshoi Bal'tisky became the basis of the larger Ilya Mourometz (S-22).
The aircraft was conceived of as a four-engine, luxury commercial passenger transport featuring such revolutionary amenities as cabin windows, an in-flight berth complete with a washroom, a heated and lit passenger compartment separated from the crew cockpit and relatively comfortable passenger seating for up to 16 on wicker chairs. The cabin floor was also glazed over to allow for a unique passenger perspective of high-altitude flight. Doors on either side of the fuselage permitted mechanics to service and repair the engines in flight.
Though hardly a spacious aircraft by today's standards, the Ilya Mourometz (IM) was ahead of its time in many ways, much to the credit of Sikorsky's forward-thinking vision. Electricity for the cabin was provided for by a wind-powered generator while heating was accomplished through radiators. First flight of the Ilya Mourometz was achieved on December 13th, 1913 while the aircrafts first passenger flight occurred a short time later on February 25th, 1914. In this event, 16 passengers were carried over Moscow for five hours, hitting an altitude of just over 6,500 feet and reaching speeds over 60 miles per hour. Production was handled at the Russo-Baltic Carriage Factory (RBVZ) in Riga beginning in 1913.
Design of the IM was dominated by her large unequal-span biplane wings with vertical struts and heavy cabling. The wings fitted over and under the central fuselage nacelle which contained the glazed cockpit (appearing much like a glass box) and passenger compartment. The four engines were placed between the top and bottom wing spars and each featured two-blade propellers. The IM proved a large design to say the least, towering some two stories from the top wing to the bottom of the wheels when at rest. The fuselage was a conventional straight surfaced design while the empennage was distinctive in incorporating three vertical tail fins along with a traditional tailplane surface. The undercarriage was fixed and featured two main landing gears and a tail skid.
During development, the IM was test flown around the nearby areas of the Russian Imperial Capital to much fanfare. Distance flights were a common display of any new aircraft development in an effort to showcase capabilities and work out any design kinks. One such flight occurred from June 30th to July 12th, 1914 - a single-stop, 14.5-hour, round trip flight from St. Petersburg to Kiev. The arrival was noted for an engine fire that was put out in-flight while the return trip was without incident. Some unstable air forced the crew to fly above the clouds at times, this being accomplished without any sort of access to visible navigation markers on the ground.
It seemed as though Sikorky's vision of the IM as the world's first dedicated, four-engine passenger transport was all but fulfilled until the arrival of World War 1. Sikorsky worked with the Russian Ministry of War to adapt his new aircraft to become the world's first purpose-designed strategic bomber. Supply of the German Argus engine series was in doubt so the Russians were forced, in some cases, to use the less-capable French-made Salmson series. Ten such aircraft were ordered for military use followed by a further 80 as the war progressed. These converted bombers were now armed with a variety of defensive machine gun and cannon armament while the passenger compartment was revised to take on internal racks for bombs. Protection in the form of armor (as thick as 5mm) was allocated to the engines. The aircraft was accepted into Russian Air Force service in August of 1914 and was formed into the world's first dedicated bomber squadron, Eskadra Vozdushnykh Korablei ("Squadron of Flying Ships") in February of 1915. Deliveries of the first of 10 IM bombers began on December 10th, 1914.
Since the origins of Sikorky's aircraft lay in a commercial passenger venture, there were no military-trained personnel available to teach incoming IM recruits the subtleties of the large aircraft. As such, civilian pilots with IM flight experience were enlisted to do just that. Despite itself being a capable and sound airframe, these sort of events were beginning to build a very negative image for the IM. Its civilian-inspired origins, lack of trained combat crews and pilots coupled with the requests of a wartime government all worked in painting a dreary early picture of the Ilya Mourometz line. Combat experience, however, would prove the IM detractors wrong.
The IM quickly became the showcase of the Imperial Russia Air Force bombing offensive. The aircraft proved her worth, capable of delivering respectable bombloads with accuracy across distances against railroad targets and key transportation routes including bridges. The lethality of a single IM element was reportedly capable of rendering enemy positions destroyed for weeks. Imagine what an entire flight of IM systems could bring. The bombsight in these militarized IM's was of particular note at a time where a pilots vision and prowess were utilized moreso than reliance on any sort of bombing assistance devices. Accuracy and resulting successes improved the opinion of IM aircrews to the point that they would sometimes give up their defensive machine gun armament to take on a greater bombload instead.
The IM doubled as an intelligence-gathering platform in the reconnaissance role. Its high-flying, long-distance and well-defended gun positions played up to the IM's strengths.
Operationally, the IM was known for her ability to withstand combat damage to the point that she reach mythical status among her supporters as a bomber that could not be shot down. Her defensive weapons arrangement no doubt made many an enemy airmen think twice before tangling with IM's. To drive the point further, it was noted that German fighter pilots oft-refused to tangle with IM's if they could avoid it, such was her reach and defensive circle created by her guns and gunners. In fact, reports show that only one (some sources state up to three) IM bombers were ever lost to enemy fire in the air while IM crews themselves were credited with at least 10 air-to-air victories - a ratio few wartime bombers can boast. The first IM bomber was lost to no less than 4 German Albatros fighters on September 12th, 1916. Despite this loss, three of the four German fighters were also destroyed. Other Ilya Mourometz's were destroyed by Russian hands to avoid capture by Germans - such was the value of the Mourometz.
IM's were seen in a myriad of production forms throughout her wartime run from 1913 through 1917. Among these were the early No 107, Kievsky No 128, Type B No 135 and Type Nos 136 through 139's. These were differentiated by their selection of engines of either the German Argus or French Salmson brands ranging from 100 to 200 horsepower. The No 107 (appearing in 1913)utilized four of the Argus 100 horsepower engines while Kievsky (1914) saw fit to give their No 128 2 x Argus engines of 140 horsepower and 2 x Argus engines of 125 horsepower. While the Type B No 135 - appearing in 1914 - fitted 4 x Argus engines of 130 horsepower each, the Type B No 136-139 (beginning use in 1914) were outfitted with Salmson series engines in a 2 x 200 horsepower and 2 x 135 horsepower arrangement. It was not wholly uncommon to mix and match engines in this fashion while finding the right balance of output and weight all the while dealing with wartime availability and shortages.
The most common model in the IM line became the Type V (S-23) to which some 32 to 34 were produced from 1914 through 1916. Type V's were the first in the Mourometz line to be developed from the start as a bomber (the previous offerings were conversions). These were fitted with 4 x Sunbeam Crusader V8 engines of 148 horsepower each. The Type V No 151's arrived in 1915 with their 4 x Argus engines of 140 horsepower each. The Type V No 159 was a crew trainer from 1915 onwards and was fitted with just 2 x 225 horsepower engines of the Sunbeam brand - no doubt in an effort to commit engines to war-bound aircraft instead. 4 x RBVZ-6 engines of 150 horsepower each were added to the design of the Type V No 167, also appearing in 1915.
1916 brought about more faces in the IM line, this including the Type G-1 with her 4 x Sunbeam engines of 160 horsepower each. Likewise, the Type G-2 "Russobalt" utilized an arrangement of 4 x RBVZ-6 series engines of 150 horsepower each (RBVZ designating its Russo-Baltic Carriage Factory origins) and were built in 30 production examples. The Type G-3 saw fit to combine 2 x Renault engines of 220 horsepower (each) with 2 x RBVZ-6 engines of 150 horsepower each. The final IM production model became the Type E "Yeh-2" model with her 4 x Renault engines of 220 horsepower, appearing in only 10 production examples. By 1918, only 13 IM aircraft were built, effectively signaling the end of the IM's wartime production run. Some 80 total Ilya Mourometz were eventually produced.
As an aside, one IM aircraft was produced with water floats and, though it provided for some successful water-based landings and take-offs, the arrangement was not found to offer much in terms of military value. Other costly (manpower and financial) developments were also pursued with many ending up abandoned. License production was undertaken by the British and French while the Sikorsky four-engine bomber concept was copied throughout the world during the war years and beyond. The aging Ilya Mourometz line was replaced during the war World War 1 by Sikorsky's new Alexander Nevsky bomber design beginning in 1916. In all, the Mourometz line served through some 400 combat sorties against targets in Germany and Latvia, mostly from Vinnitza in Poland during the war.
The Ilya Mourometz proved herself a capable mount in her relatively short time along the Front. Despite missing out on the chance to become the world's first four-engine, multi-passenger commercial airliner, the IM instead developed many "firsts" in the realm of military aviation. The IM was the world's first purpose-designed, four-engine bomber featuring an internal bomb bay, bomb sighting device and excellent defensive armament. Specialized techniques were also developed for machine gun crews when combating multiple enemy fighters at one time. As the war rolled on, bombing effectiveness from IM platforms reached near-excellent levels.
IM's enjoyed a short-lived existence after the war with a handful still being produced. From May through October of 1921, Moscow-Kharkov line was finally able to utilized the aircraft in its originally intended role of passenger transport until these aircraft were deemed too fragile for further use. The aged technology and construction methods had finally taken their toll on this fine machine. After 1921, IM's were all but extinct with the last IM's flying in 1922. Beyond the Russian Empire Air Force and soon-to-follow USSR Air Force, Poland was the only other operator of the aircraft.
Incidentally, the name, Ilya Mourometz, is a hero based in Russian mythology.