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Turboprop torque and power question (Read 1148 times)

Feb 1^st, 2010 at 4:29pm

snippyfsxer Offline
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Having spent my FSX time lately flying old piston propliners, I became intimately familiar with using torquemeters and RPM in combination to achieve the right power settings.

However, I recently got bored with the A2A stuff and started fooling around with the Digital Aviation Piper Cheyenne.

I noticed that the engine torque meters are not responsive to changes in RPM, only throttle input. This airplane has pt-6 geared turboprop engines. I am a bit confused as to what is being measured here. If a torque meter is a torque meter, the torque should increase when I decrease the RPM provided that the throttle is left where it is.

So in real life turboprops, how does this work? Am I seeing correct behavior from the guages?

Note that fuel flow does vary with changes in RPM for a fixed throttle setting, if that helps anyone to answer.

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Reply #1 - Feb 1^st, 2010 at 4:49pm

C Offline
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snippyfsxer wrote on Feb 1^st, 2010 at 4:29pm:

So in real life turboprops, how does this work? Am I seeing correct behavior from the guages?

Note that fuel flow does vary with changes in RPM for a fixed throttle setting, if that helps anyone to answer.

From memory, yes (I last flew a PT-6 nearly 3 years ago). It's an interesting engine, in that the gas turbine gases power a separate turbine ("free turbine") which is connected to the prop shaft, unlike others, such as the Garrett TPE331, as fitted to the Shorts Tucano, where the prop is geared directly to the main compressor shaft of the engine.

So, in simple terms, IIRC the torque ~~is the power the gas turbine ("gas generator" unit) is producing~~, which can remain constant despite a change in prop RPM, as the two aren't directly connected (the prop blade angle changing to keep RPM constant).

https://engineering.purdue.edu/AAE/Research/Propulsion/Info/rockets/jets/tjets/t...

« Last Edit: Feb 2^nd, 2010 at 3:54pm by C »

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Reply #2 - Feb 1^st, 2010 at 5:04pm

C Offline
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Ok, I was working on my experience, which was King Air gauging. It's all a little hazy. As I said, I haven't been near one in a few years!

Jets are soooo much less complicated! Grin

And I'm not a technical genius! Grin

« Last Edit: Feb 2^nd, 2010 at 12:56pm by C »

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Reply #3 - Feb 1^st, 2010 at 5:06pm

snippyfsxer Offline
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In the Airplane Flying Handbook, I'm reading that Gas Generator operation is monitored by the Gas generator tachometer, a separate instrument.
If the Cheyenne guages are working correctly, I must be making the dumb assumption that the torquemeter is measuring the torque on the crankshaft to the propellor and it must not be, like you said

(sorry for deleting my post above, which you answered, I realized I didn't know what I was talking about

)

EDIT: I don't know engines, but if I'm pulling together all the various snippets I have read off the net, I can only conclude that FSX and the Cheyenne are modelled wrong. I think that when reducing the RPM, the fuel flow should stay the same and the torque should change, not the other way around!

If so, that sucks, because it fundamentally changes the way that power is set from real life. I think that the default King Air is exactly the same.

« Last Edit: Feb 1^st, 2010 at 6:53pm by snippyfsxer »

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Reply #4 - Feb 2^nd, 2010 at 9:20am

Brett_Henderson Offline
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MSFS turbine modeling is pretty bad.

That accepted... the best we can do is piece things together and use what IS workable.

I'm no turbine expert, but I understand the differences between turbine and piston engines, when it comes to tourque and fuel-flow, and how they are effected by a constant-speed prop.

Your assumption:

Quote:

I think that when reducing the RPM, the fuel flow should stay the same and the torque should change, not the other way around!

should be correct. Turbine fuel-flow is more throttle-position based, regardless of RPM; where piston fuel-flow IS RPM based regardless of throttle position. If the mixture is proper, RPMs determine the amount of air that passes through a piston intake system, hence the amount of fuel.

You "throttle" a piston engine by controlling the air-flow. You "throttle" a turbine by controlling the fuel-flow.

Ideally.. torque is not RPM related. It's HORSEpower that is a function of tourque/RPM. However, the reality of piston/crank geometry makes torque a slave to RPM (ie. tourque curve). That's one of the main advantages to a constant-speed prop (especially on a piston engine). It allows you to stay on a specific part of the torque curve (RPMwise), no matter the actual torque (power (MP))

There is no appreciable geometry to a turbine.. it just spins.. Cheesy

The sweet-spot for a turbine, is a funcion of the air-flow geometry.. and to the best of my knowledge, an N1 reading is how that's monitored.

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Reply #5 - Feb 2^nd, 2010 at 3:58pm

C Offline
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On reflection on my first post I was a bit wide of the mark.

snippyfsxer wrote on Feb 1^st, 2010 at 5:06pm:

I think that when reducing the RPM, the fuel flow should stay the same and the torque should change, not the other way around!

Indeed. Scouring back to the rear of my brain, ISTR that on reaching top of climb we'd set cruise power, bring back the RPM, then adjust the power back to cruise power again, which would fit in with your logic above.

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