Definition
In reciprocating engine design, a cylinder arrangement in which cylinders are placed in two rows on opposite sides of the crankshaft, lying flat and pointing directly away from each other (also called a flat or horizontally opposed engine).
Plain English
An engine layout where the cylinders lie flat on each side of the crankshaft, pointing away from each other like a pair of outstretched arms, instead of standing upright in a line or arranged in a V.
Context Anchor
Seen when describing common light-airplane piston engines, especially horizontally opposed engines.
Derivation
From the Latin opponere, meaning 'to set against.' The cylinders are literally set against each other across the crankshaft, which is exactly what the layout looks like.
Why Pilots Care
The opposed arrangement is by far the most common engine layout in light aircraft. Knowing the term lets a pilot read engine specifications, maintenance documents, and POH descriptions without confusion.
Intuition Check
Opposed does not mean the engine is fighting or resisting something. Here it means parts of the engine are positioned across from each other.
Example Sentence 1
The Cessna 172 is powered by a four-cylinder horizontally opposed Lycoming engine.
Example Sentence 2
Many light aircraft use an opposed engine to keep the center of gravity low and reduce vibration.