The
Cirrus Minor started life as a clean-sheet replacement for the original
Cirrus and Hermes series of light aircraft engines. Design was led by Technical Director C. S. Napier, son of
Montague Napier, and was already under way when in 1934 Cirrus-Hermes Engineering was taken over by the
Blackburn Aeroplane & Motor Company and moved to Brough in Yorkshire. Napier remained Technical Director and, while he completed the development and initial sales of the Cirrus Minor and its larger stablemate the
Cirrus Major, Blackburn kept Cirrus Hermes Engineering as a separate company (though without the hyphen in its name). Although completely new designs, they were of generally similar layout to the previous inverted engines, being air-cooled inverted four-cylinder inline designs. Like the others the Minor had a robust steel five-bearing crankshaft, but unlike them the long studs bolting the cylinder heads to the crank housing were replaced by short studs and flanges at each end of the cylinder barrels. Light alloy was used for the cylinder heads and con rods, while the valve gear was adopted with little change from the Hermes IV. The Claudel-Hobson down-draught carburetter featured independent altitude control, and the dual-redundant magnetos were of B.T.H. S.G.4/2 type. The Cirrus Minor was introduced in 1935. Two years later, Cirrus Hermes Engineering lost its identity as a separate company and was adopted as an operating division of its parent company, which by then had become
Blackburn Aircraft. 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). ==Applications==