Free-turbine turboshaft

A free-turbine turboshaft is a form of turboshaft or turboprop gas turbine engine where the power is extracted from the exhaust stream of a gas turbine by an independent turbine, downstream of the gas turbine. The power turbine is not mechanically connected to the turbines that drive the compressors, hence the term "free", referring to the independence of the power output shaft (or spool). This is opposed to the power being extracted from the turbine/compressor shaft via a gearbox.

The advantage of the free turbine is that the two turbines can operate at different speeds and that these speeds can vary relative to each other. This is particularly advantageous for varying loads, such as turboprop engines.^[1]