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Radial Vs. Inline Aircraft Engines

Radial and in-line aircraft engines were the earliest engine designs used in aviation. Although the first radial aircraft engine was developed two years earlier than the in-line aircraft engine, an in-line engine was the first to successfully power an aircraft in flight. That historic first flight was made by the Wright Brothers in Kitty Hawk, North Carolina, in 1903.

Radial Engines
- Charles Manley invented the radial engine in 1901. Radial engines are wheel-shaped and composed of cylinders arranged around a crankshaft in the center. Manley's engine weighed only 2.4 lbs. for each horsepower it generated and delivered 52 horsepower at 950 rpm. It never powered an aircraft in flight because of design problems with the Langley Aerodrome for which it was built. Nevertheless, the technology was embraced and there were 22 known types of radial aircraft engines in production just prior to World War I. These engines produced from 30 to 600 horsepower.
In-line Engines
- In 1903, Charlie Taylor's in-line engine successfully enabled the Wright Brothers to make their first flight. In-line engines have cylinders arranged in a straight line, one behind another, with the crankcase beneath them. Taylor's four-cylinder engine weighed 179 lbs. and delivered 12 horsepower at 1,025 rpm. The American-made, in-line, 400-horsepower Liberty V-12 aircraft engine powered the British DH-4 combat airplane during World War I. Over 13,000 Liberty engines were manufactured for the war effort where they proved themselves to be both powerful and reliable.
Radial Engine Advantages
- The round design of the radial engine makes it more compact and lightweight than an in-line engine. Because these engines are mounted directly behind the aircraft propeller in the front of the airplane, the entire engine is exposed to the air and is easily cooled by that air alone. In-line engines have to be liquid cooled, since only the front of the engine is exposed to the air. Cooling systems make in-line engines heavier, more complex and more vulnerable to damage from enemy fire.
In-line Engine Advantages
- In-line engines generate less drag than radial engines because they have less surface area to move through the air. The biggest advantage of in-line engines over radial engines, however, is that they can produce more power. In-line aircraft engines dominated the World War II military airplane industry because of this. Leading examples of this type of engine were the Rolls Royce Merlin that powered the Spitfire and Mustang fighters, and the Daimler-Benz DB 601 engine that powered the later models of the Messerschmitt Bf109. These engines were considered to be leading-edge technology until jet engines supplanted them in the 1950s.

Aviation