The Yeoman Cropmaster is an agricultural aircraft that was developed in Australia by Yeoman Aviation. The company was set up by Kingsford Smith Aviation Services Pty. Ltd. (KSA) at Bankstown Airport to engage in agricultural aircraft production. KSA had obtained a number of CAC Wackett trainers following their retirement from Royal Australian Air Force service and had converted four for agricultural use as KS-3 Cropmasters in the second half of the 1950s. The conversion involved the installation of a hopper located in the rear cockpit of the Wackett, the cutting of a hole in the centre section of the Wackett’s wooden wing to allow for the dispersal of the chemical load, and re-routing controls to bypass the hopper.
The Cropmaster and the CA-6 Wackett shared the same tubular steel fuselage frame. However, major modifications were made to the Wackett airframe to develop the YA-1 Cropmaster. The Wackett’s wooden wing was replaced by a new metal wing, and different external panels of metal and fibreglass were used instead of fabric to cover the fuselage. The first five aircraft retained the Wackett’s wooden tail, but subsequent aircraft had a metal tail that featured a stabilator, a swept-back fin, and a larger rudder. The Wackett’s fixed tailwheel undercarriage was retained. The Warner Scarab radial engine of the Wackett was replaced by a horizontally-opposed engine; the YA-1 250 was fitted with a Lycoming O-540 engine of 250hp, while the YA-1 250R was fitted with a Continental IO-470, also developing 250 hp (190 kW), both driving a Hartzell or McCauley propeller.
The first Cropmaster, a YA-1 250, took to the air for the first time on 15 January 1960. Twenty further aircraft were converted before production ceased in 1966. By that time the company was known as Cropmaster Aircraft. Six of the twenty-one aircraft were the YA-1 250R model, and the final three aircraft produced featured relocated main landing gear. While the Cropmaster proved unsuccessful in competing with more modern agricultural aircraft, it remains an interesting aircraft historically due to its unique development from the CAC Wackett trainer of World War II.