freeturbine

A freeturbine, commonly referred to as a free turbine, is a type of gas turbine engine arrangement in which the power turbine that drives the load is mechanically independent from the gas generator turbine that drives the compressor. The engine contains two shafts: a high‑speed shaft for the gas generator (compressor and combustor) and a separate, independent power turbine shaft that delivers power to the output load, such as a helicopter rotor or a propeller through a reduction gearbox. The term “free” reflects the lack of a direct mechanical connection between these two turbines.

How it works: in operation, the combustion gases pass through the gas generator turbine to extract energy for the compressor. After passing through the gas generator stage, the gases expand through the power turbine, which is on its own shaft and is not coupled to the gas generator shaft. The power turbine’s rotational speed is matched to the load via a gearbox, allowing the gas generator to run at a different, optimally efficient speed. This decoupling enables rapid spool-up and load changes without forcing the gas generator to follow load variations.

Applications and advantages: free-turbine engines are widely used in turboshaft engines for helicopters and in many turboprop engines. The design provides easier starting and better performance across a wide range of flight conditions, as the power turbine can accelerate independently of the gas generator. It also allows simpler engine restarts under load and can improve engine tolerance to sudden load changes. Disadvantages include added mechanical complexity and weight, along with a potential reduction in peak efficiency compared with some alternative configurations.

Examples include several widely used turboshafts such as the PT6 family and other helicopter and turboprop engines that employ a two-shaft, free-turbine arrangement.