Some 70 per cent of parts were common to both the M-18 and M-21, among them the outer wing panels (including flaps and ailerons); rear fuselage (including cockpit and tailwheel); mainwheels and brakes; upper surface of hopper, including hood; parts of the fuel, oil, hydraulic and electrical systems; and some of the agricultural equipment. New to the M-21 were the PZL-3SR engine and its propeller, wing centre-section, enlarged tail surfaces, main landing gear legs, lower part of hopper, and forward fuselage. Some standard subassemblies were, however, used in the new assemblies.
Construction of two flying prototypes and static testing of the airframe started in August 1981, and the first prototype (SP-PDM c/n 1ALP01-01 (site files) flew for the first time on June 18, 1982, powered by a PZL-3S engine. First flight of the same aircraft with a PZL-3SR engine was made on October 20, 1983. The second prototype ((SP-PDN c/n 1ALP01-02), also with the PZL-3SR engine, made its initial flight on March 21, 1985, gaining a provisional type certificate. Agricultural service trials were conducted in Yugoslavia during June-September 1986 and March-June 1987. However, series production was not undertaken.
The first prototype is now a static museum piece at the premises of PZL Mielec. The pictured second prototype had been used for many years as an eye-catcher by Motortech GmbH in Celle, Germany, and was obtained by Fritz Koldehofe in late 2007. In 2008 the aircraft was dismantled and subsequently shipped to PZL in Mielec, Poland and completely overhauled. On August 13, 2009 a PZL company pilot delivered the aircraft, now registered SP-YFK, to Weser-Wümme airfield in Hellwege, Germany. The aircraft is regularly flown by Fritz Koldehofe, pictured here in front of his aircraft at the annual Yak-Treffen (Yak meeting) at Roitzschjora.
AIRFRAME: Generally similar to M-18, but main landing gear is of split-axle type with oleo-pneumatic shock absorbers.
POWERPLANT: One 600 hp PZL-3SR seven-cylinder air-cooled radial engine, driving a 10 ft 2 in (3.10 m) four-blade propeller.
SYSTEMS: Hydraulic system pressure 2,220 lb/sq.in (153 bars). Electrical system powered by 1.5kW alternator and 18Ah battery.
AVIONICS: RS6201 Unimor VHF com transceiver.
Span: 47 ft 7.25 in (14.51 m)
Length: 31 ft 1.25 in (9.48 m)
Height: 10 ft 2.5 in (3.11 m)
Tailplane span: 18 ft 4.5 in (5.60 m)
Wing area: 350.9 sq.ft (32.60 sq.m)
Wheel track: 8 ft 10.25 in (2.70 m)
Empty weight: 4,541 lb (2,060 kg)
Loaded weight: 7,495 lb (3,400 kg)
Max takeoff weight: 7,936 lb (3,600 kg)
Wing loading: 22.62 lb/sq.ft (110.4 kg/sq.m)
Power loading: 13.23 lb/hp (8.05 kg/hp)
Max level speed: 130 mph (210 kmh)
Operating speed: 97-112 mph (155-180 kmh)
Stall speed: 67 mph (108 kmh)
Climb: 985 ft (300 m)/min
Service ceiling: 13,125 ft (4,000 m)
Range: 373 mls (600 km)
Endurance: 3 hr