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Yakolev EG / Sh / Yak-M11FR-1

The Yakolev OKB started development of helicopters after the war and designed an experimental machine in 1946, the Yak-EG (Eksperimentalnyi Gelikopter) with a coaxial rotor system, and widely known as the Yak-M11FR-1.
The chief engineer was S.A.Bemov, assisted by I.A.Erlikh. From the outset it was planned as the smallest practical machine to solve basic problems. The engine was a 140hp M-11FR-1 mounted in the normal attitude with the drive taken through a cooling fan and centrifugal clutch to a 90deg bevel gearbox to co-axial vertical shafts. These turned two-blade rotors in opposite directions, at 233 rpm. After studying the possible use of pilot controlled tabs or auxiliary surfaces behind the blade tips the choice was a fully articulated hub with swashplates giving collective and cyclic pitch control. A unique feature was a spring-loaded hydraulic coupling which, upon failure of the drive torque, automatically moved the collective linkage to autorotative pitch.
The rotor blades were laminated pine and hardwood, covered in glued fabric, held in a hub of steel and duralumin. The fuselage was a simple truss of welded steel tube, with Dl skin as far back as the rear of the engine compartment, where an aft-facing gap allowed fan-induced cooling air to escape. The fuel tank was under the main gearbox and the oil tank next to the engine. For better stability in cruising flight a light fabric-covered rear fuselage with twin fins and a tailskid was added behind this point. The welded truss was extended at the front and sides to three vertical shock struts with single wheels, the nose unit having levered suspension. Track was 2.8m and wheelbase 2.68m. The side-by-side cockpit had a door on each side and the largest possible window area.
The Sh was completed in early 1947. The chief pilot was V.V.Tezavrovskii, who with others made forty tethered tests (total 5hr) followed by seventy-five free flights (total 15hr). Ground resonance, then little known, was avoided by pure chance. The centre of gravity was clearly too far aft, so the tail (and for a time the tail-skid) were removed and the oil tank relocated behind the cockpit bulkhead.
Eventually the Sh hovered under good control, but as soon as forward speed exceeded about 30km/h vibration and progressive loss of control were encountered. Following testing it was decided that the coaxial rotor layout should be developed by the Kamov bureau, and Yakolev moved on to other helicopter configurations.

Engine: M-11FR-1 5-cylinder air cooled piston, 140 hp
Rotor diameter: 10.0m
Length: 6.53m
Max take-off weight: 1020kg
Empty weight: 878kg
Payload: 142kg
Fuel: 50kg
Max speed at sea level: 150km/h
Max speed reached: 70km/h
Hovering ceiling: 250m
Service ceiling: 2700m
Service ceiling reached: 180m
Range: 235km

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