By the 1960s the converted World War II-vintage types that comprised a large part of the Australian agricultural aircraft fleet were in need of replacement.[1] Sasin Aircraft Service of Goulburn, an operator of a de Havilland Tiger Moth modified for use as an agricultural aircraft, decided to convert de Havilland Canada DHC-1 Chipmunks for agricultural use and contracted Aerostructures at Bankstown to perform the necessary modifications.[2]
The starting point was a Mk. 10 or Mk. 21 version of the Chipmunk. The modifications included a redesigned fuselage to accommodate a 50 Imperial gallon (227 L) hopper located where the front seat was found in the original aircraft, and a completely new cockpit in the location of the original rear seat.[2] An air-driven spray pump was mounted under the forward fuselage. The pilot sat 11 inches (28 cm) higher than in the original aircraft.[1] A turnover truss and a new energy-absorbing seat were provided to protect the pilot from injury in a crash; and cable cutters were mounted in front of the cockpit as protection against one of the main hazards to agricultural aircraft (flying into power or telephone lines). A small dorsal fin and anti-spin strakes were fitted to the rear fuselage as well.[1] [2]
Modifications to the wing included endplates instead of the standard wingtips, the removal of wing root leading edge extensions and drooping the trailing edges.[2] These modifications resulted in a lower stalling speed of 38 kn at a maximum takeoff weight (MTOW) of 2,300 lb compared to a 45 kn stalling speed at a 2,100 lb MTOW in the original aircraft.[1] The Chipmunk's original de Havilland Gipsy Major engine was retained, but the original metal propeller was replaced with a lighter fine-pitch wooden propeller.
The prototype was built using two Chipmunks; the then- Department of Civil Aviation (now the Civil Aviation Safety Authority) found problems with the fuselage modifications, which necessitated modifying a second fuselage. The prototype first flew in mid-1965 and was granted a Type Certificate and Certificate of Airworthiness on 1 September 1965; it was then damaged beyond repair in a crash at Goulburn the following day.[3] Two more aircraft were converted, one of which crashed in 1970.[2] The remaining Spraymaster was restored to stock Chipmunk configuration in the mid-2000s for use as a Warbird.[4]
Aerostructures proposed a more capable version with a 110-gallon (500 L) hopper, MTOW of 2,800 lb (1,270 kg), and a 210 hp Continental IO-360 engine. This was to be called the SA-29 Mk. 2.[3] A larger aircraft with four seats and a one-ton hopper was also proposed; in the event neither design left the drawing board.[3]
Aerostructures also performed a two-seat non-agricultural conversion called the Sundowner that had many of the same modifications as the Spraymaster, and which had wingtip fuel tanks as well.[5]
