page_title: Aircraft engine
section_title: Radial engine
hierarchical_section_title: Aircraft engine / Shaft engines / Reciprocating (piston) engines / Radial engine
caption_attribution_description: Pratt &amp; Whitney R-2800-21 Radial Engine
context_page_description: An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Most aircraft engines are either piston engines or gas turbines, although in recent years many small UAVs have used electric motors.
context_section_description: This type of engine has one or more rows of cylinders arranged around a centrally located crankcase. Each row generally has an odd number of cylinders to produce smooth operation. A radial engine has only one crank throw per row and a relatively small crankcase, resulting in a favorable power-to-weight ratio. Because the cylinder arrangement exposes a large amount of the engine's heat-radiating surfaces to the air and tends to cancel reciprocating forces, radials tend to cool evenly and run smoothly.  The lower cylinders, which are under the crankcase, may collect oil when the engine has been stopped for an extended period. If this oil is not cleared from the cylinders prior to starting the engine, serious damage due to hydrostatic lock may occur.
  Most radial engines have the cylinders arranged evenly around the crankshaft, although some early engines, sometimes called semi-radials or fan configuration engines, had an uneven arrangement.  The best known engine of this type is the Anzani engine, which was fitted to the Bleriot XI used for the first flight across the English Channel in 1909.  This arrangement had the drawback of needing a heavy counterbalance for the crankshaft, but was used to avoid the spark plugs oiling up.
  In military aircraft designs, the large frontal area of the engine acted as an extra layer of armor for the pilot.  Also air-cooled engines, without vulnerable radiators, are slightly less prone to battle damage, and on occasion would continue running even with one or more cylinders shot away. However, the large frontal area also resulted in an aircraft with an aerodynamically inefficient increased frontal area.
caption: A Pratt & Whitney R-2800 engine
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