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How Does A Radial Engine Work?

You've probably heard of a radial engine. They're the powerhouses of early aviation, up through the beginning of the jet age These engines are amazing. But why were they invented and how do they work? And, why did they disappear? Check it out...

Pure Power In A Circle

Radial engines entered development before the Wright brothers made their first powered flight, when C.M. Manly created a liquid cooled five-cylinder radial engine for Samuel Langley's Aerodrome aircraft.

At the time, they competed with rotary engines and inline water-cooled engines. But, by the end of World War I, rotary engines reached their peak and radial engines quickly overshadowed them.

Air-cooled radial engines have quite a few advantages over their inline cousins. They're lighter than liquid-cooled inline engines and since they don't rely on coolant, they're more damage-resistant. Radial engines are simpler - the crankshafts are shorter and they need fewer crankshaft bearings. They're more reliable and run smoother.

But radial engines do have downsides. Their massive frontal area creates drag and limits pilot visibility. Radial engines need significant airflow to cool the cylinders, so engine placement on the aircraft is limited. It's nearly impossible to install a multi-valve valve train - so nearly all radial engines use a two-valve system, limiting power. And, while a single bank of cylinders cools evenly, larger engines use rows of cylinders. The rear rows are masked by the front rows, and the air's already hot after passing the first set of cylinders - which limits cooling.

How Does A Radial Engine Work?

A radial engine works like any other four-stroke internal combustion engine. Each cylinder has an intake, compression, power and exhaust stroke. They differ from inline and horizontally opposed engines in their firing order and they way they connect to the crankshaft

A radial engine's cylinders are numbered from the top, going clockwise, with the first cylinder numbered 1. The first cylinder's connecting rod attaches directly to the crankshaft - this is the master rod. The other cylinders' rods connect to pivoting points around the master rod.

radial-engine-800px Boldmethod

Every radial engine has an odd number of cylinders, and they fire in an alternating order. So, a five-cylinder engine fires in a 1, 3, 5, 2, and 4 order. A seven-cylinder engine fires in an 1, 3, 5, 7, 2, 4, 6 order.

firing-order Boldmethod

As the cylinders fire, the rod assembly rotates around the crankshaft, spinning it like a bell crank would. A counterbalance weight sits opposite from the rod hub to prevent engine vibration.

Turbines Stole The Market

To get more power out of a radial engine, engineers added multiple rows of cylinders. The Pratt & Whitney Wasp Major uses four rows of seven cylinders (that's 28 total cylinders!) with a supercharger to generate up to 4,300 horsepower. It powered many of the last piston-powered large aircraft, including the B-36 Peacemaker (which used six Wasp Majors and four turbojets) and the Martin Mars.

B-36 Peacemaker

Ultimately, the turbine and turboprop engines developed after World War II could develop much more power than a radial engine, more efficiently and with less weight. But it doesn't change the fact that radial engines look cool, and sound even better.

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at

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