Definition
A turboprop engine design in which the turbine that drives the propeller is mounted on a separate shaft from the turbine that drives the compressor, with no mechanical connection between the two shafts. The propeller turbine is spun only by the flow of hot exhaust gases passing over it, allowing it to turn at its own speed independent of the gas generator section.
Plain English
A turboprop engine built in two separate spinning sections that are not bolted together. One section makes the hot gas. The other section catches that hot gas and uses it to spin the propeller. Because they are not connected by a shaft, the propeller side can run at its own speed.
Context Anchor
Seen when learning turboprop engine types, especially split-shaft or free turbine engines used in many training and transport turboprop airplanes.
Derivation
The word 'free' here means 'not mechanically connected.' The propeller turbine is 'free' to spin at its own speed because no shaft ties it to the rest of the engine. 'Power turbine' is the turbine that extracts power for the propeller, as opposed to the turbine that only powers the compressor.
Why Pilots Care
On engine start, the propeller may not turn until the gas generator is producing enough exhaust flow to spin the power turbine. This is normal behavior and looks very different from a fixed-shaft turboprop, where the propeller turns the moment the engine starts rotating. Knowing the engine type changes how you interpret start sequences, power response, and propeller behavior on the ground.
Analogy
Think of a pinwheel held in front of a hair dryer. The hair dryer has its own motor inside, but the pinwheel is not connected to that motor at all -- it spins only because of the air being blown at it. A free power-turbine engine works the same way: the hot exhaust does the blowing, and the propeller turbine is the pinwheel.
Intuition Check
“Free” does not mean the turbine has no limits or control. It means the propeller-driving turbine is not mechanically locked to the compressor-driving turbine.
Example Sentence 1
Because the PT6 is a free power-turbine engine, the propeller stayed nearly still during start until the gas generator came up to speed.
Example Sentence 2
During the approach, the pilot reduced power on the free power-turbine engine while keeping propeller RPM steady.