The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders point outward from a central crankshaft like the spokes on a wheel. This configuration was very commonly used in aircraft engines before being superseded by turboshaft and turbojet engines.
In a radial engine the pistons are connected to the crankshaft with a master-and-articulating-rod assembly. One piston has a master rod with a direct attachment to the crankshaft. The remaining pistons pin their connecting rods attachments to rings around the edge of the master rod (see animation). Four-stroke radials almost always have an odd number of cylinders, so that a consistent every-other-piston firing order can be maintained, providing smooth running.
Charles Manly constructed a water-cooled 5 cylinder radial engine in 1901, a conversion of one of Stephen Balzer's rotary engines, for Langley's Aerodrome aircraft. Manly's engine produced at 950 rpm.
In 1903-04 Jacob Ellehammer used his experience of constructing motorcycles to build the world's first air-cooled radial engine, a 3-cylinder engine which he used as the basis for a more powerful 5-cylinder model in 1907, which he installed in his triplane and which made a number of short free flight hops. During 1908-9 Ellehammer developed another engine, which had 6 cylinders arranged in two rows of three. His engines had a very good power-to-weight ratio but his aircraft designs suffered from his lack of understanding of control. If he had concentrated on his engines he might have become a successful manufacturer.
Another early radial engine was the 3-cylinder Anzani, one of which powered Louis Blériot's Blériot XI in his 1910 crossing of the English Channel. By 1914 Anzani had developed their range, with their largest radial being a 20-cylinder engine of , with its cylinders arranged in four groups of five.
The radial engine was not developed in Germany; two radial engines were made in Germany before World War I, but the Germans seemed to lose faith in the type under war conditions, or it may have been that insistence on standardisation ruled out any but proven engine types.
There was a debate about the merits of radial versus inline engines after World War I which continued throughout the 1930s, with both types seeing use. The radial was more popular largely due to its simplicity, and most navy air arms had dedicated themselves to it because of its improved reliability for over-water flights and better power/weight ratio for aircraft carrier takeoffs. Although inline engines offer smaller frontal area than radials, inline engines require the added weight and complexity of cooling systems and are generally more vulnerable to battle damage. A significant advantage of the air-cooled radial is that all cylinders receive equal cooling airflow.
Another advantage is that by being flat they resulted in shorter aircraft and so reduced the angular momentum enabling early fighters to turn more tightly.
The Wright company's J-5 Whirlwind radial engine of 1925 was billed as "the first truly reliable aircraft engine". Wright employed Giuseppe Mario Bellanca to design an aircraft to showcase it, and the result was the Wright-Bellanca 1, or WB-1, which was first flown in the latter part of that year. The J-5 was used on many advanced aircraft of the day, including Charles Lindbergh's Ryan NYP.
To reduce the chance of engine fires, in 1932 the French company Clerget developed the 14D, a 14-cylinder 2-stroke diesel radial engine. After a series of improvements, in 1938 the 14F2 model produced at 1910 RPM cruise power, with a power-to-weight ratio near that of contemporary gasoline engines and a specific fuel consumption of 166g/hp/hour. During WWII the research continued, but no engines were mass-produced because of the Nazi occupation, and by 1943 the engine had grown to produce over 1000 HP with a turbocharger. After the war, the Clerget company was integrated in the SNECMA company and had plans for a 32-cylinder diesel engine of , but in 1947 the company abandoned piston engine development in favor of work on the emerging turbine engines.
The Nordberg engines were initially designed for electricity production in aluminium smelters. They differed from the norm of radial design by using two opposite cylinders as a double master instead of the more usual single master rod, and managed to run perfectly circular. The engine design also permitted even numbers of cylinders in a single row with the cylinders being fired in consecutive order. The engines were a two-stroke design and were also available in a dual-fuel gas/diesel model. A number of powerhouse installations utilising large numbers of these engines were made in the US.