Curtiss H-1640 Chieftain
The H-1640 was an air-cooled, two-row 12-cylinder radial with the cylinders aligned fore and aft in a hexagonal layout. The cylinder banks were placed at 60° intervals. One piece cylinder heads shared a single overhead camshaft, the propeller was directly driven. It was thought that the use of a Townend ring would prove more aerodynamically efficient over an inline engine. The engine first ran in 1927, by aligning the cylinders the diameter of the engine was less than more conventional radial engines.
The H-1640 was the first airworthy 'inline radial' and was sponsored for flight testing in a range of aircraft by the U.S. Government. Among the types selected were the Thomas-Morse XP-13 and the Curtiss XO-18. Although the engine was also intended for civil use in the 600 horsepower class continued cooling problems with the rear cylinders caused the project to be canceled with few production engines being built.