The design of the MA-1 by Murrayair Ltd., a Hawaii-based operator, was based on an extensive modification of the Boeing Stearman 75 Kaydet biplane. Air New Zealand, under contract to Murrayair, started in September 1968 to build the prototype using a Stearman airframe and the aircraft, N101MA c/n 001, first flew on July 27, 1969. Subsequently the aircraft was dismantled and shipped to Honolulu, Hawaii, where FAA certification trials were conducted, leading to the Restricted Category Type Approval on April 14, 1970.
The agricultural MA-1 was powered by the 600 hp Pratt & Whitney R-1340-AN1 Wasp radial piston engine, had a larger wing compared to the Stearman and a 450 gal (1,703 l) hopper in front of the cockpit. The enclosed cockpit has a bench seat for the pilot, but can also accommodate a ground-crew member seated on a jump seat. The hopper could carry 3,000 lb (1,360 kg) of fluid, powder or granular products for treating crops, etc.
For manufacturing Murrayair formed Emair at Harlingen, Texas and produced the aircraft as the MA-1 Paymaster. Briefly marketed as the Agronemair MA-1 Paymaster, 28 aircraft (c/n 002 to 029) were completed by January 1976.
In the meantime the company was taken over by George A. Roth, Emair operating as a Division of Emroth Co. and the MA-1 became subject of a redesign. In addition to the 1,200 hp (derated to 900 hp) Wright R-1820 radial engine, it had new wings, a larger vertical tail of different shape and a 25 gal (95 l) enlarged hopper.
The first MA-1B Diablo 1200, as the type became known, flew for the first time August 1975; Type Approval was gained in May 1976. Emair produced 23 aircraft (c/n 30 to 52) before production was halted in 1980; several of the MA-1s produced were converted to the MA-1B Diabolo.
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Single-seat agricultural biplane
One 600 hp Pratt & Whitney R-1340-AN1 Wasp nine-cylinder radial air-cooled piston engine
41 ft 8 in (12.7 m) (upper), 35 ft 0 in (10.67 m) (lower)
28 ft 8 in (8.74 m)
11 ft 3 in (3.43 m)
400 sq.ft (37.16 sq.m)
3,746 lb (1,699 kg)
7,000 lb (3,175 kg)
148 mph (238 km/h)
59 mph (95 km/h)
1,700 ft (518 m)/min