Mi-4 HOUND
Z-5 / Z-6

The Mi-4 HOUND helicopter is a piston-engined aircraft developed for unarmed military transport. Approximately 3,000 Mi-4 Hounds were built before production ended in 1969. These piston-engined aircraft have been largely replaced by jet-powered helicopters in the transport and antisubmarine role.

The large, four-blade main rotor is mounted on top of the fuselage midsection. The single piston engine is mounted within the nose section. The fuselage is short and oval with a solid rounded nose and stepped-up cockpit. It features a high-mounted, long, thin tail boom with a gun mount under the belly (oil pan) and four-wheeled landing gear. The tail is small, with a three-blade rotor attached to right side of the thin fin and small flats forward of the fin.

The Mi-4 featured a stepped ("two-level") arrangement, with the flight deck placed above the cargo hold in a similar manner to the American Sikorsky S-55 helicopter. A powerful ASh-82V (V stands for vertolyotnyy - helicopter-specific) 14-cylinder twin-row radial with a cooling fan and a clutch was located in the front part of the semi-monocoque fuselage. The engine had a take-off and nominal rating of 1,700 hp and 1,530 hp respectively.

Behind the engine, in the area of the machine's CG there was a roomy cargo hold measuring 4.5 x 1.6 x 1.76 m (14 ft 9 in x 5 ft 3 in x 5ft 9 in). It was equipped with tip-up seats for the troops, cargo tie-down cleats and a ramp for loading wheeled vehicles. The aft end of the cabin incorporated a full-section hatch with clamshell doors. At a later stage the helicopter was equipped with an external load sling system permitting it to carry underslung bulky cargoes weighing up to 1,300 kg (2,866 lb) and perform flying crane operations. The helicopter had a ventral gondola or "bathtub" with a TKB-481M heavy-calibre machine-gun on an NUV-1 flexible mount ; the flight engineer doubled as a gunner, lying prone in this gondola to fire it. Above the cargo hold was the cockpit seating two pilots side by side. The cockpit was extensively glazed. The bay behind the cockpit housed the main gearbox and a fuel tank. To increase range, an extra fuel tank could be installed in the cargo hold.

The engine is connected with the R-5 two-stage planetary main gearbox by a shaft which passed between the pilots' seats. The main gearbox distributed power to the main rotor shaft and, via the tailboom and tail pylon shafts and associated gearboxes, to the anti-torque rotor. The four-bladed main rotor with a 21-m (68 ft 1.5 in) diameter had a three-hinge hub and a swashplate which represented scaled-up and improved versions of the same assemblies of the Mi-1 helicopter. Their design, however, underwent some radical alterations. The drag hinges of the hub were located so as to ensure that their bearings were subjected to virtually equal stresses when operating under the most difficult conditions. All hinges were provided with rubber seals. Thanks to a new design of the feathering hub the losses of lubricating oil were reduced to a minimum. Centrifugal stops were used to reduce the blades' droop. The swashplate's design was substantially simplified, in particular, by deleting the superfluous universal hinge in the blade pitch control chain.

The blades of the V-12's main rotor were initially of mixed construction similar to that of the Mi-1. The steel-tube spar was assembled from telescopically joined tubes. Attached to it by rivets was the structure consisting of wooden ribs and stringers, with fabric and plywood skinning. The V1Kh1 three-bladed pusher anti-torque rotor of 3.6 m (11ft 10 in) diameter (developed by a design bureau headed by N. I. Petrov) had all-wood trapezoidal blades which were attached to the hub by feathering and drag hinges. The rotor blades and the front glazing of the cockpit were provided with de-icing systems.

The V-12 became one of the first helicopters in the world to feature hydraulic actuators (boosters) in the control system, and it was the first aircraft in the Soviet Union to have a control system with irreversible boosters. The control linkage comprised push-pull rods and bellcranks. The cockpit had a dual set of controls. The flight instrumentation enabled the helicopter to operate at night and in adverse weather conditions. A steerable stabilizer was installed at the end of the tailboom. The wheel undercarriage included four units, the main units being strut-braced. At the end of the 1950s, flotation gear and ski undercarriage were also developed for the helicopter.

The Mi-4 helicopter saw more widespread service than the Mi-1. The capacious cargo hold of the Mi-4 could accommodate 16 troops or 8 stretcher patients; a GAZ-69 jeep or two M-72 motorcycles with sidecars and 5 troops; a field gun with its crew and ammunition or two 82-mm (3.23-in) mortars with crews and 7 cases of mortar shells. Besides transport and troop-carrying operations, the Mi-4 was used, with appropriate modifications, for search-and-rescue work and medevac duties.

The Mi-4 helicopters in Soviet Army service were initially assigned to helicopter squadrons. These were later joined into independent helicopter regiments, several such regiments being incorporated into the structure of each military district or a group of large army units. Alongside the Army aviation, the Mi-4s entered service with independent squadrons in other branches of the Armed Forces.

The Mi-4 transport helicopter laid the beginning of the Soviet Army Aviation; it was widely used both in the armed forces and in the national economy and for several decades remained the main type of helicopter in the inventory of the Soviet Armed Forces and of the Civil Air Fleet. Particularly notable was the role played by these machines in the development of inaccessible areas of Siberia, the Far East and the mountainous areas of Central Asia. The Mi-4 rendered useful service to geological prospecting parties, oilfield workers, fishery vessels, rescue teams. In the period between 1956 and 1966 more than 700 Mi-4s were sold to 34 countries of the world, including Afghanistan, Algeria, Austria, China, Cuba, Egypt, Finland, Ghana, India, Indonesia, Iraq, Mongolia, North Korea, Pakistan, Syria, Yugoslavia and the Warsaw Pact countries.

For several years the Mi-4 was the biggest production helicopter in the world in terms of all-up weight and payload. Its high performance enabled it to set eight world aviation records of speed and altitude. More than once the Mi-4 was a worthy representative of the Soviet helicopter design school at international competitions and airshows. This helicopter was awarded a diploma and a Gold medal at the Brussels World Exhibition in 1958, and two years later it won a prestigious contest of rotary-wing machines in India.

In 1958 a group of designers comprising M. L. Mil, A. K. Kotikov, V. A. Kuznetsov, G. V. Kozelkov, I. S. Dmitriyev, M. P. Andriashev and I. V. Anan'yev was awarded the Lenin Prize for the creation of the Mi-4 helicopter. Thanks to its high reliability the Mi-4 became one of the most long-lived helicopters; some examples are in operation to this day.

The Z-5 is the Mil Mi-4 built in China by Harbin, known as "Syuan Fen" (Whirlwind). The Z-6 was a prototype turboshaft version of Z-5, that did not enter production. Manufacture of the Mi-4 laid the beginning to series production of helicopters in the Chinese People's Republic. The first transfer of documentation to the Harbin aircraft factory took place in 1956, and two years later the first helicopter was assembled there. It was known locally as the Z-5 (zhonghaji - helicopter). Production of the Z-5 in China went on until 1979, totalling 545 examples.