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Fuel trimmers?

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Merlin59 View Drop Down
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    Posted: 26 Sep 2017 at 11:02pm
Are the fuel trimmers modeled on this airplane? If so how do you figure the settings? I know how it's done on the similar F27 Fokker but don't see the same gauges or whatever here. If it's not modeled I'll quit trying to figure it out!
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Merlin59 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Merlin59 Quote  Post ReplyReply Direct Link To This Post Posted: 27 Sep 2017 at 11:00pm
Anybody have an educated guess?
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snave View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snave Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 2017 at 5:28pm
No, fuel trimmers are not modelled. Unless it's been added in the latest patch.

Happy to provide details for fuel trimmers if that's what you want to know, or won't bother if the `no` rather defeats the object..?


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Merlin59 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Merlin59 Quote  Post ReplyReply Direct Link To This Post Posted: 28 Sep 2017 at 5:36pm
Any advice on proper usage modeled or not would be greatly appreciated. I always try to fly by the book even if the plane modeled falls short. The F27 has the fuel datum which helps. This one obviously doesn't. Thanks!
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snave View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote snave Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep 2017 at 5:27pm
Originally posted by Merlin59 Merlin59 wrote:

Any advice on proper usage modeled or not would be greatly appreciated. I always try to fly by the book even if the plane modeled falls short. The F27 has the fuel datum which helps. This one obviously doesn't. Thanks!

As I said earlier, the Dart were a first generation turboprop (actually THE first generation turboprop!). In consequence informed wisdom for operation was led by what had gone before which was piston engines -but impacted by the the limitations and foibles of the new technology.

The Dart does what we call today single lever control, except that it wasn't a single lever, and it wasn't a digitally precise control like FADEC. 

The Power Lever (actually still called `throttle` in those days but all T/P's have Power Levers now) is connected to the Propeller Control Unit (PCU) which controls the RPM. It is also connected to the Fuel Control Unit (FCU) which is the fuel metering system.

So the power setting is fixed in its relationship to rpm and directly with fuel flow.

To allow more flexibility under a wider range of operating conditions (sub-zero to tropics and deserts) Rolls Royce created a secondary way of subtly changing the fuel flow (and hence the TGT), for a specific throttle position - and if this seems familiar it is very similar to adjusting the mixture of a piston engine for high temperatures or density altitudes (they only had piston experience to go on). Except here they are controlled by knobs and are called fuel trimmers, 

They are calibrated from 0% to 100% - the full range regulating around 25% of total fuel flow - as a rule of thumb at 0% fuel trim and full throttle it is possible that you might only be achieving 75% power... 
This is why the correct simulation of the Fuel Trims is very important for the Dart and the 748 and it is highly disappointing if it is just a mock-up gauge.

Generally you have a table or mechanical computer where you enter with airfield elevation & temperature, that gives you the correct fuel trim setting for take off and landing or go-around. 

Theoretically you then just push the Power Levers forward to the stops to get full power.

Heres a chart from the Viscount, but it's a useful illustration and of course, if all you want to do is twiddle these settings will do nicely...

Sea level at +20°C OAT = 86% trim (100% = full fuel with the throttle set at the ground idle position)
1000 ft at +20°C OAT = 80% trim
5000 ft at +20°OAT = 58% trim
Sea level at +35°C OAT = 50% trim 
1000 ft at +35°C OAT = 45% trim 
5000 ft at +35°OAT = 25% trim 

How you use them: 

For start set to the above settings. Once you have set a fixed RPM on the throttles, you only adjust the TGT with the trimmers. The TGT's are very sensitive and any time the OAT changes, it requires a fuel trim change (in order to get the TGT back to the correct figure). This means pilots are constantly monitoring TGT and adjusting, especially during the climbout! 

Dart starts are usually best as cool as possible. Either start at 100% fuel trim or trim of the day from the chart. While you get a higher peak start TGT because of the `extra` fuel, the engine accelerates faster and the TGT will peak for a shorter time. 
A second school of thought says to start at 0% fuel trim with start peak TGT lower, but as the engine doesn't accelerate as  effectively, the TGT spike will last longer. 
Once at cruise set the appropriate RPM via the P/L's then tweak the TGT via the fuel trimmers - per previous advice earlier in the topic.

Finally, trim should be >50% for normal descent - not because the engine needs it for the descent, 0% would be fine - but if you need to add power for a go-around you can risk over-temping or under-powering at either extreme. 

Hope this helps.
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Merlin59 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Merlin59 Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep 2017 at 6:30pm
All helps very much! Thank you Snave I appreciate it. Shame they didn't accurately model this but that seems typical. I'll try and fly it correctly modeled that way or not. Again thanks! ☺
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snave View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snave Quote  Post ReplyReply Direct Link To This Post Posted: 29 Sep 2017 at 8:45pm
You're very welcome.

It is disappointing to witness attempts at even the slightest simulation of the Dart turboprop without understanding - or at least attempting to represent - fuel trimming. It is a fundamental part of the engine simulation. One that represents the difference from what came later. 

Ouch
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