Inline engine (aeronautics)

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An inline engine in aviation is a reciprocating engine with its cylinders arranged in one or more straight rows along the crankshaft. A single row is called a bank; more than six cylinders in a bank is uncommon. The main rival is the radial engine, where cylinders form a circle around the crankcase.

In aircraft, “inline” is used a bit differently than in cars. Car engines called inline are usually straight engines. In aircraft, inline can refer to engines that are not strictly straight, including V, H, or horizontally opposed layouts. Some engines with a single bank are considered variants of radial engines.

An upright engine has cylinders above the crankshaft; an inverted engine has cylinders below. Inverted designs offer advantages for aircraft: easier access to parts for ground crews, a lower center of mass, and better pilot visibility for nose-mounted engines, while keeping the widest part of the engine near the fuselage’s midline.

Notable inverted inline engines include the Hirth HM 60 (four cylinders, 1924) and the de Havilland Gipsy Major (widely used in the Tiger Moth). The Daimler-Benz DB 601 is an inverted V engine used in the Messerschmitt Bf 109.

Inline engines can have many banks, but cost, complexity, and reliability limit practical designs.