Definition
An arrangement in a split-shaft turboprop engine in which two turbine sections spin in opposite directions on separate, mechanically independent shafts. One turbine drives the engine's compressor; the other (the free or power turbine) drives the propeller through a reduction gearbox. The two shafts are not physically connected, so each turbine is free to turn at its own speed and in its own direction.
Plain English
Two sets of turbine blades inside the engine that spin opposite ways on separate shafts. One spins to keep the engine running, and the other spins to turn the propeller.
Context Anchor
Seen when studying split-shaft or free-turbine turboprop engines and how their internal turbine sections drive different parts of the engine.
Derivation
Counter-rotating' simply means rotating in opposite directions. The term highlights that the two turbines, although stacked in the same engine, are not linked and turn against each other.
Why Pilots Care
The opposing rotation reduces overall engine torque and gyroscopic forces transmitted to the airframe, making the airplane easier to control.
Analogy
Think of two fans mounted in line, with one fan spinning clockwise and the other spinning counterclockwise. They are both part of the same airflow path, but they do not turn the same direction.
Intuition Check
Do not assume this means the airplane has two propellers rotating opposite directions. Here, it refers to turbine parts inside one engine rotating opposite ways.
Example Sentence 1
In the PT6 free-turbine engine, the counter-rotating turbines allow the propeller shaft to remain still during start while the gas generator spools up.
Example Sentence 2
During the power check, the pilot noted smooth operation from the counter-rotating turbines in the free-turbine engine.