V-8A Twin-Turbine Prototype

Both the EDB and the customers placed their bets on the twin-turbine variant. The second single-engine V-8 prototype completed in November 1961 was never flown, serving only as a ground endurance test prototype. From the outset it was destined to serve as a basis for subsequent conversion into the twin-engine variant.

The new TV2-117 engines and the VR-8 main gear­box developed by the Izotov EDB were delivered to Plant No. 329 in the summer of 1932. The engines had a take-off power rating of 1,500 hp each and possessed relatively good specific performance data. The twin-turbine power plant assured a high power output-to-weight ratio which was sufficient for the helicopter to continue horizontal flight if one of the engines failed. The emergence of the first dedicated helicopter engines was a notable event in the history of the Soviet rotorcraft technology because the TV-2VM and D-25V engines used on the Mi-6 were derivatives of engines originally designed for fixed-wing aircraft. The VR-8 was a three-stage planetary gearbox with a reduction ratio of 1:62.6. The increased power output of the power plant prompted the developers of the Mil and Izotov bureaus to redesign not only the main gearbox but also other elements of the transmission. Minor changes were introduced into the design of the upper fuselage and the engine mount. The number of seats in the passenger cabin was increased from 18 to 20.

On August 2, 1962, the twin-engine prototype lifted off for the first time with test pilot N. V. Lyoshin at the controls, and on September 17 the helicopter made the first free flight. A few days later the V-8A together with the V-8 was shown to a group of Government leaders of the Socialist countries headed by N. S. Khruschov at the Central Airfield in Moscow and won high praise. The factory tests lasted throughout the autumn and the winter. Besides N. V. Lyoshin, they were conducted by test pilots G. V. Alfyorov, I. N. Dryndin, V. P. Koloshenko, Yu. S. Shvachko and others. A. Ya. Choolkov and V. A. Izakson-Yelizarov were chief project engineers.

Concurrently with the testing and development of the prototype, the EDB flight test facil­ity conducted development of the V-8 airframe and its subassemblies, using a Mi-4 converted to a test bench for research work. The test objects included the main rotor head with hydraulic dampers of the drag hinges, experimental rotor blades with a steel tube spar and glass fibre leading-edge section, an electric de-icing system, a three-blade anti-torque rotor with a cardan-mounted hub and all-metal blades, a four-blade anti-torque rotor featuring a hub with drag and flapping hinges and elastic links between the blades, and other ele­ments of the airframe and equipment. Gradually, as they were brought up to scratch, they were mounted on the V-8A prototype, but far from all of them would eventually find use on the new machine.

In March 1963 the yellow-painted V-8A helicopter wearing Aeroflot titles went to the first stage (Stage A) of the joint State trials which, on the whole, proceeded smoothly, even though flights were suspended from time to time in order to make improvements and elim­inate the faults discovered. In the summer of 1963 it proved necessary to suspend the tests for as long as two months in order to effect some improvements on the engines and the main gearbox. Simultaneously with the obligatory test programme the test pilots and specialists from the LII (Flight Research Institute) and GNIKI VVS (Red Banner State Research Institute of the Air Force) conducted various experiments on the V-8A in accordance with their own research plans.

Alterations were constantly introduced into the design of the prototype, accentuating the difference between the new machine and its predecessor, the Mi-4. In particular, the EDB designers had to develop a new five-blade main rotor in an effort to reduce vibrations. The rotor's diameter and the design of the basic assemblies and components of the rotor hub remained unchanged; the designers merely drew the 'pipes' on the rotor hub apart from each other in azimuth so as to insert one more 'pipe'. The Mi-4-style all-metal rotor blades were retained, but some assemblies of their structure were strengthened and a new electric de-icing system was installed. The old tail rotor with wooden blades was replaced by a new rotor with metal blades and a cardan shaft-mounted hub.

The single-chamber struts of the main under­carriage units inherited from the Mi-4 proved somewhat unsafe, harboring the risk of ground reso­nance during a rolling take-off. Hence they had to be replaced by new ones featuring two-chamber oleo-pneumatic shock absorbers which ruled out dynamic instability. The tail bumper design was changed. The undercarriage struts were provided with fairings, and spats were fitted to the wheels. Increased load-lifting capacity made it possible to install a reinforced external sling system capable of handling loads of up to 3 tonnes. The four-channel autopilot type AP-34 incorporated into the flight control system on a differential basis helped to improve considerably the V-8A piloting characteristics.

In the course of the tests and development the new power plant was fitted with an automatic engine control system which assured that main rotor RPM remained within the specified limits and synchronized engine RPM. In the event of an engine failure the con­trol system caused the functioning engine to automatically increase of its power setting. All the improvements were promptly incorporated in the third prototype which was being assembled at the experimental production facility of Plant No. 329. In accordance with the Government directive this example of the new helicopter was manufactured in the transport and troop-carrier variant and was allocated the designation V-8AT. Twenty folding seats for troops were provided along the cargo cabin side walls. Other four seats could be added in overload configuration.

The customers used the full-scale mock-up to test and perfect the loading of various kinds of military equipment and engineering machinery and their fastening in the cargo cabin, as well as the installation of an armament system similar to the one used on the Mi-4AV. The V-8AT had some external differences compared to the V-8A: the side doors of the cockpit were replaced by sliding blisters; the forward-hinged portside door of the cargo cabin gave way to an aft-sliding door.

Assembly of the V-8AT was completed in the summer of 1963 and the helicopter replaced the V-8A in the State trials programme. The V-8A was subsequently used for flight testing and ground endurance tests. In the spring of 1964, while undergoing State trials, the V-8AT was converted experimentally into a VIP version for Government use, featuring an appropriately furnished interior and powerful commu­nications equipment. Later still, it was reconverted to transport configuration for further testing. On 19th April 1964 a crew captained by test pilot V. Koloshenko set two world records on the V-8AT in the course of the trials: a closed-circuit distance record 2,465.7 km and a speed record over a 2,000 km (201.8 km/h). Several years later, in the years between 1967 and 1969, crews captained by woman pilots Inna Kopets and Lyubov' Isayeva set five world records for women.

In May 1964 the new V-8AP passenger helicopter was completed as a VIP version for Government use. In most respects it was identical to the V-8AT and served as a test bench for the updated AP-34B autopilot and an automatic main rotor speed governor. The V-8AP was demonstrated to leading members of the Communist Party and the Government. In September 1964 the V-8AP flight tests marked the beginning of the second stage (Stage B) of the joint State trials programme. A month later it was joined by the V-8AT. The helicopters demonstrated excellent flight performance and economic efficiency. They had twice the cargo-carrying capacity of the Mi-4, one and a half times its speed and three times the productive capacity.