Definition
A bell crank is a mechanical lever, pivoted at a fixed point, with two arms set at an angle to each other. It is used in flight control systems to change the direction of motion of a control input — for example, converting a push-pull movement from a control rod into rotational or angled motion that drives another component.
Plain English
A bell crank is an angled lever that turns on a pivot. When you push one arm, the other arm swings in a different direction. Pilots see it used inside aircraft to redirect the pull of a control cable or rod so it can act on something at a different angle.
Context Anchor
Seen in flight control system diagrams, especially where cables, rods, springs, or control surfaces connect and change direction.
Derivation
Named after the lever shape originally used in old house bell systems. Pulling a cord at one end of the lever made the bell at the other end ring, even though the cord and the bell were aligned at right angles. The word stuck because the shape and function are the same in mechanical engineering.
Why Pilots Care
It allows compact routing of control linkages through tight airframe spaces while preserving precise and reliable movement of flight controls.
Analogy
A bell crank works like a small mechanical elbow. Movement comes in from one direction, the elbow pivots, and movement goes out in another direction.
Intuition Check
Do not read “bell” as meaning an aircraft warning bell, or “crank” as meaning an engine crankshaft. Here, a bell crank is simply a pivoting lever used to pass control movement through the airplane.
Example Sentence 1
The push-pull rod from the control column moves a bell crank, which then transfers the motion to the elevator cable.
Example Sentence 2
The bell crank in the wing linkage converts yoke input into opposite up-and-down movement of the ailerons.