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Mi-4 HOUND - Development

At the end of 1940s the Mil design team started work on the development of multi-seat helicopters of various layouts. At first it was envisaged that they would be powered by a helicopter version of the 1,000-hp Shvetsov ASh-62 radial engine used on a number of fixed-wing aircraft. Consideration was given to projects of machines featuring a twin-rotor tandem layout, this layout being in vogue at the time. However, in a short while it became evident that, despite all its apparent advantages, the tandem layout harboured a great number of serious problems, and in 1949 M. L. Mil took the decision to design a multi-seat single-rotor helicopter powered by a single ASh-62 engine. Unfortunately, an attempt to develop a helicopter-specific version of this engine ended in a failure, and M. L. Mil had to pursue further project development work with a considerably more powerful Shvetsov ASh-82 engine in mind.

The idea of creating a helicopter with a high load-lifting capacity did not find due support on the part of the customer (i.e. the Armed Forces) at the time. The situation changed in the early 1950s when the USA began using helicopters with notable success in the Korean war. In September 1951 a conference presided by I. V. Stalin was held in the Kremlin; the question on the agenda was how to overcome the Soviet Union's lagging behind in the field of helicopter construction. At this conference the go-ahead was given to the project of the V-12 (V stands for vertolyot - helicopter) twelve-seat helicopter developed in the Mil Design Bureau (it was also designated VD-12 - vertolyot desahntnyy, troop-carrier helicopter, for 12 persons). According to the directive issued by the Council of Ministers of the USSR on 5th October 1951, the transport and troop-carrier helicopter was to carry 12 troops, a 57-mm or 76-mm (2.24-in or 3-in) light field gun, or a GAZ-67B or GAZ-69 army jeep internally. The overall cargo-carrying capacity was specified as 1,200 kg (2,646 lb) for a normal payload and 1,600 kg (3,528 lb) in overload confi-guration.

The designers were given just one year to create the machine. The OKB personnel worked for 14 to 16 hours a day and stayed overnight at the factory. The new helicopter with an all-up weight three times that of the Mi-1.

M. N. Pivovarov was responsible for the design and construction of theV-12 as chief project engineer; G. V. Remezov was chief engineer in charge of the flight tests. All the detail drawings were issued in early March 1952, and at the end of April the first flying prototype was completed at the experimental shop of Plant No. 3. At the same time a fatigue test airframe was assembled at the production Plant No. 292 in Saratov.

As distinct from its forbear, the Mi-1, the new helicopter proved to be a more troublesome machine. In the course of testing and development the Mil Design bureau engineers assisted by specialists from LII and TsAGI had to solve a number of complex problems associated with ensuring the structure's fatigue strength, with the dynamic strength of the anti-torque rotor and with the elimination of flutter and ground resonance.

The Mi-4 may well be unique among Mil helicopters with regard to the magnitude of difficulties encountered when developing main rotor blades with a long service life. The early blades of mixed construction had a life of 150 hours, and for a long time this figure could not be bettered. Only in 1955, as a result of several steps taken to reduce stresses in the blades, and some alterations in the production methods, Mil engineers succeeded in increasing the service life to a stable 300 hours. A further increase of the blades' life, however, was not reached until 1957 when blades of mixed construction were developed, featuring a continuous steel tube spar with machined holes in the shank The introduction of such improvements as tempering by means of high-frequency electric current, the use of adhesives instead of rivets for attaching the ribs and stringers to the spar, the polishing of the inner surface of the tube and the hammer-hardening of the outer surface, allowed the life of the main rotor blades to be progressively increased to 500, 600, 800 and 1,000 hours.

By the end of the 1950s, after many years of work, the Mil Design Bureau finally achieved the desired result, creating reliable all-metal blades. New main rotor blades for the Mi-4 were developed, tested and put into production in 1959-60, featuring a pressed continuous duralumin spar and a three-layer sandwich construction trailing-edge section with a metal honeycomb filler. Their life reached 2,000 to 2,500 hours. The reliability of these blades was enhanced by installing pneumatic sensors signaling the appearence of cracks in the spar. The design of rotor blades which had been developed to perfection on the Mi-4 was adopted as standard for Mil helicopters for many years to come, placing them among the most reliable helicopters in the world.

No less important was the problem of ensuring the strength of the Mi-4's anti-torque rotor. Attempts to reinforce the V1Kh1 rotor failed to bring the desired result, and in 1960 the Mil OKB engineers together with specialists from a design bureau headed by G. M. Zaslavskiy, the Flight Research Institute and TsAGI developed a new anti-torque rotor designated V531-Kh3 which was successfully used on this helicopter throughout the rest of its service career.

A dramatic increase in the dimensions of the Mi-4's main rotor compared to its predecessor called forth a new phenomenon which revealed itself the very first time the rotor was set into motion. This was rotor blade flutter, something that had not happened before. Installing counterbalances on the blades alleviated this problem to some extent but could not eradicate it completely. The "soft" flutter manifested itself more than once and made it imperative that specialists thoroughly study the nature of this phenomenon and work out methods of dealing with it. Yet another phenomenon - ground resonance - was repeatedly encountered in the course of the Mi-4's operational life. The main means of counteracting it was the installation of dampers on the drag hinges of the blades, supplemented by introducing damping elements into the design of the undercarriage oleos and by selecting the characteristics of the shock absorbers and wheel tires. Considerable experience was also accumulated in solving the problems associated with reducing the intensity of vibrations. The work on refining the Mi-4 was a good school for the Mil OKB designers; later it enabled them to successfully solve extremely complicated problems that arose in the process of designing subsequent machines.

The Mi-4 was used for conducting a number of unique experiments which were of great significance for the development of helicopter construction. Thus, in 1955 the Mi-4AP avionics testbed was used for investigating in principle the possibility of using an autopilot on a helicopter (the AP-5 autopilot was fitted). Two years later production Mi-4s received the AP-31, the first Soviet autopilot designed specially for helicopters. In the same year of 1957 , experiments were carried out for the first time, in which the main rotor blades were blown off in the air to permit the crew to bail out in an emergency. In 1964, a Mi-4 was used as a testbed for the AP-34 autopilot which, unlike the AP-31, had a consecutive, not parallel, procedure of coupling to the control system. The Mi-4 was the first helicopter to be used as a flying crane in construction operations in the USSR. One Mi-4 was fitted experimentally with exhaust mufflers/flame dampers to investigate ways of making helicopters less observable to the enemy.

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