If we wish to vary the power output of an engine in flight we have three options.
1. Vary the mass flow rate of the air passing through it (this is usually done by varying the RPM).
2. Vary the amount of energy that we give to the airflow by varying the amount of fuel we burn (this results in variations in gas temperature).
3. Vary the pressure rise within the engine (this is usually done by varying the RPM, but can also be done to some extent by increasing fuel flow rate at constant RPM).
To achiveve maximum operational flexibility we must vary all three factors simultaneously.
The first tuboprop engines were produced simply by extending the compressor drive shaft forward and connecting it through a reduction gearbox to the propellor.
This meant that if we wanted constant propellor RPM then we would have to accept constant engine RPM. Although this worked (and still does in some engines) it made the engines more prone to problems such as compressor surge, particularly at high power settings.
By disconnecting the gas generator from the output shaft, a free power turbine permits us to vary all three of the above factors simultaneouly while keeping our output shaft RPM constant. This gives us maximum operational felxibility.
If for example we select a power increase, the fuel flow is increased. This increases the gas generator RPM, which increases air mass flow rate and also increases the pressure rise and temperature rise. At the increased RPM the compressor is more able to cope with the increased pressure ratio that is being demanded of it.
All of this occurs while the free power turbine and output shaft RPM remain constant. The overall effect is that the gas generator is permitted to operate far more efficiently that it would have done, had we demanded a power increase at a constant gas generator RPM.
Brian Abraham's statement that "Whether it be a Free Power Turbine or Fixed Turbine, all helicopters require a clutch mechanism." is untrue.
A single engine helicopter with a free power turbine does not require a clutch. But all helicopters require a free-wheel unit to permit autorotation following a turbine seizure.
Twin engine helicopters usually have a clutch on one engine. This permits that engine to be started first to drive accessories such as hydraulics, with the main rotors stationary. After the second engine has been started and the rotors run up to normal RPM, the clutch is then used to connect the first engine to the rotating rotors. Both engine swil then be driving the rotors.
Each engine will be connected to the main transmission through a separate free-wheel unit. But only the engine that is to be started first needs to have a clutch.