Definition
The main rotating shaft of a piston engine that converts the up-and-down (reciprocating) motion of the pistons into rotational motion. In an aircraft engine, the crankshaft transmits this rotation to the propeller, either directly or through a reduction gear.
Plain English
The long shaft inside the engine that turns the push-and-pull motion of the pistons into a spinning motion, which then drives the propeller.
Context Anchor
Seen in engine and propeller discussions, especially when learning how a piston engine turns fuel-burning power into propeller rotation.
Derivation
From 'crank' (a bent arm or lever used to turn something) and 'shaft' (a long rotating rod). The shaft has bends, or 'throws,' built into it that the pistons push on, the same way you'd turn a hand crank.
Why Pilots Care
A properly functioning crankshaft is essential for transmitting engine power to the propeller; failure results in immediate loss of thrust.
Analogy
Think of pedaling a bicycle. Your legs push straight up and down, but the pedal arms (the cranks) convert that into the round spinning of the wheel. A crankshaft does the same job for the pistons.
Intuition Check
Do not think of the crankshaft as a handle the pilot cranks by hand. In this context, it is an internal engine part that rotates while the engine is running.
Example Sentence 1
After the prop strike, the mechanic recommended a full engine teardown to inspect the crankshaft for hidden damage.
Example Sentence 2
Mechanics inspect the crankshaft bearings for wear during an engine overhaul.