NG Governing on a PT6

[Dax]

Can someone who knows PT6’s tell me how exactly the NG is governed at its limit? If the NG is limiting out and you attempt to give it more power what happens to the torque and ITT?

Thank you

Dax

[Goodwrench]

Ng is not a parameter that is directly controlled by any input. If you take a running engine at a given power setting, the Ng will reflect the gas generator speed required to sustain that power. On a power assurance check (maintenance) you will take the pressure altitude and ambient temperature of the run and plot it on a chart. This will give you the allowable limits for I.T.T., Fuel Flow, Ng and Torque for that run. The idea being that Ng and fuel flow should be below the target value when the temp or torque limit is reached. How much below the chart limit is an indication of engine efficiency, along with the other parameters.

To answer your governing question, yes there is a fuel governor in the fuel control unit. It governs at a limit slightly above 100%, I don't have the value memorized, but if the Ng ever gets there, you have big problems. It is designed more as a fail safe at the actual R.P.M. limit of the compressor section.

[bubba070]

To answer your governing question, yes there is a fuel governor in the fuel control unit. It governs at a limit slightly above 100%, I don't have the value memorized, but if the Ng ever gets there, you have big problems. It is designed more as a fail safe at the actual R.P.M. limit of the compressor section.

I think he is asking when the NG limit is reached, does more input on the throttle have any consequence on torque and ITT.

[amclean]

Ok, I am just a pilot, but I have had a bit of experience teaching the theory behind the pt6, so here's how I understand it.

The FCU is an NG governor. As you adjust the power levers, you are moving a linkage that adjusts the speed scheduling cam in the FCU. This adjusts the spring tension that the flyweights are trying to overcome, and in turn the speed at which it is governing. (Think prop governor -- same concept). Since the FCU is attached to the AGB, it is turning at a speed that is proportional to Ng, so this is a direct control of Ng.

The FCU control arm (connected to the power lever) has a physical stop that is adjusted to give you your max Ng (somewhere at or slightly over 100%, depending on model). Once it hits this stop, the control arm can move no further, thus your Ng will not increase any further. As torque, itt, and ff are by products of trying to maintain the selected Ng, at a constant altitude, they will not change.

Hope that makes sense.

[david_351]

If you reach your max NG stop, the mechanical stop on the FCU, any further throttle input will do nothing, PT6s govern at 101.5%, and you should not need that much NG to make take off torque, if you do you either have other problems, or you are at a very high(6000 feet and up airfield) or you are in an extremely hot and humid place, like Panama City.

Im not sure exactly what your asking, I'm not sure if my answer clarified anything.

[GA MX Trainer Dude]

DAX

The gas generator speed is controlled by 2 things - the setting of the fuel control governor - which has a maximum speed adjustment that is set mechanically and the action of the fuel topping governor that controls the gas generator speed as a function of the propeller speed.

So it is very important to know which system you are talking about.

Once you have reached the maximum speed setting on the fuel control governor - no further fuel is provided and further movement forward of the power lever will produce no increase in torque, ITT - or fuel flow as the constant speed governor inside the FCU controls the fuel schedule.

If you reach the fuel topping governor setting - the pneumatic control pressure (Py) is bled through the valve - this directly controls the position of the fuel metering valve limiting the fuel flow to the now constant speed control of the propeller by limiting the fuel flow. Further movement of the power lever under this condition will result in no added fuel flow - thus no increase in gas generator speed - no increase in torque or ITT. The gas generator speed under this condition can be substantially less than the governing maximum as set on the fuel control unit flyweight governor.

Changing the position of the propeller lever under the above condition may change the gas generator speed as there is a mechanical reset linkage connecting the propeller lever control to the fuel topping governor.

If you have a complete Py bleed - (Think disconnected line) - the fuel control will roll all the way back to the minimum flow setting and the power lever will have no effect due to the over-ride nature of the pneumatic control. The engine will stabilize at a speed slightly less than the idle speed. Idle speed is controlled by adjusting the FCU governor speeder spring input - maximum Ng speed is adjusted at the other end of the speed range by the max speed stop setting which is the highest speeder spring pressure. When the pneumatic control system is active it is what is controlling the maximum fuel flow allowed in the FCU.

All this described in detail here:

http://freebee.ucoz.com/load

Hope this helps.

Mx

[Dax]

Thanks you all for the answers. I think I have a grasp on how it works now.