The Blackburn Cirrus Minor is a British four-cylinder, inverted, in-line air-cooled aero-engine that was designed and built by the Cirrus Engine Section of Blackburn Aircraft Limited in the late 1930s.
The Cirrus Minor started life as a development of the original Cirrus series of engines which progressed through a number of variants Cirrus I, II, & III. Each with slightly different displacement and power. Later Cirrus was bought by Hermes Engine Company and they produced the Cirrus Hermes I, II, III and IV. Again each differing slightly in displacement and power.
The Minor was known for excellent reliability, and had a major "win" when it was selected to power the RAF's Taylorcraft Auster observation aircraft. The RAF's version had several modifications, known as the Series I. Although externally identical, the Series II engine was redesigned to operate on 77 octane fuel, as opposed to the original's 70, increasing power to 100 hp (75 kW).