The Bartel M.4 was a primary trainer designed to use a war surplus Le Rhone engine and which was produced in significant numbers, outliving the Samolot company that had developed it.
The M.4 was developed from the Bartel M.2, a biplane trainer that had only reached the prototype stage, making its maiden flight on 7 December 1926. This came a few months after Marshal Josef Pilsudski had come to power in Poland, bringing with him a more positive attitude to the Polish aircraft industry. By early 1927 the Department of Aeronautics had issued two requirements for new training aircraft, one for an intermediate trainer and one for a primary trainer to use war surplus 80hp Le Rhone rotary engines.
The Samolot Company, which employed Ryszard Bartel as their chief designer, decided to produce designs for both requirements. The Bartel M.3 was produced for the intermediate project (later being developed into the M.5), while the Bartel M.4 was produced for the primary trainer requirement.
The resulting aircraft was know as either the M.4 or BM 4. It was a two-bay unequal span biplane with heavily staggered wings with rounded tips. Most of the rest of the airframe was based directly on that of the M.2, so most of the aircraft was of wooden construction, apart from the steel tube framework for the tail, a fabric covering for most of the wings and tail (plywood on the leading edges and undersurfaces) and sheet duraluminum for the forward part of the cockpit. The M.4 was smaller than the M.2, with a shorter wingspan and length, but also somewhat heavier.
Samolot was so confident in the design that they decided to fund construction of two prototypes, one flying and one static test airframe, in April 1927. A few months later the Department of Aeronautics selected the BM 4 as winner of the design contest and placed an order for two prototypes. This ended up including the original flying prototype and one extra aircraft.
The BM 4 was produced in a number of different versions. The Le Rhone powered version was designated as the BM 4a. However neither of the two prototypes was originally built as a BM 4a. The first prototype, which made its maiden flight on 20 December 1927, was powered by an 85hp Walter seven-cylinder air-cooled radial engine, and was designated as the BM 4b. This aircraft underwent official evaluation trails on 20-27 January 1928, the Service suitability trails in March. The aircraft was later given to King Amanulla of Afghanistan.
The second prototype was originally going to be powered by the Le Rhone engine, but was completed with an 80hp Zalewski Avia W.Z.7 seven-cylinder air-cooled radial engine as the BM 4d, making its maiden flight on 2 April 1928. The airframe was 55lb lighter than on the first prototype, and the engine was also lighter, but its performance figures were almost identical to those of the first prototype. This second prototype was then returned to the workshop and given a Le Rhone rotary engine, becoming a BM 4a. It made its first flight with the new engine on 27 April 1928.
The BM 4a was officially accepted by the Polish Air Force on 20 June 1928 and an order for 22 BM 4a primary trainers was placed. The first five aircraft were completed in February 1929 and the other seventeen by May. The first pilots to be trained on it made their solo flights in July 1929. The BM 4a was the first Polish designed aircraft to enter service in significant quantities.
The BM 4 was produced in a number of further variants over the next few years, but not by Samolot. In September 1929 a fire damaged the firm’s buildings, and early in 1930 it entered voluntary liquidation. Bartel stayed on as chief designer until 31 August 1930 and then went on to produce a number of new variants on the aircraft for other companies. The most significant of these was the BM 4h, which was produced in large numbers by P.W.S. in 1932
The BM 4 was used at the training school at Deblin before being withdraw in the late 1930s and sent to air cadet flying schools. Some of these aircraft were used as liaison aircraft after the German invasion of 1939.
The BM 4a was the original primary trainer, powered by an 80hp Le Rhone radial engine. 22 were ordered and delivered by May 1929. In 1930 they were all given the new undercarriage introduced on the BM 4e, new rounded wing tips, mass balanced ailerons and the tail was reshaped. These changes improved the aircraft’s handling characteristics.
The BM 4b was the first prototype, powered by a 85hp Walter seven-cylinder air-cooled radial engine.
The BM 4c was a touring aircraft produced at Samolot for their staff. It had an 110hp Lorraine five-cylinder radial engine and extra fuel tanks.
The BM 4d was the designation given to the second prototype while it was powered by a 80hp Zalewski Avia W.Z.7 seven-cylinder air-cooled radial
The BM 4e was the designation given to the second prototype when it was given an 85hp Peterlot engine and a new divided undercarriage with Vickers oleo-pneumatic shock-absorbers. The new engine was heavier than the original engines, and performance was poor.
This designation was given to one aircraft that was given a 120hp Polish Skoda Works G.594 seven-cylinder engine in the spring of 1931.
This designation was given to a single aircraft that was given a 85-100hp de Havilland Gipsy I four-cylinder inline air-cooled engine and had its fuselage cleaned up.
The BM 4h was the second main production version, and was built in response to a shortage of primary trainers. The prototype was produced by fitting a 105hp Walter Junior four cylinder inline air-cooled engine to one of the BM 4as. This prototype was built under licence by P.W.S. and in service trials in the summer of 1931 showed an improvement over the BM 4a. An order was then placed for 50 aircraft, powered by either the 105hp Walter engine or 120hp Gipsy III. These aircraft were delivered in 1932.
Engine: Le Rhone radial engine
Span: 33ft 5in
Length: 23ft 8.25in
Height: 9ft 7.5in
Empty Weight: 1,187lb
Gross Weight: 1,744lb
Maximum Speed: 77.7mph at sea level
Climb rate: 9m 42s to 3,280ft
Endurance: 3 hours