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The Mil Mi-1 "Hare", Mi-2 "Hoplite", & Mi-4 "Hound"

v1.0.6 / 01 oct 16 / greg goebel

* Soviet efforts in helicopter design after World War II paralleled those of the West, with a range of experimental rotorcraft finally leading to workable production machines. The most prominent of Soviet first-generation helicopters, the piston-powered "Mi-1 Hare", was the product of the experimental design bureau (OKB) led by Mikhail Mil. It led in turn to a much larger helicopter, the piston-powered Mil "Mi-4 Hound", which was strongly influenced by the American Sikorsky S-55, and to a turbine-powered derivative of the Mi-1, the "Mi-2 Hoplite". This document provides a history and description of these early Mil helicopters, as well as the Polish PZL "W-3 Sokol" follow-on to the Mi-2.

Mil Mi-2 Hoplite


[1] IVAN BRATUKHIN & THE ORIGINS OF SOVIET HELICOPTERS
[2] MIL MI-1 "HARE"
[3] MIL MI-2 "HOPLITE"
[4] MIL MI-4 "HOUND"
[5] FOOTNOTE: PZL SWIDNIK W-3 SOKOL
[6] COMMENTS, SOURCES, & REVISION HISTORY

[1] IVAN BRATUKHIN & THE ORIGINS OF SOVIET HELICOPTERS

* Soviet rotary-wing efforts began in the 1920s, under the direction of the Central Aero-Hydrodynamic Research Institute (TsAGI in its Russian acronym). Initial work focused on autogyros, then moved on to helicopters. Early Soviet helicopter experiments were conducted by TsAGI staff such as Boris Yuriev, who had built primitive helicopters before World War I in parallel with Igor Sikorsky's early efforts, as well as by three men who would establish the Soviet helicopter industry: Ivan Bratukhin, Mikhail Mil, and Nikolai Kamov.

The early history of Soviet helicopter design is muddy and confused. It appears that a number of autogyros were developed at TsAGI during the 1930s, leading to a set of more or less unworkable experimental helicopters. The historical trail becomes more solid with Bratukhin's "Omega" demonstration prototype, with work on this machine formally authorized at Bratukhin's "OKB-3" experimental design bureau in the summer of 1941, shortly after the Nazi invasion of the USSR. The Omega had a general configuration not too different from the contemporary German Focke-Achgelis helicopters, with side-by-side rotors.

Development was protracted, but considering the magnitude of the military emergency being faced by the Soviet Union at the time, it is surprising that any work was being done at all on such a speculative technology. In any case, by 1944 Bratukhin was flying an improved prototype, the "Omega II", also known as the "G-2", where "G" meant "helicopter". The Omega 2 led in turn to a handful of "G-3" machines, flown in 1945, which differed mostly in being powered by two Pratt & Whitney Wasp Junior radial engines salvaged from US-supplied Lend-Lease aircraft, instead of less powerful Soviet-built MG-31F radials available to Bratukhin. The G-3 was supposed to be used for artillery spotting, but it saw little or no operational use.

Bratukhin continued to refine his designs, but none of them would enter mass production. A handful of "G-4" helicopters, much like the G-3 but with more powerful Soviet-built engines, were built in 1947 and 1948, to be followed by the scaled-up "B-5", with the "B" in honor of Bratukhin. Only one B-5 was built, flying in 1947 and proving unsatisfactory. A single "B-9", which was a medical evacuation version of the B-5, was also completed in 1947 but it is not thought to have flown. One "B-10", which was a largely redesigned derivative of the B-5 with an entirely new fuselage, was completed in 1947 as well and did meet expectations. However, by this time, the authorities were becoming discouraged with progress in helicopters in general, and the side-by-side rotor scheme in particular.

Bratukhin B-11

Bratukhin's final helicopter was the "B-11", with two built in 1948. It was apparently intended just to provide a reference design for more promising helicopters then being built by Mil and Kamov. One of the B-11s crashed on 13 December 1948, killing both aircrew on board. By that time Soviet officials had generally written off Bratukhin's efforts. His OKB was disbanded in 1951, and he went to work for other organizations.

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[2] MIL MI-1 "HARE"

* The most prominent early Soviet helicopter designer, Mikhail Mil, began work on rotorcraft in the 1930s, when he was on the staff of TsAGI. In 1947, the Soviet state authorized Mil to form his own OKB to design helicopters. In September 1948, the Mil OKB performed the initial flight of the "GM-1" helicopter; in 1950, the state began a competition for a fully operational helicopter, and Mil submitted the GM-1 design. The GM-1 contended against two rivals for the award, including the Bratukhin B-11 and the "Yak-100", designed by the Yakovlev OKB, well-known for their piston-engine fighters of World War II.

The Mil design won the competition. Yakovlev remained in the helicopter business for a few years longer, designing the "Yak-24", which was a tandem-rotor helicopter along the lines of the American Piasecki "Flying Bananas" but substantially bigger, with a capacity of 40 passengers. It was unsuccessful, with only 100 built, and the Yakovlev OKB then got out of the helicopter business.

The first two Mil GM-1 prototypes were lost in accidents, the second killing the pilot, but the bugs were worked out, with the machine going into Red Air Force service in 1951 as the "Mi-1". It was publicly unveiled at the Tushino Air Display in 1951. NATO assigned the type the reporting name "Hare".

The Mi-1 was comparable to the Sikorsky S-51 Dragonfly in size, capability, and broad configuration, though it was clearly different in appearance (arguably more attractive, in fact) and absolutely not a clone. It featured a main rotor / tail rotor configuration, with a three-blade wooden main rotor and a similar small tail rotor; a fully-enclosed, metal-skinned fuselage; fixed tricycle landing gear, with single wheels on all gear assemblies; and accommodations for a pilot and three passengers. It was powered by a seven-cylinder air-cooled radial Ivchenko AI-26V piston engine, providing 430 kW (575 HP).

An estimated total of 1,800 Mi-1s was built in the USSR up to 1955. Along with the standard three-passenger Mi-1, a number of variants were built in Russian production:

The Mi-1 was used as a light general-purpose helicopter, operating in a wide range of roles such as observation, liaison, search and rescue (SAR), air ambulance, and flight training. The Mi-1 was used by several branches of the Soviet armed forces, as well as Aeroflot, the state airline, and DOSAAF, the youth air corps. It was exported to Warsaw Pact nations, Soviet client states around the world, and Finland.


   MIL MI-1 "HARE":
   _____________________   _________________   _______________________
 
   spec                    metric              english
   _____________________   _________________   _______________________

   main rotor diameter     14.3 meters         46 feet 11 inches
   fuselage length         12.0 meters         39 feet 4 inches
   footprint length        16.95 meters        55 feet 7 inches
   height                  3.30 meters         10 feet 10 inches

   empty weight            1,800 kilograms     3,970 pounds
   max loaded weight       2,400 kilograms     5,290 pounds

   maximum speed           185 KPH             115 MPH / 100 KT
   service ceiling         3,000 meters        9,840 feet
   range                   620 kilometers      385 MI / 335 NMI
   _____________________   _________________   _______________________

In 1955, production was transferred to the PZL factory at Swidnik in Poland, as per a Soviet custom of assigning production of various light utility and training aircraft to Warsaw Pact nations. The PZL Swidnik factory built an estimated additional 1,700 Mi-1s up to 1965. They were given the designation "SM-1" and were powered by a license-built copy of the AI-26V engine with the designation of "LiT-3". PZL Swidnik built an improved variant, the "SM-1W", with metal blades, which was used as the basis for the "SM-1WS" air ambulance, the "SM-1WZ" cropduster, and the "SM-1WSZ" dual-control trainer.

Mil Mi-1 / PZL SM-1, PZL SM-2

The Poles also built a substantially redesigned version of the SM-1 designated the "SM-2", featuring a "fat" cabin with accommodations for a pilot and four passengers. Initial flight was in 1959, with the SM-2 going into production in 1961 and being built in a number of different versions.

* The Mi-1 was not very powerful, but that was true of many of the early helicopters in its class such as the Bell Model 47 and the Hiller Model 360, and that didn't stop the Mi-1 from being put to extensive good use in light transport, medical evacuation (medevac), and agricultural roles. It remained in service long after the end of production. It is unclear if any remain in operational service.

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[3] MIL MI-2 "HOPLITE"

* In September 1961, the Mil OKB test-flew the first of two "V-2" or "Mi-2" prototypes. The Mi-2 was the USSR's answer to the US Bell UH-1 Huey and was more or less a turbine powered derivative of the Mi-1 with twin Isotov GTD-350 turboshaft engines, each providing 300 kW (400 SHP). The Mi-2 leveraged off some Mi-1 assemblies, but the engines were mounted on top of the fuselage, while the Mi-1's engines had been inside the fuselage. The new engine arrangement provided much more cabin space and also made the machine less sensitive to changes in center of gravity from load arrangements.

The twin engines drove a three-blade main rotor, and a two-blade tail rotor. The main rotor blades featured a metal spar with either metal or fiberglass skinning. Both the main and tail rotors had electrical de-icing. The Mi-2 had fixed tricycle landing gear, with twin wheels on the nose gear and single wheels on the main gear. Skis could optionally be fitted over the wheels. There were two forward-hinged doors on the right and one on the left. There was a rubber fuel tank under the floor with a capacity of 600 liters (158 US gallons), while an auxiliary fuel tank with a capacity of 238 liters (63 US gallons) could be fitted on each side of the fuselage.

Mil Mi-2 Hoplite

The Mi-2 had seating for a pilot on the left, but no second seat for a copilot. As a utility transport, it could carry eight passengers plus the pilot in an air-conditioned cabin; or 700 kilograms (1,545 pounds) of cargo internally; or 800 kilograms (1,765 pounds) of cargo externally on a sling hook. It could be fitted with a door winch with a lifting capacity of 150 kilograms (330 pounds) for the SAR role. In the medevac role, it could carry four stretchers and a medical attendant, and in the agricultural role it could be fitted with a hopper on each side with a capacity of 450 kilograms (990 pounds) of dry chemicals or 500 liters (132 US gallons) of liquid chemicals. When the type became known in the West, it was assigned the NATO reporting name of "Hoplite".


   MIL MI-2 "HOPLITE":
   _____________________   _________________   _______________________
 
   spec                    metric              english
   _____________________   _________________   _______________________

   main rotor diameter     14.5 meters         47 feet 7 inches
   fuselage length         11.4 meters         37 feet 5 inches
   footprint length        17.42 meters        57 feet 2 inches
   height to rotor head    3.75 meters         12 feet 4 inches

   empty weight            2,424 kilograms     5,344 pounds
   max loaded weight       3,700 kilograms     8,157 pounds

   maximum speed           200 KPH             125 MPH / 110 KT
   service ceiling         4,000 meters        13,125 feet
   range, max payload      170 kilometers      105 MI / 92 NMI
   _____________________   _________________   _______________________

* The Mi-2 was only produced in Poland. An agreement was signed with the PZL Swidnik organization in January 1964 for production, with PZL building the Isotov engines as well. The first PZL Mi-2 performed its first flight in November 1965. At least 5,520 were built in all to end of production in the early 1980s. As with the Mi-1, the Mi-2 went into widespread service with Soviet forces, Warsaw Pact nations, and Soviet client states. Some Mi-2s operated by Arab forces may have fallen into Israeli hands.

Civil Mi-2s were used as utility transports, cargolifters, air ambulances, and photographic survey platforms. Military variants included:

A slightly improved "Mi-2B" was introduced late in production, featuring a better electrical system and new navigation aids. It was manufactured in a range of variants similar to those of the original Mi-2, though apparently the number of Mi-2Bs built was relatively small. In the late 1970s, PZL Swidnik also developed a variant of the Hoplite with US Allison turboshafts, known as the "Kania (Kittyhawk"), with about a dozen built.

The Mi-2 appears to have been a straightforward, reliable, and effective helicopter, and many still remain in service. Indeed, the Mil and Rosvertol organizations are now offering an "Mi-2A" upgrade, with modern turboshaft engines and avionics.

* The Mil organization did design an "Mi-3" variant in the mid-1950s, but sources are confused and confusing on the matter. Descriptions give it as an enhanced Mi-1, presumably piston-powered, with a four-blade rotor, bigger cabin, and improved avionics. Whatever it really was, it did not enter production.

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[4] MIL MI-4 "HOUND"

* The Mi-2 was actually produced out of numeric sequence, with the chronological successor of the Mi-1 being the "Mi-4", which resembled a scaled-up Sikorsky S-55 and had a similar engine arrangement, with a piston engine providing 1,270 kW (1,700 HP) mounted in the nose and driving the four-bladed rotor through a driveshaft running up between the pilot's and copilot's seats. It also had a four-wheel landing gear arrangement along the lines of that of the S-55. Initial flight of the Mi-4 was in April 1952, with the type going into service in 1953. It was assigned the NATO reporting name of "Hound" -- later "Hound-A", after other variants were introduced.

The Sikorsky S-55 had gone into production in 1950, and it's hard to believe that the Mil OKB engineers weren't heavily influenced by it -- but all that is saying is that the Soviets were smart enough to adopt good ideas when they saw them. The S-55-type engine arrangement permitted a large cabin and eased maintenance, since the service crew didn't have to clamber up on top of the helicopter to work on the engine.

In any case, the Mi-4 was different from the S-55 in almost all specific details, as well as substantially larger, empty weight being almost twice as much. The Mi-4 was powered by a Shvetsov ASh-82V two-row 14-cylinder air-cooled radial providing 1,270 kW (1,700 HP). It was originally fitted with wooden-skinned rotor blades with an unsatisfactory lifetime of only 100 hours, but that was increased to 300 hours in 1954, doubled to 600 hours in 1957, and then extended to 1,500 hours in 1960 with the introduction of all-metal rotor blades.

The Mi-4 was the first Soviet helicopter to feature hydraulically-boosted flight controls. The pilot and copilot climbed up kick-out steps to enter the helicopter through the large sliding cockpit side windows, or they could enter through the passenger door on the left. The landing gear could be fitted with skis or inflatable pontoons.

Mil Mi-4 Hound-A

The standard Mil-4 military transport version also had features not found on the S-55, including clamshell rear loading doors and a ventral tub, with the tub used for navigator observation. Typical loads for the Mil-4 included 14 fully equipped troops, or 1,600 kilograms (3,530 pounds) of internal cargo; loads could also be carried on a sling hook.


   MIL MI-4 "HOUND-A":
   _____________________   _________________   _______________________
 
   spec                    metric              english
   _____________________   _________________   _______________________

   main rotor diameter     21.0 meters         68 feet 11 inches
   fuselage length         16.79 meters        55 feet 1 inches
   footprint length        25.02 meters        82 feet 1 inches
   height                  4.40 meters         14 feet 5 inches

   empty weight            5,355 kilograms     11,805 pounds
   max loaded weight       7,800 kilograms     17,195 pounds

   maximum speed           210 KPH             130 MPH / 115 KT
   service ceiling         6,000 meters        19,700 feet
   range                   590 kilometers      370 MI / 320 NMI
   _____________________   _________________   _______________________

A confusing range of different variants was built. Sources tend to disagree in their descriptions and designations, but highlights included:


Mil Mi-4P heli-liner, Mi-4M Hound-B

There were a number of special fits, including winterized and high-altitude machines, a SAR variant, a battlefield minelayer, a variant intended to help lay gas pipelines, trials platforms, and even target drone conversions.

* About 3,500 Mi-4s were built in the USSR to end of production in 1968, and served with Soviet forces, Aeroflot, Warsaw Pact nations, and Soviet client states. The Chinese built about 545 Hounds as the Harbin "Z-5", and the type was heavily used by the People's Liberation Army.

The Soviets provided drawings to China in 1958 and the first Z-5 performed its initial flight at the end of 1959. However, Mao Zedong's government was pushing a aggressive "Great Leap Forward" economic development program at the time -- which turned out to be a fiasco that emphasized quantity at the expense of quality, and the Z-5 program suffered accordingly. Essentially the Harbin factory had to start all over again, receiving new drawings in 1961 and flying a second "first prototype" in August 1963. Metal rotors were introduced in 1966.

The Chinese appear to have built a range of variants of the Z-5 similar to the Mi-4 variants built by the USSR, and may have built a few unique variants as well. They did re-engine at least one Z-5 with a Pratt & Whitney Canada PT6T-6 "TwinPac" turboshaft in 1979.

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[5] FOOTNOTE: PZL SWIDNIK W-3 SOKOL

* The Polish PZL Swidnik organization followed up production of the popular Mi-2 Hoplite with a much improved successor, the "W-3 Sokol (Falcon)". The first of five prototypes made its initial flight on 16 November 1979, with the second, which featured many changes, following on 6 May 1982. The other three flew in 1984 and production began in 1985.

The Sokol had certain broad configurational resemblance to the Hoplite but was clearly more modern and capable. Like the Hoplite, it had a main-tail rotor configuration, twin side-by-side turboshafts on top of the fuselage, and a tricycle landing gear arrangement that seemed to have been inherited from the Mi-2 with little change. The two PZL Rzeszow TWD-10W turboshafts were much more powerful than the Hoplite's Isotov engines, providing 769 kW (900 SHP) each; and the rotorcraft was fitted with a modern four-blade main rotor system, the blades featuring swept tips, along with a three-blade tail rotor.

The fuselage featured hinged cockpit doors, sliding doors on both sides, and accommodation for 12 passengers. Including fuel, the Sokol could handle a load of up to 2,100 kilograms (4,630 pounds). There were fuel bladders under the floor providing a total internal fuel capacity of 1,700 liters (449 US gallons), and an auxiliary fuel tank with a capacity of 1,100 liters (290 US gallons) could be carried as well.


   PZL SWIDNIK W-3 SOKOL:
   _____________________   _________________   _______________________
 
   spec                    metric              english
   _____________________   _________________   _______________________

   main rotor diameter     15.7 meters         51 feet 6 inches
   fuselage length         14.21 meters        46 feet 8 inches
   footprint length        18.79 meters        61 feet 8 inches
   height to rotor head    4.12 meters         13 feet 6 inches

   empty weight            3,300 kilograms     7,275 pounds
   normal takeoff weight   6,400 kilograms     14,110 pounds

   maximum speed           235 KPH             145 MPH / 125 KT
   service ceiling         4,650 meters        15,250 feet
   range                   1,225 kilometers    760 MI / 660 NMI
   _____________________   _________________   _______________________

A total of 51 standard W-3 transports were built, with this total including 13 sold to Myanmar / Burma, and the rest flying in Polish service. Two of the Burmese and one of the Polish machines were built as VIP transports, sometimes referred to with the designation "W-3P". Other variants of the baseline W-3 included:

PZL W-3A Sokol

* With the fall of the Soviet Union, the Poles quickly turned their attention to the West, with following Sokol variants built with Westernized avionics. The type is still in production and it seems that PZL Swidnik is making a tidy business from it, though its sales are modest relative to those of helicopter giants such as Sikorsky and Airbus Helicopter. Modern variants include:

Studies were conducted for a stretched "Sokol Long" assault transport, with seating for 14 fully-equipped troops. It didn't happen, but with the Sokol still in production, it may not be entirely dead.

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[6] COMMENTS, SOURCES, & REVISION HISTORY

* Soviet-era aircraft tend to be "under-documented" in the West, though that has been changing since the fall of the USSR, and documentation on helicopters in general tends to be scarce as well. The result is that this document still leaves much to be desired in terms of completeness, and likely accuracy as well. In its defense, for the moment there doesn't seem to be much better data available, and it also provides a useful framework for plugging in more information as it becomes available.

* As concerns copyrights and permissions for this document, all illustrations and images credited to me are public domain. I reserve all rights to my writings. However, if anyone does want to make use of my writings, just contact me, and we can chat about it. I'm lenient in giving permissions, usually on the basis of being properly credited.

* Sources include:

Some details were also scavenged from various editions of JANE'S ALL THE WORLD AIRCRAFT.

* Revision history:

   v1.0.0 / 01 feb 05 
   v1.0.1 / 01 feb 07 / Review & polish.
   v1.0.2 / 01 feb 09 / Review & polish.
   v1.0.3 / 01 jan 11 / Review, added Ansat helicopter.
   v1.0.4 / 01 dec 12 / Review & polish.
   v1.0.5 / 01 nov 14 / Review & polish.
   v1.0.6 / 01 oct 16 / Review & polish.
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