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The Radial Engine

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 large aircraft engines before most large aircraft started using turbine engines.

In a radial engine, the pistons are connected to the crankshaft with a master-and-articulating-rod assembly. One piston, the uppermost one in the animation, 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. Four-stroke radials always have an odd number of cylinders per row, so that a consistent every-other-piston firing order can be maintained, providing smooth operation. This is achieved by the engine taking two revolutions of the crankshaft to complete the four strokes, (intake, compression, power, exhaust), which means the firing order is 1,3,5,2,4 and back to cylinder 1 again. This means that there is always a two-piston gap between the piston on its power stroke and the next piston to fire (i.e., the piston on compression). If an even number of cylinders was used, the firing order would be something similar to 1,3,5,2,4,6, which leaves a three-piston gap between firing pistons on the first crank shaft revolution, and only a one-piston gap on the second crank shaft revolution. This leads to an uneven firing order within the engine, and is not ideal. 

Most radial engines use overhead poppet valves driven by pushrods and lifters on a cam plate which is concentric with the crankshaft, with a few smaller radials, like the five-cylinder Kinner B-5, using individual camshafts within the crankcase for each cylinder. A few engines utilize sleeve valves instead, like the very reliable 14 cylinder Bristol Hercules (built up to 1970' under licence in France by SNECMA) and the powerful 18 cylinder Bristol Centaurus.


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 52 hp (39 kW) at 950 rpm.

In 1903-04 Jacob Ellehammer used his experience 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. This was installed in his triplane and made a number of short free-flight hops. During 1908-9, Ellehammer developed another engine, which had six 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, originally built as a "semi-radial" W3 configuration design, one of which powered Louis Blériot's Blériot XI in his July 25, 1909 crossing of the English Channel. By 1914 Anzani had developed their range, their largest radial being a 20-cylinder engine of 200 hp (150 kW), with its cylinders arranged in four groups of five. One of the three-cylinder "fully radial", 120º cylinder angle Anzani powerplants still exists today, in fully running condition, in the nose of Old Rhinebeck Aerodrome's restored and flyable 1909 vintage Blériot XI. There is also another running Anzani at Brodhead airfield to go on a replica Blériot XI.

Radial engines are regarded as being air-cooled almost by definition—so that it is interesting that one of the most successful of the early radial engines was the Salmson 9Z series of 9 cylinder water-cooled radial engines that were produced in large numbers during the First World War. Georges Canton and Pierre Unné patented the original engine design in 1909, offering it to the Salmson company—and the engine was often known as the Canton-Unné.

The radial engine was not developed at this time in Germany: two radial engines were made there 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 standardization ruled out any but proven engine types.

During the decade 1910-1920 the radial engine was largely overshadowed by its close relative, the rotary engine—which differed from the so called "stationary" radial in that the whole engine revolved with the propeller. In WWI, many French and other Allied aircraft flew with Bentley, Clerget, Gnome and Le Rhone rotary engines, the ultimate examples of which produced about 240 hp (180 kW), with the Germans either making close copies of the Gnome and Le Rhone powerplants built by the Oberursel firm, or, late in the war, using the unique Siemens eleven-cylinder rotary engine. By the end of the war the rotary engine was already essentially obsolete, being superseded as a type by rapid development of true radials.

Posted Wed, Apr 20 2011 12:06 PM by flyvertosset | Report Abuse