#biplane

The Polikarpov I-153DM was one of the Soviet Union’s most unusual experimental fighter aircraft, combining the proven Polikarpov I-153 “Chaika” biplane with pioneering ramjet propulsion. The suffix DM, short for Dopolnityelnyi Motor (“supplementary engine”), referred to the installation of two underwing ramjet engines designed to boost the aircraft’s speed without replacing its conventional piston engine. Developed in 1940, the project reflected Soviet interest in advanced propulsion systems during the years immediately preceding the widespread introduction of turbojet aircraft.

The standard I-153 was powered by a Shvetsov M-62 radial engine driving a two-bladed propeller and was renowned for its exceptional manoeuvrability despite its biplane configuration. Engineers fitted experimental Merkulov DM-2 and later DM-4 gasoline-burning ramjets beneath the lower wings. Unlike turbojets, ramjets contain no moving compressor and rely entirely on the aircraft’s forward speed to compress incoming air. As a result, the ramjets could not operate from take-off and only became effective after the aircraft had reached sufficient speed under piston-engine power.

Flight testing demonstrated that the concept worked. Aircraft fitted with the smaller DM-2 ramjets achieved an increase in maximum speed of approximately 30 km/h (19 mph), while the more powerful DM-4 installation produced gains of up to 50 km/h (31 mph). A total of 74 experimental flights were conducted, proving that auxiliary ramjets could provide a measurable performance improvement without fundamentally altering the aircraft’s design.