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Propeller system

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Forum Name: Flight1 ATR General Discussion
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URL: http://atr.flight1.net/forums/forum_posts.asp?TID=6633
Printed Date: April-02-2020 at 8:58am
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Topic: Propeller system
Posted By: Diegoair72
Subject: Propeller system
Date Posted: July-17-2016 at 10:31am
Dear All
First of all allow me to introduce myself, my name s Diego and i am a challenger captain - the shorter version of the CRJ for those of you who are not familiar with private jets - and even though i never had the chance to fly any turboprop i have always been in love with the ATR.
I went through the FCOM and i read the section pertaining to the powerplant trying to figure out how the propeller system works and i came to very few conclusions which i am not even sure are correct! Lol
As far as i have understood pitch control is achieved by routing control pressure and supply pressure through an electro hydraulic valve or EHV, respectively to the decrease and to the increse side of a servo piston mechanically connected to the prop shanks, causing them to rotate along their longitudinal axis travelling from the flat picth setting to the feather setting, based on inputs from a propeller electronic control or PEC.
The Supply pressure is supplied from a mechanical propeller pump on the RGB and from an electrical auxiliary feathering pump. The mechanical pump gets its oil from a small reservoir kept full with pressurized oil from the engine whenever the associated engine is operating, and via a standpipe preventing the mechanical pump from sucking the oil from the bottom of the same reservoir and thus making the remaining of the oil available to the electrical feathering pump, allowing to feather the propeller in the event of an engine failre as the Np's drop.
Control pressure whereas is supplied froma governor integral to a PVM and controlled by the PEC.
The PEC is designed to feed control and the supply pressures via an oil transfer tube to the increase/decrease side of the pitch change mechanism or PCM housed in the hub and basically consisting of a piston that as it slides, it drags with it an off center trunnion at the base of each blade, causing the blade to twist. The oil transfer tube is connected to a servo piston on the PVM end and to an acme screw on the PCM end in such a manner that as it is rotated by the PEC it opens and closes holes in the PCM venting oil from one chamber and feeding it to the other, controlling the balance between the supply and the control pressure and thus the position of the piston - When the force exerted by the control pressure balances the force by the supply pressure the piston is at rest and the blades pitch is constant.
The twisting of the oil transfer tube and thus the blade angle is controlled by the PEC via the servo valve through a ball screw mechanism, and acted upon supply pressure on one side and upon metered control pressure on the other side. The way that the system operates depends upon in which mode the PEC is operated: Np governing, 100% and Beta.
The Np governing mode is selected by advancing the CL's to the AUTO detent: during Np governing mode the PEC adjusts the pitch to the Np selected via the PWR MGT selector - 100% when in either T/O or MCT and 82% when in CLB or CRZ - and therefater it maintains it as the torque and/or airspeed vary, by the usage of a governor housed in a propeller variable module or PVM and based on the tension applied to a speeder spring on the governor itself resulting in an onspeed/underspeed/overspeed condition - increasing the control pressure will cause the blades to move towards a flat pitch and a higher Np while decreasing the control pressure will cause the supply pressure to move the blades towards a higher pitch lower Np.
When the CL's is pushed forward to the OVRD position in the event of an engine failure, the PEC adjusts and maintains the Np to 100% regardless of the PWR MNG selector setting and/or torque setting.
On the ground during Beta mode governing the CL's have no affect on the blade pitch which is controlled directly by the PL's in order to generate an immediate response - the EEC will then adjust the fuel flow as to cope with the aerodynamic loads and to maintain a minimum Nh of 61% and a minimum Np of 70.8% to ensure that the torque output of the engine and of the prop is sufficient to drive the DC and the AC wild generators.
The PVM is also comprised of a least pressure valve as a part of an overspeed protection system designed to prevent the blades from overspeeding and basically consisting of a shuttle valve acted upon the metered control pressure from the integral governor and upon the metered pressure from an overspeed governor: the least pressure valve will automatically feed to the servo valve controlling the twisting of the oil transfer tube, the lower of the two metering pressure and thus allowing the supply pressure to drive the baldes towards a higher pitch lower Np setting.
The overspeed governor is used in conjuction with a pitch lock mechanism basically consisting of a bulkhead in the PCM that in the event of a complete loss of oil pressure to the PCM, will lock the blades to 1 degree from the selected pitch, preventing them from moving towards a flat pitch anf thus from overspeeding. Even though a pitch locked propeller will behave like a fixed pitch propeller it might be feathered provided that oil is still available to the electrical pump and that the plumbing is not compromised.

Now the questions:
The EHV is the servo valve controlling the twisting of the oil transfer tube via the ball screw mechanism or the valve in the PCM or a different animal?
During beta ops is the Np kept at 70.8% at all time - apart from when PL's are adjusted and for the time it takes for the EEC to adjust the ff?
Where does the beta tube fit in the picture?
What about the feathering solenoid?
At which stage of the after start are the CL's set to AUTO position?

If you have anything to add or to correct please do so as i am very eager to learn a bit more about the systems of the ATR72

Many thanks




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