hi folks here I am with a couple of new questions I'd love to have your expert advice about.
As far as I know in the ATR42 the prop blade angle is changed by means of a hydraulic device called Hydromatic consisting of a piston actuated by pressurized oil from the RGB to change the prop pitch. The side where the oil is routed is controlled by a Prop Control Unit or PCU mechanically connected to the Cl or to the PL allowing the pilot to change the pitch with one set of levers or the other according to the phase of the flight. When the airplane is airborne the system operates into the blade angle mode where the PL controls the output of the engine by controlling the amount of fuel delivered by a hydromechanical unit or HMU and fine tuned by an Engine Control Unit or ECU that via a torque motor enriches the fuel flow to assure a quickest response of the engine to the lever movements, while the CL controls the blade angle by means of the PCU.
When the power lever is retarded aft the FI position, correct me if I am wrong, the prop is automatically set to the lowest blade angle or LBA, the CL is set full forward and it is disconnected from the PCU: during this phase referred to as the fuel metring mode the blade angle is maintained at a constant value, and the prop rpm is changed by controlling the amount of fuel delivered into the combustion chamber and thus the flow that runs through the power tuerbine, the only engine control during the fuel metering modeis the PL connected to the PCU by a LBA PCU cam,besides the ECU has a special program that anytime the PL is retarded aft the FI position and the engine begins to operate in the fuel metering mode, matches the PL position to a given prop rpm.
Now here come the questions:
Why is the PL connected to the PCu if the blade angle is maintained constant during the fuel metering mode, since the prop rpm is changed only by controlling the fuel flow?
When does exactely the fuel metering mode engage?
Is it correct to say that the beta range, corresponding to the fuel metering mode has to be set cos the turbine engine even when opearted at idle generates a huge amount of thrust, due to the minimum compressor speed, and thus to lish that thrust the blades are set to a low angle?
I know there is a transition mode but I don't know exactely what it is and how it operats, can anyone clear this area?
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