A radial engine is an internal combustion engine which has its pistons arranged in a circular pattern around a central crankshaft. The connecting rods of the pistons are connected to a master rod, which, in turn, is connected to the central crankshaft. A bank of cylinders on a radial engine is always composed of an odd number of cylinders, which ranges from 3 to 13 ones or more.
A radial engine operates in a four-stroke cycle, with every other cylinder firing and having a power stroke as the crankshaft turns around, giving the engine an smooth operation. Since it has four-stroke cycle, radial engine crankshaft requires two revolutions to complete the four strokes of each piston, which are intake, compression, combustion, and exhaust. Internal combustion radial engine has been used mostly on medium and large size propeller-driven aircraft. For large aircraft, two or three banks of cylinders are mounted together one behind the other on a single crankshaft.
The British Sopwith Camel biplane had the engine so mounted with the propeller fastened to the rotating bank of cylinders. One of the most famous radial engine used in aviation is the Continental R-670, which was made up of seven cylinders, displacing 668 cm³. However, it was the Yakovlev M-501 the biggest and most powerful radial engine ever designed. The gyroscopic forces generated by the large rotating engine mass allowed aircraft fitted with these engines perform astonishing maneuvers which was not possible with engines whose pistons were arranged in a line.
Below, an example of aviation radial engine; a Bristol Pegasus IIIM, 9-cylinder engine, which powered the Fairey Swordfish biplane.
Schematic drawing of a radial engine.
