Design and Development
The aircraft was designed by Ryszard Bartel in the Samolot factory in Poznań. It was a development of the Bartel BM-2, which did not advance beyond the prototype stage. Thanks to a lower weight than the BM-2, it could use lower-powered engines, so its performance was actually improved. Its performance was also superior to the Hanriot H.28, used by the Poles and licence-built by Samolot. The BM-4 prototype was flown on 20 December 1927 in Poznań. It had good handling and stability and was resistant to spinning. A distinguishing feature of all Bartels was an upper wing of a shorter span, because lower and upper wing halves were interchangeable (i.e. the lower wingspan included the width of the fuselage).
The first prototype was designated BM-4b and was fitted with 90 hp Walter Vega radial engine. The second prototype, flown on 2 April 1928, was designated BM-4d and fitted with the Polish experimental 85 hp WZ-7 radial engine, then refitted with 80 hp Le Rhône 9C rotary engine and redesignated BM-4a. The BM-4a became a production variant, because the Polish Air Force had a store of Le Rhône engines. 22 aircraft were ordered and built in 1928–1929. This variant had a cowled engine which made it different from all other BM-4s with radial engines.
Next several variants remained experimental. The BM-4c with a 125 hp Lorraine-Dietrich 5Pb radial engine, built as a one-off in 1928, was supposed to be used for long-distance flights to advertise the engines, but was finally used as the factory's aircraft. Three BM-4a's were converted to BM-4e of 1930 with the Polish experimental 85 hp Peterlot radial engine, the BM-4f of 1931 with the Polish experimental 120 hp Skoda G-594 Czarny Piotruś radial engine, and the BM-4g of 1931 with 100 hp de Havilland Gipsy I inline engine. The last one competed against the RWD-8 in a search for a standard trainer aircraft, but was not selected. After tests in 1932, all three were converted back with Le Rhône engines.
The second series variant became BM-4h, with 120 hp de Havilland Gipsy III or 120 hp Walter Junior 4 inline engines. Like late BM-4a's, they had a rounded tailfin and a modified undercarriage. Due to the Samolot factory's closure in 1930, the BM-4h was developed at the PWS (Podlaska Wytwórnia Samolotów) and built there in 1932 in a series of about 50 aircraft.
Read more about this topic: Bartel BM-4
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