Last design authorized from OKB-3, to provide comparision against Mil and Yak single-rotor submissions to VVS requirement for three-seat all-weather communications helicopter. Dynamic parts as before except for hydraulic rotor-hub dampers and faired masts with oil-cooler inlets. Wing of lifting section, set at zero incidence. Fuselage of improved form, with round instead of polyhedral top and bottom. Tail basically as B-5. Pilot in B-5 type nose, main cabin for two seats, with space at rear for freight ot two stretchers (in lieu of passengers) loaded through enlarged door. Improved differential rotor controls with reduced friction.
Two prototypes completed in April 1948, respectively flying in June and September. Made good progress with measured performance, sustained (47 min) single-engine flight, autorotative landings and in eradication of various faults stemming mainly from vibration. One problem was lack of wing lift resulting from incidence setting, causing rotors to stall at high forward speeds. Further snag was hydraulic leak, difficult to rectify. On 13 Decemeber 1948 first machine shed blade from right rotor, killing Ponomaryov and I.G.Nilus. Numerous modifications suggested which did not receive support.
Bill Gunston "The Osprey's Encyclopedia of Russian Aircraft", 2000
Aerodynamically refined development of the B-10. B-11 was equipped for blind flight. Rotor manipulation system was also more advanced. Cargo section in the central fuselage had a big side door, could be used also to house one patient on ladder and a doctor.
Flight tests of the first B-11 started in June 1948, the second one - in September. Tests include flight up to 47 minutes with only one engine running. Some vibrations were observed on the maximum speed. Wide research program started, involving recording of vibrations. During those tests one rotor blade broke out (unhappy coincidence of manufacturing defect and minor design error), pilot K.I.Ponomarev was killed in a crash.
After several modification, including installation of new engines (same series), it was presented on an Aviation Parade. Unfortunately, the favored scheme of twin-rotor helicopter became "The Flying Banana" (due to less drag), and the transversal scheme was not used until the enormous Mi-12 came to live.
Technical data for B-11
Crew: 3, passengers: 1-3, engine: 2 x AI-26GRF pistone engine, rated at 575hp, rotor diameter: 10.0m, take-off weight: 4150kg, empty weight: 3398kg, max speed: 155km/h, service ceiling: 2550m, range: 328km