Mi-8 Hip - Development
The first Soviet second-generation helicopter made its appearance in 1957. It was the Mi-6 heavy transport and troop-carrying helicopter. In the second half of the 1950s Mikhail Leontyevich Mil, Chief Designer of Plant No. 329, considered developing qualitatively new machines in the light and medium-lift helicopter classes as well; these would supersede the first-generation Mi-1 and Mi-4 series production helicopters. Experience gained in the process of developing the Mi-6 held promise of success. As early as 1957 Mil Design Bureau (the so called «Mil OKB») personnel made some rough calculations of the parameters of new machines.
Whereas the MI-1 was expected to be replaced by a single-engine helicopter, the Mi-4's successor was regarded from the outset as a twin-turbine machine. This was designed to enhance safety and survivability, because in the event of failing one of the engines the second one would pass to emergency mode making it possible to continue the flight of the helicopter.
Unfortunately, the social and economic situation in the country and the low-priority funding of the helicopter industry made it rather unrealistic to expect that radically new machines had a chance of being ordered. High-ranking military and civil officials were quite satisfied with the Mi-1 and Mi-4 which had a good service record and were well established in production. To obtain a government order and funding, M. L. Mil once again had to resort to a compromise.
Mikhail Mil originally approached the Soviet government with a proposal to design an all new two-engined turbine helicopter after the success of the Mil Mi-4 and the emergence and effectiveness of turbines used in the Mil Mi-6; but the military argued against a new helicopter, as they were content with the current Mil Mi-4. To counter this, Mikhail Mil proposed that the new helicopter was more of an update to new turbine engines rather than an entirely new helicopter; this persuaded the council of ministers to proceed with production. Due to the position of the engine, this enabled Mikhail Mil to justify redesigning the entire front half of the aircraft around the single engine (designed by Alexander Ivchenko, originally for fixed wing aircraft as all other Soviet turbines had been up to that point).
The new helicopter was presented as a further modification of the Mi-4 with a gas tubine power plant. Main and tail rotors, tail-boom and pylon, transmission, control system and most of other components and aggregates remained virtually unchanged. Alterations were made to the forward and central fuselage. The power plant was placed above the cargo cabin, while the cockpit was moved to the forward fuselage. In other words, the helicopter's layout was altered to follow the style of Mi-6 which had shown itself well. Besides, M. L. Mil succeeded in proving the expediency of redesigning the fuselage for the purpose of increasing the cargo/passenger cabin capacity. The project provided for the carriage of up to 20 passengers or troops, or long-barrel guns and other bulky materiel. In addition to the baseline passenger and transport/troop-carrier versions, development of an armed version and an ASW variant, as well as of a VIP transport version was planned from the outset. The latter was proposed primarily as a prestigious means of transport for the heads of the Soviet Government and the US administration during the US President visit to Moscow which was being prepared at the time.
The leadership of the Soviet Ministry of Civil Aviation was the first to evince interest in the development of a new medium-lift helicopter. At its insistence, on 20th February 1958 the Council of Ministers of the USSR adopted a directive calling for the development of the V-8 helicopter with a cargo-carrying capacity of 1.5 to 2.0 tonnes, powered by a helicopter version of the Ivchenko AI-24V engine. About a year later, the Soviet Armed Forces command expressed its support for the V-8 project. Development work on the V-8 was headed by deputy Chief Designer V. A. Kuznetsov. G. V. Remezov was appointed chief project engineer (V. A. Nikiforov currently plays this role). Scrupulous designing work on the V-8 began, when in 1959 the advanced development project (ADP) and the full-size mock-up were approved.
The AI-24V engine (V = vertolyotniy, helicopter version) developed 1,900 shp, which permitted to retain on the V-8 the transmission of its predecessor Mi-4. However, other characteristics of the AI-24V, especially the specific fuel consumption, left much to be desired. And, most importantly, the designers were well aware of the necessity to switch to the safer and more reliable twin-turbine variant. Admittedly, this would require a redesign of the main gearbox, but M. L. Mil succeeded in convincing the governing bodies of the aircraft industry that it would be expedient to study a twin-turbine version intended for operations on local passenger services, the development of which enjoyed much attention at the time.
The granting of government support was due not least to Nikita S. Khruschchov's wish to have a new, modern and safe VIP helicopter for official use. A Council of Ministers directive was issued tasking several engine design bureau with the design of a gas turbine engine delivering 1,250 shp. The warmest response to this task came from the Design Bureau headed by Chief Designer Sergey P. Izotov (OKB-117). The same design team was entrusted with designing a two-shaft main gearbox. The prospect of getting a power plant with greater output enabled the designers to increase the number of passengers and the load-carrying capacity of the machine under development. The customer approved the project of the proposed modification, and on 30th May 1960, following the official approval of the full-size mock-up, a Council of Ministers directive was issued calling for the construction of a twin-engine V-8A version in parallel to the single-engine V-8.
At the initial and detail design stages the Mil EDB (Experimental Design Bureau) engineers got the idea of improving not only the transmission but also a number of other assemblies and systems of the V-8. In particular, it was decided that the following alterations should be made: the quadricycle landing gear should be replaced with a tricycle undercarriage, the nose leg to be provided with twin castoring wheels; the friction dampers of the drag hinges of the main rotor head should be replaced by hydraulic ones; the liquid de-icing system should be replaced by an electric de-icing system; the hydraulic actuators of all four control circuits should be mounted in a single hydraulic package, on the main gearbox, together with other parts of hydraulic system; trim tabs with electric artificial feel mechanisms should be incorporated into the flight control system; a fairing was to be fitted to the tail rotor pylon etc.
The designers pledged to introduce most of these new features on production Mi-4, so as to achieve maximum interchangeability between this machine and the one under development. In this way M. L. Mil and his associates gradually transformed a thoroughly modernized Mi-4 into a radically new advanced helicopter.
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