A gas turbine engine-free turbine does not drive a compressor, i.e. there’s no mechanical connection between the compressor and the power output shaft. The energy delivered by the free turbine stages is only dependent on the rotational speed of the HP spool. In this configuration, the engine will simply act as a gas generator to supply high-energy gases to an independent free power turbine driving a propeller or rotor via a reduction gearbox. This is why this type of turbine is often used on turboprop engines and helicopters.
In fact, there will be two major assemblies: one shaft known as the HP spool and connecting the compressor and the compressor-driving turbine, and a free turbine placed after the compressor-driving turbine that is connected to a propeller. Because these two assemblies are not interconnected in any way, the propeller and free turbine assembly will be able to rotate at an independent speed from that of the HP spool. The most suitable control system for a turboprop fitted with a free turbine is the direct control of fuel flow. In this system, the gas generator (the core of the engine) is controlled in the same manner as a turbojet, and the power available to the free turbine assembly is governed by the fuel flow. For instance:

• If the power setting is increased (due to an increased fuel flow), the RPM of the HP spool will also increase and the axial airflow will be momentarily lagging, causing the EGT to increase. However, the free turbine and propeller assembly will be maintained at constant RPM by the Constant Speed Unit (CSU) that will do so by increasing the propeller’s blade angle.
• If the power setting is decreased, the RPM of the HP spool and the EGT will also decrease. However, the free turbine and propeller assembly will be maintained at constant RPM by the Constant Speed Unit (CSU) that will do so by decreasing the propeller’s blade angle.