Definition
Pendulum-style weights attached to the crankshaft of certain reciprocating aircraft engines that swing on pins rather than being rigidly bolted, allowing them to absorb and dampen the torsional (twisting) vibrations produced by each cylinder firing. Because they pivot freely within designed limits, they can be detuned or damaged by abrupt power changes, sudden throttle movements, or operating the engine outside approved RPM and manifold pressure combinations.
Plain English
Small swinging weights inside the engine that smooth out the twisting pulses caused by the cylinders firing. They are loosely mounted so they can move, which makes them effective at damping vibration but also vulnerable to damage from rough handling of the throttle.
Context Anchor
Encountered in piston-engine operating guidance, especially when discussing smooth power changes and engine care in multiengine airplanes.
Derivation
Dynamic, from Greek dynamis (power, force), refers to something that moves or responds to force rather than being fixed. Counterweight is literally a weight that counters or offsets another force. Together: weights that move in response to engine forces in order to counteract vibration.
Why Pilots Care
Excessive vibration from an unbalanced crankshaft can accelerate engine wear and mask other mechanical issues during single-engine operations.
Grounding Statement
Each power pulse from the engine can slightly twist the crankshaft, and these weights help absorb that repeated twisting motion.
Intuition Check
Do not read “dynamic” as meaning electronically controlled or pilot-adjusted. Here it means the counterweights can move with engine forces instead of being fixed solidly in one position.
Example Sentence 1
The POH warned against rapid throttle movements at low RPM because they can damage the dynamic crankshaft counterweights.
Example Sentence 2
After an engine failure the pilot noticed increased vibration until the propeller was feathered, confirming the remaining engine's dynamic counterweights were still working correctly.