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The ADC Cirrus and Cirrus-Hermes are British aero engines of the mid-1920s. Sometimes known as the Blackburn Cirrus,[1] examples remain airworthy today.
Design and development ADC Cirrus engines were originally built by ADC Aircraft until Cirrus Aero Engines Limited was formed in 1927. The company became Cirrus-Hermes in 1931 when it was bought by the Cirrus-Hermes Engineering Company and later became the Cirrus Engine Section of Blackburn & General Aircraft Limited in 1934, and operated as a separate division until production ended in the post-World War II era.[2] Cirrus's first product was the 90 hp (67 kW) Cirrus I, which passed its 50 hour type rating in 1925. It was the first air-cooled inline engine, a design by Frank Halford that proved extremely popular for light aircraft. The basic layout (using one cylinder bank of an ADC Airdisco V-8 engine)[3] was quickly copied by a number of other manufacturers. Later versions named the Cirrus II, and Cirrus III were produced each with slightly greater displacement, and power (Cirrus II - 85 hp, Cirrus III - 90 hp). The next model line, Cirrus-Hermes I, II, and IV were produced ranging in power from 105 hp to 140 hp depending on type. The later Cirrus engines were designed to run inverted.[4] Variants A comparison of Cirrus Hermes I (left) and Cirrus III (right) - Cirrus I
- (1925)
- Cirrus II
- (1926)
- Cirrus III
- (1929)
- Cirrus IIIA
- (1933)
- Cirrus-Hermes I
- (1929)
- Cirrus-Hermes II
- (1930)
- Cirrus-Hermes IIB (inverted)
- (1931)
- Cirrus-Hermes IV
- (1930)
- Cirrus-Hermes IVA
- (1929) Inverted engine
Applications List from Lumsden.[5][6] Cirrus - Cirrus I
- Cirrus II
- Cirrus III
- Cirrus IIIA
Cirrus-Hermes - Cirrus-Hermes I
- Cirrus-Hermes II
- Cirrus-Hermes IIB
- Cirrus-Hermes IV
- Cirrus-Hermes IVA
Engines on display  A Cirruss Hermes Specifications (Cirrus I) Cirrus III fitted to a de Havilland DH.60 Moth See also References Notes Bibliography - Gunston, Bill. World Encyclopaedia of Aero Engines. Cambridge, England. Patrick Stephens Limited, 1989. ISBN 1-85260-163-9
- Lumsden, Alec. British Piston Engines and their Aircraft. Marlborough, Wiltshire: Airlife Publishing, 2003. ISBN 1-85310-294-6.
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