Topic Author
Posts: 135
Joined: Mon Apr 23, 2001 7:33 pm

Prop Control Unit

Sat Aug 11, 2001 3:58 am

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?
Regards Diego
Please do not hesitate to contact me on diegoair@yahoo.it
T prop
Posts: 915
Joined: Tue Apr 03, 2001 4:33 pm

RE: Prop Control Unit

Sat Aug 11, 2001 6:52 am


I'm more familiar with the Dash 8 than the ATR and although some of the terminology may be different, the aircraft use the same propeller system. Anyway let me see if I can answer this.

You asked why are the PL's connected to the PCU if the blade angles are held constant, well that's because the blade angles are not held constant.
First off the blades do not automatically go to the lowest blade angle when the PL is moved below flight idle. The blade angles are being controlled directly by the pilot through the power levers. Actually the power levers are controlling blade angle at a point slightly above FI. This point above flight idle to max reverse is known as beta range. Just remember that in beta range blade angle is controlled by the pilot through the power levers.

On this propeller as you know, there is a restricted rpm range on the low side. To keep the props out of this range when engine torque is minimal on the ground, the ECU's are controlling fuel scheduling. This is called underspeed fuel governing. (fuel metering mode on ATR?) When the pilots pull the PL's back below flight idle they are changing blade angle, the prop rpm will change because of this. The ECU will sense this rpm change and adjust fuel metering to compensate and keep the prop rpm just above restricted range, 785 rpm on the Dash 8. On the Dash 8 there is a point below flight idle called DISC. This is the point at which there is zero thrust, the blade angle at this point is about 1.5 degrees. You ask when is underspeed fuel governing or fuel metering mode is in effect, It would be in effect from flight idle to disc.
As far as a transition mode, I'm not sure if I understand you correctly, but when going from disc to max reverse the blade angles go from 1.5 degrees to -11 degrees (these angles are probably similar in the ATR) the ECU will increase engine power as the pilot pulls toward max reverse. The pilot is changing blade angles and the ECU is programmed to add fuel till max NP is about 1140 at max reverse. So at disc the ECU is still providing underspeed governing to keep the prop at 785 rpm, once you go below disc towards max reverse the ECU starts adding fuel to increase prop rpm and torque. Disc would be the transition point.

Hope this helps.

T prop.

Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Prop Control Unit

Sat Aug 11, 2001 1:09 pm

Nice answer T Prop! And just like I remember it as well.

Best Regards,


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