Cirrus Aero Engines is a British manufacturer that has produced a series of aero engines from the 1920s until the 1950s under various changes of ownership. All engines were air-cooled, four-cylinder inline types, with earlier models upright and later designs inverted. The first Cirrus engine was created for the de Havilland Moth light aeroplane in 1925, which created a market for private flying. The engine, along with its successors, was widely used for private and light aircraft from that moment on.
The Cirrus engine originated in Geoffrey de Havilland’s 1924 quest for a powerplant suited to a light two-seat sports biplane which would become the de Havilland Moth. The Aircraft Disposal Company (ADC), was producing low-cost Airdisco V8, which was developed by Frank Halford from their large stocks of war surplus Renault V8 aero engines. De Havilland realized that half of this engine would make an air-cooled four-cylinder inline engine of just the right size and at low cost. He persuaded Halford to undertake its design and development.
Cirrus Aero Engines Limited was formed in 1928 at Croydon to manufacture the Cirrus models from scratch, as ADC began to run out of the Renault engines. Although Halford was no longer associated with it, the Cirrus company continued to develop new models, with the uprated Hermes model being the last of the line. The Cirrus engines were simple enough to be understood and looked after by the private owner, while their reliability made private flying safe for the first time, all at an affordable cost.