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Mi-6 HOOK - Development (MIL)

Having successfully completed the construction of the Mi-4, the Mil OKB started designing a new rotorcraft with an even greater load-carrying capa-city. The development of combat tactics employed in military conflicts of the early 1950s was analysed; this enabled the Chief Designer to draw the conclusion that the need had arisen for a heavy transport and troop-carrying helicopter capable of airlifting combat materiel weighing up to six tonnes (13,228 lb), such as artillery pieces with tractors, heavy trucks and self-propelled guns for the airborne assault troops. All previous attempts to build such a big machine undertaken by helicopter firms both at home and abroad had failed dismally. Nevertheless, the young team of designers confidently took on the task. Already at the end of 1952 the first project versions of the new machine were brought out by the OKB's General Arrangement Section; the new machine was allocated the manufacturer's designation VM-6 (Vertolyot Mil'a - Mil's helicopter, load-carrying capacity 6 tonnes).

Despite the opinion of the most authoritative Soviet and foreign specialists who expressly recommended a twin-rotor tandem layout for heavy helicopters, M. L. Mil opted for a single-rotor layout for this machine. He took the bold decision to design a five-blade main rotor with a diameter in excess of 30 metres (98 ft 5 in). By comparison, the rotor diameter of the biggest helicopters of the time did not exceed 25 m (82 ft). No experience was available in designing the main gearbox for such a heavy helicopter, either. Besides, preliminary calculations showed that piston engines would be unsuitable for this kind of rotorcraft. The OKB was facing the task of getting to grips with the introduction of new turboshaft engines. The VM-6 was designed around a single TV-2V gas turbine engine then under development at OKB-19 headed by Chief Designer Pavel A. So-lov'yov. The TV-2V was based on the TV-2F turboprop engine developed for fixed-wing aircraft by OKB-276 headed by Nikolay D. Kuznetsov. The free-turbine layout of the helicopter-specific gas turbine engine made it possible to regulate the main rotor's RPM within a range which ensured maximum fuel efficiency and the greatest possible range for the helicopter. The project envisaged placing the engine above the cargo hold. Its location forward of the main gearbox was expected to ensure the required CG position. The VM-6 was meant to be the first gas turbine-powered helicopter in the world, and the powerplant layout developed for this helicopter came subsequently to be regarded as a classic one.

In the course of consultations with the customer on the parameters of the project the required payload was increased half as much again - which, accordingly, led to substantially greater dimensions. Hence the OKB had to undertake a redesign, adapting the machine to a powerplant comprising two TV-2V turboshafts. Besides, the customer envisaged the use of the transport and troop-carrying helicopter in some other roles requiring the ability to fly at speeds of up to 400 km/h (249 mph). Consequently, the OKB had to consider a compound helicopter layout, which was in vogue at the time, for the VM-6. In this version the machine was to feature detachable wings equipped with powerful high-lift devices and provided with two tractor airscrew units for forward propulsion. Such wings not only made it possible to reach forward speeds comparable to those of fixed-wing aircraft; they also off-loaded the main rotor in forward flight. This reduced the stresses in the main rotor blades and decreased the level of vibrations in the airframe. The preliminary design project of the VM-6 transport powered by two TV-2Vs was ready by the end of 1953, but M. L. Mil still had to convince the customer of its feasibility. A directive of the Council of Ministers calling for the development of the giant rotorcraft was issued half a year later, on 11th June 1954. In accordance with this directive the V-6 (as the VM-6 came to be designated since then) was regarded as a "new means of airlifting army units... and almost all types of artillery pieces employed at the [army] division level" and was intended to transport a 6-tonne (13,228-lb) payload at the normal all-up weight, 8 tonnes (17,637 lb) in overload configuration and 11.5 tonnes (25,358 lb) in the event of flights over short distances. The helicopter was designed simultaneously in transport, troop-carrying and medevac versions. For the first time the carriage of underslung loads was envisaged from the outset. At the same time, the design bureau headed by N. I. Kamov was allotted the task of designing the Ka-22 rotary-wing aircraft of approximately the same class.

By the end of 1954 the Mil OKB had completed the advanced development project of the V-6. The machine's layout was frozen, and on 1st June 1955 the Government Commission gave its approval to the full-scale mock-up. Shortly afterwards Plant 329 began manufacturing the parts of the first prototype of the helicopter which was now officially designated Mi-6 (izdeliye 50 - article 50). These parts were so big that the walls of the workshops had to be dismantled to extract them for final assembly. M. N. Pivovarov was chief project engineer for the Mi-6; the subsequent flight test programme was the responsibility of engineer D. T. Matsitskiy. N. G. Roosanovich was appointed vice-chief project engineer, supervising the construction of the new heavy transport and troop-carrying helicopter.

The project was unique, which meant that virtually all parts and assemblies of the Mi-6 had to incorporate novel design features. The OKB engineers developed a radically new all-metal main rotor blade design which utilised a steel spar and a rib-and-stringer structure divided into separate sections. The latter were attached to the spar only in one point (at mid-span) and were not rigidly interconnected; hence they were not subjected to stresses as the blades flexed, which relieved the blade structure of considerable variable loads. The spar consisted of three tubes bolted together through flanges. The blades were of trapezoidal planform. Later, in 1959-62, production was mastered of a one-piece extruded tubular spar with variable cross-section and variable wall thickness; it superseded the three-piece spar which was heavier and more labour-intensive to manufacture. Production methods were constantly improved so as to increase the tubular spar's dynamic strength and service life; changes were also introduced into the design of the rib-and-stringer sections and their attachment to the spar. Trailing-edge pockets with a honeycomb filler made of metal foil were introduced. The blades became rectangular in planform. As a result of these modifications, the performance of the blades was improved and their service life was increased from 50 hours in 1957 to 1,500 hours in 1971. The high forward speed specified for the helicopter dictated the use of high-speed airfoils at the blade tips. The rotor hub of the Mi-6 was fully articulated and, for the first time in Soviet helicopter design practice, equipped with hydraulic dampers. The AV-63 anti-torque rotor had all-wooden blades of trapezoidal planform. The main rotor blades were electrically de-iced; the tail rotor had alcohol de-icing.

The power of the engines was distributed through the R-6 four-stage planetary main gearbox to the main and tail rotor shafts, as well as to the drive of the cooling fan and to the generators, hydraulic pumps and other accessories. The torque at the gearbox output shaft reached 60,000 kg.m (433,980 lb.ft). Engineers abroad did not succeed in developing a gearbox of the same capacity until 17 years later. The first Mi-6 prototypes were powered by two TV-2VM helicopter-specific gas-turbine engines with a take-off rating of 5,500 shp and a nominal rating of 4,700 shp. In 1959 these were supplanted on production Mi-6s by the new D-25V turboshafts with an identical power rating; the new engine had been developed by the same Solov'yov OKB as a derivative of the D-20P turbofan engine intended for fixed-wing aircraft. The D-25V was shorter and lighter than the TV-2VM, and the free turbine's direction of rotation was changed. Hence the helicopter was equipped with a new R-7 main gearbox featuring a modified lubrication system.

The aerodynamically clean fuselage was of all-metal semi-monocoque riveted construction. It comprised four main subassemblies: the forward fuselage, the centre fuselage with hydraulically-actuated aft loading doors and ramps, and the tailboom terminating in a tail fin. The fin-shaped part of the tail boom off-loaded the anti-torque rotor and improved the Mi-6's handling in flight. The forward fuselage housed the flight deck for the crew comprising two pilots, a navigator, a flight engineer and a radio operator. It also housed most of the basic equipment and an NUV-1M flexible mount with a forward-firing Afanas'yev TKB-481 (A-12.7) heavy-calibre machine-gun. The centre fuselage was occupied by the upper compartments: engine bay, main gearbox bay and fuel tank bay. Beneath them was the cargo hold with a volume of 80 m3 (2,825 cu ft). Measuring 12 x 12.65 x 2.5 m (39 ft 5 in x 8 ft 8.5 in x 8 ft 2.5 in), the cargo hold had dimensions similar to those of the Antonov An-8 and An-12 troop-carrying aircraft. 61 easily detachable tip-up seats could be installed along the sides and in the centre of the cargo hold. Their number was in-creased to 65 and then to 90 on later versions. In the ambulance version 41 stretcher patients and two medical attendants could be accommodated in the cargo hold. This was by no means the limit of the helicopter's capacity. Under extreme conditions, when operating in third-world countries, the helicopter could accommodate as many as 150 persons. The stressed cargo floor with cargo tie-down cleats allowed various kinds of vehicles and heavy cargoes to be transported. Wheeled and tracked vehicles could be driven into the helicopter under their own power. The Mi-6 could airlift and unload at the place of an assault operation two ASU-57 self-propelled guns, or a BTR-152 armoured personnel carrier, or guns and howitzers of different types with their tractor vehicles, or materiel for engineer troops with a total weight of up to 12 tonnes (26,455 lb). The floor of the cargo hold had a hatch which could be equipped with an external cargo sling system for the carriage of underslung loads or a winch for rescue or cargo-lifting operations. The detachable sling system for the carriage of underslung loads permitted the helicopter to transport bulky loads weighing up to 8 tonnes (17,637 lb).

The space under the cargo hold floor housed eight bag-type main fuel tanks; the three service tanks were placed above the cargo hold ceiling. Provision was made for two external strap-on tanks, each holding 2,250 litres (495 Imp gal), which could be mounted on the fuselage sides on special brackets.

To improve the helicopter's stability and handling, the tailboom was fitted with variable-incidence stabilizers. Stabilizer incidence was controlled from the flight deck by the collective pitch lever. The flight control system included powerful hydraulic boosters powered by the main and back-up hydraulic systems. In addition, the helicopter had an auxiliary hydraulic system which actuated the loading ramps, aft loading doors, windscreen wipers etc. The Mi-6 had a tricycle undercarriage with a tail bumper. The main units and nose unit had oleo-pneumatic shock absorbers. The mainwheels were provided with brakes and, on some versions, with spats. The castoring nose unit had twin wheels.

The flight instrumentation, navigation equipment and radio equipment installed on the Mi-6 enabled the helicopter to operate by day and night and under adverse weather conditions. The Mi-6 project provided for the installation of an AP-31V three-channel autopilot which had been tested on the Mi-4 and was capable of stabilising the helicopter in the yaw, pitch and roll planes. In 1962 it was replaced by the improved four-channel AP-34V autopilot which, in addition, provided altitude stabilization. As distinct from its predecessor, this autopilot was incorporated into the control system in parallel, not consecutively, which made the helicopter much easier to fly.

The Mi-6 prototype was completed in the Design Bureau's assembly workshop at the Zakharkovo airfield. Alongside with the construction of the prototype the OKB engineers conducted thorough and time-consuming fatigue tests of load-bearing units: the rotor head and main rotor blades; the tail rotor; the main gearbox mount and the swashplate. In October of 1956 the first example of the Mi-6 was ready in wingless configuration; only the manufacturing of the main rotor was lagging behind. For this reason the helicopter was initially used for fatigue tests, the main rotor being temporarily replaced by an aerodynamic brake ("club rotor"). Not until June of the following year was the main rotor finally assembled and installed on the Mi-6. Thus, the fatigue test airframe was turned into a flying prototype. On 5th June 1957 test pilot R. I. Kaprelian took the Mi-6 into the air for the first time, and on 18th June he already made a circuit of the airfield. Two days later the Mi-6 was flying in formation with two Mi-1s acting as chase aircraft.

The design of the machine proved to be sound, and it successfully passed the first stage of the preliminary tests. On 30th October 1957, in one of the test flights, a crew captained by R. I. Kaprelian lifted a payload of 12,004 kg (26,469 lb) to an altitude of 2,432 m (7,977 ft). In this flight the Mi-6 displayed a cargo-lifting capacity amounting to twice the record figure attained previously by the Sikorsky S-56 heavy helicopter in the USA. Since 1957 the Soviet Union/Russia has always ranked first in the development of helicopters with the biggest dimensions and cargo-lifting capacity. The first record-breaking flight was followed by others. On 16th April a crew headed by test pilot S. G. Brovtsev lifted a 5-tonne (11,022-lb) payload to an altitude of 5,584 m (18,316 ft), and a crew captained by R. I. Kaprelian lifted 10 tonnes (22,050 lb) to 4,885 m (16,023 ft). In September of 1962 the Mi-6 reached an altitude of 2,738 m (8,981 ft) with a payload of 20.1 tonnes (44,320 lb); the crew was captained by R. I. Kaprelian. In these record flights the helicopter's take-off weight reached 48 tonnes (105,840 lb). The take-off weight of helicopters abroad to this day does not exceed 34 tonnes (74,970 lb). Only 12 years later did the Mi-6 cede the "title" of the biggest helicopter and most potent cargo-lifter to its stablemate - the giant Mi-12 twin-rotor helicopter which made use of the powerplants and some other proven components of the Mi-6.

In its time the Mi-6 was arguably not only the helicopter with the highest lifting capacity but also the fastest one. The machine's high power-to-weight ratio, coupled with the well-chosen aerodynamic layout, enabled the Soviet pilots to use it for setting several world speed records. On 21st September 1961 a crew headed by N. V. Lyoshin reached 320 km/h (200 mph) on a Mi-6 - a speed which had long been considered unattainable for helicopters. In this connection the American Helicopter Society awarded the Mil Design Bureau the prestigious Sikorsky Prize in recognition of the "outstanding achievement in the development of the art of helicopter construction". Two years later, a crew captained by B. K. Galitskiy attained an even better result - their Mi-6 covered a 100-km (62-mile) distance with an average speed of 340 km/h (211 mph). In all, the Mi-6 set 16 world records.

In February 1958 Plant No. 329 completed the assembly of the second flying Mi-6 prototype. It differed from its predecessor in being fully equipped in accordance with the original project - that is to say, it was fitted with two-position steerable wings, a sling system for external loads, the AP-31 autopilot etc. The wing panels of torsion-box construction, with a total area of 35 m2 (376.5 sq ft), had a different angle of incidence to port and starboard to allow for the asymmetrical main rotor downwash. When entering the autorotation mode the pilot could set the wings at a lower incidence. The manufacturer's tests of the Mi-6 were successfully completed in December 1958.

The joint State trials commenced in the summer of 1959. However, they had to be suspended shortly afterwards due to a decision to use D-25V engines on the Mi-6 forthwith instead of TV-2Vs. While the prototype was being re-engined, the Government decided to save time and, repeating what had been done with the Mi-4, ordered the Mi-6 into production without waiting for the official acceptance of the helicopter. The Armed Forces were so much in need of heavy helicopters that production of the Mi-6 was started simultaneously at two factories: Plant No. 23 named after Mikhail V. Khrunichev in Moscow and Plant No. 168 in Rostov-on-Don. In Moscow, the Mi-6 had a small production run: 50 machines were built in 1960-62. The Khrunichev plant then switched to manufacturing rocketry and spacecraft, and Plant No. 168 remained the sole manufacturer of the Mi-6. The first four production machines from the Rostov plant were completed in 1959. A subsidiary branch of the Mil Design Bureau was established at the factory to deal with production, development and further modification of the Mi-6. Production of the type in Rostov continued until 1980, when the Mi-6 was supplanted in the workshops of the enterprise (by then known as the Rostov Helicopter Production Association - RVPO) by a new-generation rotorcraft, the Mi-26. Sometimes production reached as many as 74 machines a year, as was the case in 1974, for example.

The first Mi-6 re-engined with the new D-25Vs made its appearance at the factory flight test facility in the spring of 1959. The previously suspended joint State trials were resumed without delay; in the course of these trials the work of the new assemblies was subjected to close scrutiny and unorthodox flight modes were mastered. Tests with the carriage of underslung loads were undertaken in 1960, and in the following year a landing was performed in autorotation mode. The final stage of the joint State trials was completed in December 1962, and in the following year the Mi-6 officially joined the Soviet Air Force inventory. The flight testing of the Mi-6 was conducted by well known Soviet test pilots G. V. Alfiorov, S. G. Brovtsev, B. V. Zemskov, R. I. Kaprelian, V. P. Koloshenko, N. V. Lyoshin, Ye. F. Milyutichev and others.

As the trials progressed, Air Force units took conversion training to the Mi-6. The type was put on the strength of independent helicopter regiments. In the subsequent years the Mi-6 was successfully used in combat operations by the Soviet Air Force and by air forces of a number of other countries; this included operations during the Arab-Israeli conflict, the Indo-Pakistani war, the wars in Vietnam and Afghanistan. Along with the baseline versions (assault, cargo and ambulance versions), some other variants were supplied to the Air Force. From 1963 onwards the Mi-6 came to be widely used in the national economy. Introduction of the Mi-6 into Aeroflot service contributed to the development of the remote areas of the Far North, Siberia and the Far East. These helicopters transported underslung outsize cargoes, took part in the construction of oil pipelines, erected high-voltage power line pylons, installed heavy and bulky equipment at major industrial sites. The use of the Mi-6 in the flying-crane role helped to save time and cut costs during installation and construction work.

Export deliveries of the Mi-6 began in 1964, more than 60 machines being supplied to a number of countries. Indonesia, the United Arab Republic and Pakistan were the first foreign customers. Later, Mi-6s were operated in Algeria, Bulgaria, Ethiopia, India, Iraq, Peru, Poland and Vietnam. In 1965 the Mi-6 was demonstrated with great success at the International Aerospace Show in Le Bourget (Paris). On numerous subsequent occasions the Mi-6 represented the Soviet helicopter construction at major international exhibitions and air shows.

Concurrently with the service introduction of the Mi-6, the Mil OKB engineers continued upgrading and perfecting the units and assemblies of the helicopter. The main undercarriage units were modified to include two-chamber shock absorbing struts featuring a fluid by-pass system and a spring-loaded damper; this helped eliminate the possibility of ground resonance. From 1967 onwards the Mi-6 was provided with an Ivchenko AI-8 auxiliary power unit (APU) for starting the engines; two supplementary fuel tanks holding 2,260 litres (497 Imp gal) apiece were installed in the cargo hold, giving the helicopter a ferry range of 1,450 km (901 miles). The introduction of fixed wings instead of steerable ones simplified operational procedures and reduced airframe weight. In the following year the stabilizers were reinforced, and 10 years later a new stabilizer design of the "elastically attached" type was introduced.

In the mid-1960s the first experimental metal/plastic rotor blades were developed and tested in the Mil OKB. They featured a one-piece glassfibre-plated steel spar. In 1968 some Mi-6s were fitted with experimental blades featuring a steel spar and glassfibre ribs and stringers. Lightened blades featuring a spar with thinner walls were tested in 1972. During this period several experimental versions of the anti-torque rotor were tested, featuring co-located hinges and glassfibre blades; four types of intake dust filters for the powerplant were tried. Changes were introduced several times into the flight deck and cargo hold interior and avionics suite. Following the introduction of a new autopilot, a device for stabilising the main rotor speed was installed on the Mi-6 in 1967. Five years later the Mi-6 received a system for pressurising the fuel tanks with inert gas to reduce the danger of explosion if hit by enemy fire. From 1974 onwards production Mi-6s were equipped with mechanical devices to facilitate the loading and unloading of wheeled vehicles. Several experiments were made with the external load sling system, its capacity being increased to 12 tonnes (26,455 lb); studies were made to check the possibility of transporting extra-heavy loads jointly by several helicopters on a common sling system, etc. Mi-6s used in the flying-crane role were operated with the wings removed. The Mi-6 keeps soldiering on to this day in Russia and abroad. In the course of 40 years a great number of various military and civil versions of the Mi-6 were produced.

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