During testing of the prototype aircraft RWD-16 bis Eng. Andrzej Anczutin and Eng. Tadeusz Chylinski developed a draft version of this airplane with an engine more powerful, which received the designation RWD-21. With a more powerful performance of the airplane rose substantially. The design of the airplane was not fundamentally different from RWD 16 bis. RWD-21 and RWD 16 bis could distinguish between the different engine cover and windows.
The RWD 21 was a development variant with a stronger 90 hp engine Cirrus Minor and some minor changes, mostly to a canopy.
A wooden construction low-wing cantilever monoplane, conventional in layout, with a fixed landing gear and a closed cockpit. Fuselage semi-monocoque, plywood-covered, duralumin in front engine section. Single-part trapezoid wings with rounded tips, two-spar, plywood (in front) and canvas covered, fitted with split flaps. Conventional cantilever empennage, plywood (fins) and canvas (elevators and rudder) covered. Two seats side-by-side, with twin controls, under a canopy, with a fixed windshield. Behind a cockpit, a place for a baggage. The control column was placed between pilot seats which were situated side-by-side. Conventional fixed landing gear with a rear skid, the main gear in covers. 4-cylinder straight engine in front, driving a two-blade tractor wooden propeller Szomański of a fixed pitch. Fuel tank 73 l in mid-wing section, under the crew seats.
The prototype was flown in February 1939 (registration SP-BPE), the pilot being Eugeniusz Przysiecki. 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).
The aircraft were found by the LOPP paramilitary organization as successful economical planes, suitable for the plan of subsiding a development of the private aviation in Poland. A series of RWD 16bis was ordered by the LOPP at a price of 17,800 złoty (including the engine 6,200 zł), in order to sell airframes to private owners for 9,500 zł (the price of a mid-class car), lending them engines. The price of the RWD 21 was 20,500 zł (including the engine 8,000 zł).
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). After the German invasion on Poland, in September 1939, two RWD 21s (SP-BPE and BRM) were evacuated from the factory to Romania (one of them by a glider pilot Bronisław Żurakowski, who had not flown a plane before). At least one of them (SP-BPE) was used in Romania with markings YR-VEN and returned to Poland after the war. It was next used until the mid-1950s with new markings SP-AKG. It is currently restored and preserved in the Polish Aviation Museum in Kraków. The aircraft was restored then at the No 3 Military Aviation Works in Dęblin. The engine was also restored and started. Since 1995, it is presented in the pre-war colours.
One RWD-21 was commissioned as a show for Egypt. It was not delivered because of the war.
One RWD 21 was evacuated to Latvia, its further fate is unknown.
Engine: 1 x Blackburn Cirrus Minor, 65kW / 90hp
Wingspan: 11.00 m / 36 ft 1 in
Wing area: 14.95 sq.m / 160.92 sq ft
Length: 8.40 m / 28 ft 7 in
Height: 2.12 m / 7 ft 11 in
Max take-off weight: 685 kg / 1510 lb
Empty weight: 425 kg / 937 lb
Max. speed: 210 km/h / 130 mph
Cruise speed: 180 km/h / 112 mph
Stall speed: 72 kph
Ceiling: 5500 m / 18050 ft
Range: 650 km / 404 miles
Climb rate: 4.7 m/sec