Definition
An early aircraft engine in which the cylinders are arranged radially around a central crankshaft, and the entire cylinder assembly — along with the propeller bolted to it — rotates around a stationary crankshaft. Used widely in World War I aircraft, it was largely replaced by the static radial engine, in which the crankshaft turns and the cylinders remain fixed.
Plain English
An old type of airplane engine where the whole engine spins around with the propeller, while the crankshaft in the middle stays still. The cylinders are arranged in a circle, like spokes on a wheel.
Context Anchor
Seen in powerplant maintenance texts and historic aircraft discussions when comparing early engine designs with later radial engines whose cylinders stay still.
Derivation
Rotary means turning around a center. Radial means arranged like rays or spokes from a hub. Together the name describes both the cylinder layout (spokes) and the unusual fact that the whole engine rotates.
Why Pilots Care
Understanding the rotary radial helps pilots and mechanics see why modern radial and horizontally opposed engines were developed. The rotating mass of a rotary radial produced strong gyroscopic forces that made early aircraft difficult to handle, especially in turns.
Intuition Check
Do not read “rotary radial engine” as just any round engine with a spinning propeller. In this term, “rotary” means the cylinders and crankcase rotate with the propeller, while the central crankshaft stays fixed.
Example Sentence 1
Many World War I fighters were powered by a rotary radial engine, which gave them good power-to-weight but tricky handling.
Example Sentence 2
Early biplanes used a rotary radial engine because its spinning design helped with cooling during low-speed flight.