Definition
A turbine in a gas turbine engine that is not mechanically connected to the compressor. It is driven by the exhaust gases from the gas generator section and turns independently to drive an output shaft, typically connected to a propeller, rotor, or fan through a reduction gearbox.
Plain English
A turbine wheel inside the engine that spins on its own shaft, separate from the part of the engine that produces the hot gases. Hot gas blows through it and makes it spin, and that spinning is what drives the propeller or rotor.
Context Anchor
Seen in descriptions of turboprop and helicopter turbine engines, especially when explaining how engine power reaches the propeller or rotor.
Derivation
Called 'free' because the turbine is mechanically free of the compressor — it is not joined to the compressor by a shaft, so it can spin at its own speed independent of the gas generator.
Why Pilots Care
The design lets the propeller turn at its most efficient speed regardless of gas generator RPM, improving performance and enabling safe feathering.
Analogy
Think of one pinwheel placed behind another in a stream of air. The first part of the engine creates the fast-moving gas, and that gas spins the free turbine behind it without the two being tied together by the same rod.
Grounding Statement
In a free-turbine engine, gas flow is the connection between the engine core and the power-producing turbine, not a solid mechanical link.
Intuition Check
Free does not mean loose, uncontrolled, or disconnected from the aircraft. Here it means the turbine is not mechanically tied to the compressor-driving shaft.
Example Sentence 1
The PT6 is a free turbine engine, so the propeller can remain stationary on the ground while the gas generator section is running.
Example Sentence 2
During a normal shutdown the free turbine coasts to a stop once fuel is cut and gas flow drops.