On contrary, it appeared a successful design, with good handling and performance and ease of flying.

The first series of 10 aircraft was ordered and at least six were completed and registered before the war outbreak (SP-BRE, BRF, BRG, BRH, BRM, KAR)[2] Both aircraft were found by the LOPP paramilitary organization as successful economical planes, suitable for the plan of subsidising the development of private aviation in Poland.

At the outbreak of World War II, one RWD 21 was owned by Wilno Aero Club (SP-BRF), three by the LOPP (SP-BRE, BRG, BRH), one by private owner (SP-KAR) and two remained in the factory (SP-BPE, BRM).

[3] After the German invasion on Poland, in September 1939, two RWD 21s (SP-BPE and BRM[3]) were evacuated from the factory to Romania (one of them by a glider pilot Bronisław Żurakowski, who had not flown a plane before).

Wooden construction low-wing cantilever monoplane, conventional in layout, with a fixed landing gear and a closed cockpit.

Single-part trapezoid wings with rounded tips, two-spar, plywood (in front) and canvas covered, fitted with split flaps.