Vittorio Isacco was Italian designer who worked on a helicopter in USSR 1932-1936. In 1936 he was arrested and worked in a Special KB possibly at GAZ-39.

I-4, also called Isacco-4 and Gyelikogyr, this unusual helicopter was only Isacco design actually to be completed. Fuselage of KhMA welded steel tube with fabric covering. Tail, pilot-operated vertical and horizontal surfaces of light alloy, fabric covering. Tailskid and wide-track fixed main wheel. Cabin amidships for pilot and five passengers. On nose, main propulsion engine. Four-blade lifting rotor, each blade having constant profile with light-alloy ribs (welded from 12x10mm elliptical tubing) located at intervals along light-alloy box spar with two main webs and upper/lower booms, fabric covering overall. Blades supported at rest by bracing ties from central cabane pyramid. Pilot controls to ailerons on pairs of arms behind outer trailing edge for cyclic/collective pitch control. Rotor driven by separate engines on tip of each blade. I-4 began late 32 and built at ZOK NII GVF. Prof. B.N.Yuriev acted as consultant. Designer’s calculations found unreliable, delaying completion until 1935. Ground tests in that year caused deformed dural trunnion on engine, remanufactured in steel, but went on to discover severe blade flutter resulting in departure of one of tip engines and severe straining of whole machine. Never flew, and final conclusion was that tip-mounted engine idea was not practical.

Bill Gunston "The Osprey Encyclopedia of Russian Aircraft", 2000

In the period between the world wars several aircraft designers in Italy and France, all avowed Communists, decided that their duty lay in emi-grating to the Soviet Union. I don't know how many of them regretted it, but almost to a man they fell foul of Stalin's terror in the mid-1930s and found themselves behind bars. One such was Italian Vittorio Isacco. During the late 1920s he worried away at the problem of designing a helicopter, and at last, in 1932, he was able to start building it. The unfortunate constructors were ZOK NII GVF, the civil bureau of special construction. Isacco had as consultant Professor B.N. Yuriev, who had been trying to build a helicopter since 1912. Clearly, he didn't know much about the subject either.

Isacco's machine was finally finished in 1935. It was enormous. The four-bladed rotor had a dia-meter of just over 24m. Small by comparison, the fuselage had a 300-hp Wright Whirlwind J-6 on the nose, driving a four-blade propeller. Above the rigid rotor hub towered a mast from which bracing cables extended down to support the massive blades when at rest. The cables were certainly needed, because not only did each blade have a servo control surface carried on struts behind the trailing edge at the tip but, also at the tip, each blade carried a 120-hp de Havilland Gipsy III engine, driving a four-blade propeller! Isacco was by no means the only designer to try this arrangement, but his must surely have been the only helicopter ever built to have five four-blade propellers!

Isacco left documents outlining a succession of problems, some fundamental to the basic arrangements and the rest more intermediate difficulties with the hardware. He realised that, though this form of tip drive meant that the only torque imparted to the fuselage was that due to friction in the main hub bearing, it was still essential to be able to control the direction in which the fuselage was pointing as one reason for having the propulsion engine blowing slipstream over the rudder. In translational flight the aerodynamics of the tip propellers was complex. Isacco thought centrifugal force along the spinning blades would help feed petrol, but appeared to overlook what it might do to the flow of lubricating oil inside each of the tip-mounted engines. Moreover, his calculations were found to contain errors, which held up the work until 1935. Then ground testing started.

Anyone with much imagination will see that it must have been hilarious. Have you ever started a Gipsy? You have to open a little access door to prime the carburettor, shut the door and lock it, and then go through the rigmarole of "switches off ... suck in ... contact". At which point the propeller has to be swung as hard as possible by hand, the helper then getting clear the moment the engine fires. How do you do all this four times at once when you are 4m up on a ladder?

But that was only part of the problem. The most serious difficulty was that as soon as the mighty rotor really started going round it suffered severe blade flutter, which caused one of the engines to come completely adrift and go sailing off into the sunset, leaving the off-balance machine almost coming apart.

One of the last reports on this programme states that Isacco eventually concluded that the basic concept was not practical. But he didn't really deserve to be arrested in 1936 and sent off to one of Stalin's special (i.e. prison) design bu-reaux. Or did he?

Bill Gunston "Back to the Drawing Board", 1996

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Technical data for I-4 Engines: 1 x 225kW Wright J-5 radial driving four-blade propulsion propeller, 4 x 90kW D.H.Gipsy III each driving small four-blade propeller, rotor diameter: 24.4m, take-off weight: 3000kg

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