Mi-8M / Mi-8MT - HIP H

Studies of the Mi-8M project - a modernized Mi-8 with uprated engines and greater load-carrying capacity - were initiated in the preliminary design section of the Mil OKB as early as 1964, in parallel with the studies concerning another upgraded variant - the high-speed Mi-8S compound helicopter fitted with wings and a jet engine for forward propulsion. After a short while the compound helicopter idea had to be abandoned, but the project of the Mi-8M with accom­modation for 40 passengers became more and more topical with every passing year. The designers proposed increasing the capacity of the helicopter's cargo/passenger cabin by inserting additional sec­tions in the cargo cabin. The increased output of the power plant permitted to expect an increase in payload to a figure exceeding 4 tonnes and, most impor­tantly, improvement in high-altitude performance (in this respect the Mi-8 multi-purpose helicopter was infe­rior not only to contemporary foreign machines in the same class but also to its predecessor, the Mi-4).

In November 1967 the Central Committee of the Communist Party and the Council of Ministers issued a joint directive calling for the development of the upgraded helicopter, and by the end of the year a pro­visional full-size mock-up was ready in the assembly shop of the Moscow Helicopter Plant. The project provided for installation of two TV3-117 engines rated at 1,900 hp. The TV3-117 turboshaft was developed in the second half of the 1960s by the Izotov EDB in Leningrad and was intended for the Mi-14 amphibious ASW helicopter, a derivative of the Mi-8. A modification of this engine was to power the Mi-24 combat hel­icopter. A tempting idea cropped up - to achieve maximum commonality of the dynamic systems (power-plant, transmission and rotors) for all three helicopters.

In 1971 the advanced development project of the Mi-8M was approved by appropriate governmental bodies and, in accordance with recommendations from TsAGI, the decision was taken to upgrade the Mi-8 in two stages. This meant that before embarking on the construction of a 'stretched' variant, an intermediate 'minimum change' version was to be developed. It was decided that the TV2-117 engines and the old transmission as installed on the Mi-8T would be replaced by TV3-117MT engines, a VR-14 main gearbox and a beefed-up transmission. Besides, the modernized machine was to be fitted with the Ivchenko AI-9 auxiliary power unit with starter/generator. A new anti-torque rotor rotating in the opposite direction was to be installed; this meant that the rotor would be of a tractor type instead of the previous pusher type. As a result of the changed direction of rotation, the lower blade of the anti-torque rotor was moving against the main rotor down-wash; coupled with the increased chord of the blades, this considerably enhanced yaw control efficiency.

Despite the Government directive, work on the upgraded Mi-8 proceeded at a slow pace. The EDB was overburdened with the testing and development of the V-12, the Mi-14 and the Mi-24; besides, the chief executives of the Ministry of Aircraft Production were not eager to interfere with the well-established production of the Mi-8. This coincided with the time when the Mi-4 began to be phased out en masse. The Mi-8 with TV2-117 engines could not replace the Mi-4 in mountainous areas and countries with hot climate. For this reason the work on the new machine had to be speeded up. The prototype of the modernized helicopter was completed in the summer of 1975 in Kazan, making its first flight on 17th August. Flight tests revealed a considerable improvement in perform­ance, especially as regards service ceiling and rate of climb. The upgraded helicopter was fitted with three external stores pylons on outriggers either side of the fuse­lage instead of two.

After its entry into service the helicopter received the designation Mi-8MT; it entered series production at the Kazan Helicopter Plant in 1977. From the following year onwards it was powered by modernized TV3-117MT Series III engines. At first the number of helicopters powered by TV3-117 engines was consider­ably smaller than the number of the preceding versions with TV2-117s, but hostilities in Afghanistan prompted the Soviet military to reconsider their ordering policy, and by the mid-1980s the Mi-8MT and its derivatives became the plant's main product. By 2001 the Kazan plant had turned out more than 3,000 helicopters powered by TV3-117MT and TV3-117VM engines.

In 1981 the Mi-8MT made its international debut at the 34th Paris Air Show. For advertising and marketing reasons the export version was allocated a new des­ignation, Mi-17, under which it has gained a foothold in the world market. A passenger version with an inte­rior similar to that of the Mi-8P was named Mi-17P. Like its predecessor, the baseline Mi-8MT served as a basis for developing numerous variants for various military and civil duties.