Radial engine

The radial engine is a configuration of internal combustion engine, commonly used in aircraft, in which the cylinders are arranged pointing out from a central crankshaft like the spokes on a wheel.

Related Topics:
Configuration - Internal combustion engine - Aircraft - Cylinder - Crankshaft

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The cylinders are connected to the crankshaft with a master-and-articulating-rod assembly. One cylinder has a master rod with a direct attachment to the crankshaft. The remaining cylinders' connecting rods have pinned attachments to rings around the edge of the master rod (see animation below). 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.

Related Topics:
Connecting rod - Four-stroke - Odd number - Piston - Firing order

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For aircraft use the radial has several advantages over the inline design. With all of the cylinders at the front of the engine (in effect), it is easy to cool them with airflow. Inlines require a cooling fluid to remove heat or complicated baffles to route cooling air, as the rear-most cylinders receive little airflow. Air cooling saves a considerable amount of complexity, and also reduces weight to some degree.

Related Topics:
Aircraft use - Inline

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In addition the radial is far more resistant to damage; if the block cracks on an inline that entire cylinder bank will lose power, but the same situation on a radial will often only make that individual cylinder stop working.

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These sorts of advantages – light weight and reliability – suggest that the radial layout is a natural fit for aircraft uses. However the radial design also has two important disadvantages. One is that any supply of compressed air (from a turbocharger or supercharger) has to be piped around the entire engine, whereas in the inline only one or two pipes are needed, each feeding an entire cylinder bank. The other disadvantage is that the frontal area of the radial is always much larger than the same displacement inline, meaning that the radial will often have greater drag. For a low-speed plane this is not very important, but for fighter aircraft and other high-speed needs, this was initially a "killer problem," but was mitigated significantly with the introduction of the NACA cowling in the late 1920s. The large frontal area combined with the durability of radial engines proved advantageous to fighter aircraft at times though, particularly those in the attack role where the engine would act as an additional layer of armor for the pilot.

Related Topics:
Turbocharger - Supercharger - Cylinder bank - Fighter aircraft - NACA cowling - 1920s

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The debate about the merits of the radial vs. the inline continued throughout the 1930's, with both types seeing at least some use. The radial tended to be more popular largely due to its simplicity, and most navy air arms had dedicated themselves to the radial because of its improved reliability (very important when flying over water) and lighter weight (for carrier takeoffs).

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In the mid-1930s a new generation of highly streamlined high-speed aircraft appeared, along with more powerful inline engines like the Rolls Royce Merlin and Daimler-Benz DB 601. This re-opened the debate anew, with the needs of streamlining often winning out. However the Focke-Wulf Fw-190 and the Lavochkin La-5 and La-7 showed that a radial engine fighter could compete with the best of the inlines, given a proper installation. From that point on many new designs used radials, and after the war the inlines quickly disappeared from the now-smaller aircraft market.

Related Topics:
Rolls Royce Merlin - Daimler-Benz - DB 601 - Streamlining - Focke-Wulf Fw-190 - Lavochkin La-5 - La-7

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Originally radial engines had but one row of cylinders, but as engine sizes increased it became necessary to add extra rows. Most did not exceed two rows, but the largest radial engine ever built in quantity, the Pratt & Whitney Wasp Major, was a 28-cylinder 4-row radial engine used in many large aircraft designs in the post-World War II period.

Related Topics:
Pratt & Whitney Wasp Major - World War II

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