In the mid-1950s, when development of the Mi-6 was basically completed, the Mil OKB began studying prospective ways of further increasing the lifting capacity of rotary-wing aircraft. Among the areas of research enjoying top priority in those years was the development of helicopters with jet-powered main rotors where torque is produced by the efflux of jet engines or jet nozzles mounted at the tips of the rotor blades. Deleting the mechanical transmission was expected not only to make the airframe simpler and lighter, but also to improve the payload-to-weight ratio considerably. Besides, in this layout the main rotor was free from torque, which ob-viated the need for power-consuming and cumbersome means of counteracting it. This made it easier to develop a rational airframe layout. Of all the available means of jet propulsion, the use of turbojet engines appeared to be the most efficient option. The OKB was at one time engaged in the design of an ultra-heavy-lift helicopter utilising this concept with a main rotor measuring nearly 60 m (196 ft 10.2 in) in diameter. Each blade was to carry on its tip two1,750-kgp (3,859-lbst) turbojets, with the turbines rotating in opposite directions. But as a preliminary step M. L. Mil intended to build a small experimental four-seat helicopter which would serve as a proof-of-concept vehicle for the jet-propelled main rotor, helping to solve a number of problems inherent in this layout. He managed to get the Civil Aviation authorities and the Armed Forces interested in this idea, and on 20th December 1956 a Government directive was issued calling for the development of the V-7 experimental helicopter with a jet-driven main rotor.
The design and construction of this rotorcraft, the smallest and lightest among the progeny of the Mil OKB, proceeded at a brisk pace. In Decem-ber 1957 the detail design was almost completed, and the experimental workshop of Plant No. 329 started manufacturing a trials batch of five machines. A. V. Kochkin was appointed the V-7's chief project engineer, later succeeded by G. G. Lazarev.
The helicopter's structure was extremely simple. The main part of it was the teardrop-shaped all-metal fuselage of semi-monocoque riveted construction. Bolted to the top of the mainframes was a cast-metal slab; this carried a gearbox which comprised the main rotor shaft and the accessory drive shafts. The main rotor shaft carried the rotor head with blades and the swashplate. Joined to the front edge of the slab was a bracket with control bellcranks and hydraulic actuators. There were three entry doors. Besides the pilot, the cabin could accommodate three passengers or a stretcher patient and a medical attendant. The fuel tank was beneath the floor. A pump fed the fuel into the fuel regulator, from where it went into the main rotor shaft manifolds and thence was supplied by the centrifugal force to the turbojets mounted at the rotor blade tips.
The blades were rectangular in planform and had a steel spar, wooden ribs and plywood skinning. They were attached to the rotor head by feathering hinges and a common flapping hinge. Two fuel feed pipes ran along the blade leading edges through the front parts of the ribs. Electric wiring was placed inside the spar. Placed on top of the rotor shaft were electric contacts for supplying signals from the powerplant to the engine instruments. When it came to designing the directional control system, the original intention was to install a simple empennage placed in the slipstream. However, thorough wind tunnel tests showed it was necessary to retain a tail rotor for directional control. It was mounted behind the fuselage on a short tubular truss. Thus, it proved impossible to dispense with a transmission on this helicopter after all. The V-7 was the first Mil helicopter to be fitted with a tubular skid undercarriage. Hydraulic shock absorbers were mounted on the rear transverse tubes to prevent ground resonance. The V-7 was fitted with a lightweight set of flight instruments; an armed military version was envisaged.
The successful implementation of the jet-driven main rotor concept depended primarily on the development of sufficiently lightweight and compact engines able to run reliably during constant centrifugal forces and high G loads, and on the development of fuel feed systems and their controls. Several engine design bureaux were approached with a view to engaging their services. Of these, only Chief Designer Aleksandr G. Ivchenko undertook to design the special turbojets. The AI-7 engine developed in his OKB-478 and delivering 70 kgp (154 lb st) was a turbojet of the simplest kind with a centrifugal compressor and a single-stage turbine. To compensate for the gyroscopic forces created by the spool the engine featured three flywheels rotating in the opposite direction to the turbine. This was the simplest solution but a bad one, as subsequent events showed.
The AI-7 engines were delivered to the Plant No. 329 only in December 1959, long after the helicopter's airframe had been completed. The very first start-up of the turbojets revealed problems: the engines failed to reach the RPM specified for normal operation and produce the stipulated thrust because of a big power drain caused by the flywheels. Consequently, the latter had to be removed from then engines. To cool the engine's lubrication system which was prone to overheating, Mil OKB engineers designed a unique tubular oil cooler placed in the engine's air intake. After this the AI-7 turbojet produced the required thrust, but all the stresses caused by the gyroscopic force came to affect the main rotor.
Implementation of the jet-driven main rotor concept proved to be much more complicated than might have seemed at first. The development of the V-7 and its powerplant proved to be a protracted affair. The OKB had to enlist the assistance of specialists of TsIAM for speeding up the development of the AI-7. It took several years to get the engines to run reliably in the field of centrifugal forces, and it was not until 19th February 1962 that the first tethered hover was attempted. However, the V-7 would not leave the ground. The main rotor blades were twisted by the gyroscopic torque to a negative angle of incidence; the resultant buckling of the skin produced a strong drag, further aggravated by the absence of engine cowlings. Besides, the power of the collective pitch hydraulic actuator proved insufficient to overcome the loads in the control system. The helicopter was subjected to strong vibrations. After this the engines were sent to the Ivchenko OKB for modification, the rotor blades were repaired, the hydraulic pitch actuator was replaced by a more powerful one, and cowlings were designed for the engines.
Tethered tests of the V-7 resumed in April 1965, but one of the engines jammed at the very first start-up. It had to be returned to the manufacturer. Finally, on 20th September a stable tethered hover was achieved. The tests were conducted by technician V. A. Koloskov. The hover was performed with the main rotor rotating at reduced RPM because in this case the engines created a lesser twisting moment and the blades did not warp and buckle. The programme for 1965 envisaged evaluating the real output of the engines at different rotor RPM. That year, however, became the last one in the V-7's development history. On 11th November 1965, when a test was conducted at maximum RPM and the engines running at take-off power, both engines disintegrated almost simultaneously. As it turned out later, the take-off power setting of the AI-7 engine was also a critical setting for it. The engine compressors entered into resonance oscillations, broke through the engine casings and departed together with the rear portions of the engines. As for the helicopter, it suffered no further damage and made a gentle touch-down.
The designers had to admit that any further development work on the AI-7 was pointless. They placed their hopes on the new MD-3 turbojet developed by TsIAM; in this engine the gyroscopic force was counteracted by having the compressor and the turbine rotate in opposite directions. However, this engine, too, needed a lengthy development effort. The same was true for several other units of the helicopter. The fuel consumption of a helicopter featuring a jet-driven main rotor turned out to be considerably higher than initially presumed. The noise level was very high, too. Then M. L. Mil came to the conclusion that a more rational way to increase the load-lifting capacity of helicopters would be to use multi-rotor layouts with a mechanical transmission. Development of the V-7 was discontinued.
The V-7 was the first and only helicopter in the world featuring turbojet engines on the main rotor blade tips, and the Mil design team obtained a wealth of experience in developing rotorcraft of this type.
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