201C - One 335kW (450hp) Pratt & Whitney R-985 nine cylinder radial piston engine driving a two blade constant speed Hartzell propeller.
620 - Same except for a three blade constant speed prop.
620TP - One 375kW (500shp) Pratt & Whitney Canada PT6A-11AG turboprop.
Pilot only in all models.
Hopper capacity in 201C is 1022 litres (270US gal/225Imp gal);
Hopper capacity in 620 is 1268 litres (335US gal/280Imp gal);
Hopper capacity in 620TP is 1287 litres (340US gal/283Imp gal).
The three model types were built in relatively small numbers (including more than 100 Model 201s). By mid 2000 at least 155 Model 620/620TPs had been built.
The Weatherly 201 and 620 agricultural aircraft were built in relatively small numbers from the late 1960s.
The Weatherly 201 and its successors date back to the Weatherly WM-62C agricultural aircraft, 19 of which were built between January 1961 and late 1965. The WM-62s were agricultural conversions of the basic Fairchild M-62 airframe, fitted with a chemical hopper, spray booms and either a Wright W-670 or Pratt & Whitney R-985 radial engine.
Weatherly used its experience in designing and converting the WM-62s in developing its own agricultural aircraft design. The resulting Model 201 was a larger aircraft overall of conventional ag aircraft configuration, with a low wing, a Pratt & Whitney R-985 radial, integral chemical hopper and spray booms and an enclosed cockpit for the pilot. The initial production 201, the 201A, was certificated in 1967, followed by the 201B, and the improved 201C was granted its type certificate in 1975. A unique feature was the use of wingtip vanes which were developed to increase the effective swath width, while reducing the amount of chemicals lost from the spraying swath (they could be folded back beneath the wings for hangar storage.
The Model 620 replaced the 201C in production from 1980. The 620 featured a longer span wing with optional wingtip vanes and a larger chemical hopper, but the same R-985 engine. The 620A had a 65USgal fuel capacity, the 620B had a 4in stretched fuselage to give a 88USgal capacity.
The Pratt & Whitney Canada PT6A-11AG turboprop powered 620TP joined the 620 in production in 1980, and featured a longer nose and a slightly larger chemical hopper. This 620TP was available as the 620A-TP and 620B-TP.
The 620B-TP was replaced by the 620B-TG which is powered by a 496kW (665shp) Honeywell TPE331-1 turboprop. The 620B and 620B-TG are still in low-rate production.