baobab72:
An example of using a turboshaft eneine to provide power to a helicopter.
In the Sikorsky S-70 series of Helicopters, the power control levers levers, (PCLs) are situated on the overhead/ceiling of the cockpit. They are connected directly to the HMU (part of the fuel control/governor, which is finely controlled by the ECU or DECU (ecu = electronic control unit, another governor), and then fuel is metered in gross terms by the Load Demand Spindle. (see below) Engine speed signal is feed back into the HMU and ECU as a part of this design. (And IIRC, so is Nr, but memory is a bit sketchy here).
What you typically do is start the engines, make sure all is running OK, and then move the power levers into a "fly" detent with the idea being that you will keep the Nr at 100% rated speed by use of the governor in the HMU and the ECU/DECU. (D for digital)
This means that you rarely, if ever, touch the power control levers in flight. You instead increase or decrease demand for engine power to the transmission and rotor system by moving the collective up and down, which is where the Load Demand Spindle comes in. Pull up, earth gets smaller, lower and earth gets larger. TC, are you still with us?
If I pull the collective up, the HMUs fuel metering valve changes position to allow more fuel to flow to the engines, though the the ECU fine tunes the fuel flow to keep the Nr as close to 100% (258 RPM in a Blackhawk or Seahawk, and I suspect most S-70s) as it can. It's quicker than the HMU.
If I lose electrical power, my ECU or DECU can't fine tune the fuel flow, so that Nr will vary a bit as I change power demand with the collective. The HMU slowly catches up in its governing functions to keep Nr at 100%. This leads to operator manuals usually advising to move collective slowly when in more or less 'manual' fuel mode, to avoid surges in NR and engine speed.
If your ECU or DECU begins to go wrong, you can lock it out and revert to 'manual" power control (which is actually just using a gross governor rather than a fine governor) which will again be driven by collective position and the HMU's gross governing function.
In other turbine engine helicopters I have flow, there is a fine tuning "beep" button for the ECU to fine tune or control the fuel flow depending upon demand, or in a case where the fuel control malfunctions and you need to adjust Nr as you fly to avoid overspeed or underspeed of Nr or engines.
Each helicopter manufacturer who installs turboshaft engines will adopt a particular combination of controls so that governing of engine power in flight is mostly done without the pilot constantly having to adjust fuel flow manually.
There are a lot of good design reasons to do this, which inclue overspeed and overtemp protection.
I hope that helps.