SFKB Kb-3/4/5 Chuchuliga

 

The SFKB Kb-3 was a Bulgarian biplane two-seater trainer that was built in 1937. It was designed by engineer Lazarov who also designed the previous Kb-2 and incorporated various improvements into the Kb-3 on the basis of flight tests. It was nicknamed as "Chuchuliga I" (Bulgarian word for Skylark) and only 20 were completed in 1937. 

It was powered by a single Walter-Castor II radial engine driving a two-bladed propeller. Wing system was identical to that of the Kb-2A, but hull chassis and tail section underwent significative changes to improve overall aerodynamics. 

It had a crew of two and served with the Royal Bulgarian Air Force as trainer until 1944. 

The SFKB Kb-4 was a development of the Kb-3 based around various improvements. Engineer Lazarov left Kaproni-Kazanlak (manufacturers of the type) for DAR-Bozurishte in 1938 so Kaproni-Kazanlak designed an improved variant of the Kb-3 called Kb-4 "Chuchuliga II". This was powered by an American Wright E1P-975 radial engine rated at 220 hp of power , covered in a Townend ring, driving a two-bladed propeller. The Wright engine turned to be very appreciated by the Bulgarians, however, when Bulgaria declared war on the United States, their supply was cut, so some of them were completed with the Czechoslovak Walter-Castor II. A total of 28 Chuchuliga II machines were completed.

The Chuchuliga II was the first in the series to feature defensive armament as it was equipped with a 7,7 mm FK-33 machine gun which was the Bulgarian copy of the British Vickers F machine gun. 

It served with the Royal Bulgarian Air Force in the trainer, liaison and reconnaissance roles until 1944.

The Kb-5 Chuchuliga III, was a further development of the type made in 1939. The first prototype was powered by the Walter-Castor II engine, covered in a NACA cowling, however, flight testing was not satisfactory so it was refitted with an Italian Piaggio R.VII C1 engine, driving a three-bladed propeller, achieving the desired performance rates, so it was decided to put it into production. A total of 45 machines of this type were manufactured (making a total of 93 Chuchuligas of every type), however, the production version of the Kb-5 differed from the modified prototype in being powered by a single Walter-Pollux engine rated at 444 hp of power, fitted in a Townend ring and driving a two-bladed propeller. 
The Chuchuliga III featured increased fuel capacity and was fitted with radio and photographic equipment. It was armed with a single 7.7 mm FK-33 machine gun mounted in the nose, synchronised to fire through the propeller. It was equipped with a second FK-33 placed in the observer's position and could carry up to 200 kg (441 lb) of underwing bombs, 100 kg (220 lb) under each lower wing. 

The Kb-5 was the most used variant of the Chuchuliga, as it served with 113, 123, 453 and 463 reconnaissance squadrons of the Royal Bulgarian Air Force. Three aircraft also served with the 5th Bomber Regiment, stationed at Plovdiv, equipped with the Dornier Do-17M and the Avia B-71. Some few machines were also employed in the liaison role by the Naval Squadron at Varna, equipped with the Arado Ar.196 and the Heinkel He.60. 

When the USSR entered Bulgaria, they were employed actively in combat and the type was kept in active as a trainer until the early 1950s.

 

Sources:

1st http://www.airwar.ru/enc/other2/kb3.html (translated)

2nd http://www.airwar.ru/enc/spyww2/kb4.html (translated)

3rd http://www.airwar.ru/enc/spyww2/kb5.html (translated)

4th https://www.armedconflicts.com/SFKB-KB-5-Chuchuliga-SFKB-KB-5-Chuchuliga-III-t167476

5th https://www.nevingtonwarmuseum.com/sfkb-kb-4.html

6th https://en.wikipedia.org/wiki/Caproni_Ca.113