Definition
The main rotating shaft inside a piston engine that converts the up-and-down motion of the pistons into rotational motion. In an aircraft engine, the propeller is mounted directly to the crankshaft (or to a gearbox driven by it), so the crankshaft's rotation is what turns the propeller.
Plain English
The central spinning shaft inside the engine. The pistons push it round and round, and that spinning motion is what turns the propeller.
Context Anchor
Seen in piston-engine and constant-speed propeller discussions, especially when describing how engine power reaches the propeller.
Derivation
From 'crank,' meaning a bent arm or handle used to turn something, plus 'shaft.' The shaft has bends (cranks) along its length that the pistons push against, the same way you turn a hand crank to make a wheel spin.
Why Pilots Care
Any issue with the crankshaft directly affects propeller control, engine smoothness, and flight safety, making its condition critical during preflight and operation.
Analogy
It works like the pedals on a bicycle: your up-and-down leg motion is turned into circular wheel movement through a similar offset mechanism.
Intuition Check
Do not think of the crankshaft as a starter handle or an outside crank. In this context, it is an internal engine part that turns continuously while the engine is running.
Example Sentence 1
On a fixed-pitch propeller, the propeller is bolted directly to the engine crankshaft, so propeller and engine RPM are the same.
Example Sentence 2
In constant-speed propeller systems, the engine crankshaft delivers power to the propeller while the governor maintains selected RPM.