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STAVKA

FF vs Prop changes

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Hi Finn,

The engine model changed from the old Aerosoft Twin Otter to the new Extended version?

I noticed that the original Aerosoft Twin Otter has one of the best PT6 model because it has a realistic and unique feature in the world of FSX, when you change the Prop, the Fuel Flow not change, only the RPM and the Torque. Saddly this new extended version, that was suposed to be a evolution of the older one, uses the fsx core engine to model the aircraft engine. With that it have an unrealistic working of a PT6 engine, in this new version when you change the Prop, the FF also change, that is wrong in a PT6.

It is possible to correct this using the know-how from the older version?

I hope so, because the last version proofs that its possible to walkaround the problematic core engine of fsx to model a PT6 and also will be great to have this feature again to put this extension version as the best implementation of a Twin Otter (a lovely and amazing aircraft).

Thanks

Regards

Tércio

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Hello,

did you already try to run the programm linked in this thread?
http://forum.aerosoft.com/index.php?/topic/71731-something-to-play-with-pt6-42-simulation/

This is a PT6 engine simulation which we took (of cause togreather with other sources of cause) to build our new engine model. Please compare what you see in your sim with what can be seen within this simulation.

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Hi Emi,

yes, i tried before, using the Finn advices (selecting "SELKIRK PROGRAMMING" and using the condition levers on Low idle), it reinforces more my post. In the original Twin Otter it was like this simulation that are as the real one, in the extended version, no.

In new version the FF change with the Prop as if the engine have a mechanical link between the propellor and compressor. In a real PT6, a free spin propeller engine, you can change only the propellor without change the fuel flow.

Regards

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I noticed that the original Aerosoft Twin Otter has one of the best PT6 model because it has a realistic and unique feature in the world of FSX, when you change the Prop, the Fuel Flow not change, only the RPM and the Torque.

I´m pretty sure that this is not true.

The original Twin Otter uses the stock FSX turboprop model with no additional coding.

This means that changing Prop RPM won´t affect torque, nor give any real torque increase when going into reverse.

So thats definitly not very realistic.

It should be no secret that we still use the default FSX turboprop mode, but with some additional tweaks and enhancements.

Unfortunatly there is no way we can change the fuel flow.

Finn

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Hi Finn,


"The original Twin Otter uses the stock FSX turboprop model with no additional coding." about this i not argue because i don't know, i was not the dev of it.


I'm not talking about when going into reverse. In normal operation. In a real PT6 when Prop change (increase or decrease) the torque and RPM also change but FF stay exactly the same. This happens in the original Twin Otter and in a real PT6, in this extended version, no. When i change the Prop, the FF also moves. Something was changed to wrong from original to the extended version.


I have both products and even today i re-test both!

To sum up:
1) The original shows Torque variation as you cycle only the Prop, which is correct.
2) The original keeps the FF when you vary only Prop RPM
3) The extended version shows FF variation as you cycle only the Prop, which is wrong.


Regards

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I can also confirm this, in as far as the original model is concerned. I do not own the extended version.

It's visible, during prop variations, that the FF gauge closely mantains FF readings, and even at extreme variations, which would not be the case in RL, it shows a tide tendency to vary that is immediately compensated probably by the gauge code itself, since the internal MSFS turboprop model ( for a free-running turbine ) is wrong ( and, BTW, so is the one in X-Plane 10 )

An excellent modelling of a PT-6 free-running is, for instance, the one used in the ELITE B200. Also, the simulator linked above in this thread, showing alternative implementations by ALSIM and other company, also clearly depicts there being no FF variation, at constant altitude, when Prop RPM is cycled.

For the sake of accuracy, I think it would be great to consider coding the FF gauge at least as close as possible to the one on the default Twotter, with the necessary adaptations to the new, certainly more sophisticated model used in this extended version, that I will probably end up buying :-)

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So....

It´s some time since I worked on that part of the Twin Otter, so I digged up the notes and performance tables we used when coding and setting up the engines for the Twin Otter.

First I must emphasize that the PT6 simulator found under the mentioned link isn´t a PT6-27, but the -42, so there are some differences, which we accounted for in our simulation.

Prop RPM change with a fixed power setting does affect FF on the PT6-27 engine as You can see in this performance table:

post-7458-0-53119900-1379351988_thumb.jp

As You can see - altitude (8.000 ft), Torque setting of 42 ft/lb and RPM at 91% will give a fuel flow of: 310 lb/hr per engine

While You can see - altitude (8.000 ft), Torque setting of 42 ft/lb and RPM at 75% will give a fuel flow of: 260 lb/hr per engine

On this video You can see the difference between the old and the new extended Twin Otter.

Moving the Prop lever on the old version does neither affect torque or fuel flow, whereas it does on the extended.

On the extended You will see that with the above altitude, torque and Prop RPM settings we get appr. 300 lb/ft per engine at 91% RPM and 220 lb/ft per engine at 75%.

Not exactly to be called accurate, but still not that far way considering the way FSX calculates fuel flow. But maybe we can tweak fuel consumption towards more accurate values.

Finn

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Hi Finn,

Thanks for the reply, yes exist differences from the older version.

"But maybe we can tweak fuel consumption towards more accurate values." That will be great to improves even more this addon, looking forward to it.

Regards

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Fuel Flow variation with Prop RPM variation is one of turboprop model's known bug.

The Extended version uses FSX's standard vars for FF gauges, so this behavior is indeed noticeable.

Finn's clarification is true for different RPM values but constant torque, in RL the reduction in FF occurs because, when reducing RPMs, Throttle/power -and in consequence FF- has to be reduced to maintain torque constant.

Unfortunately FF is not simple to tweak; to obtain accurate results it is necessary to go beyond XML and use other techniques.

Tom

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Tom, excellent answer, and one I was about to write as well.

This is exactly the same problem that was present in Flight1's B200. I remember one of the devs kindly offering the performance tables, just as the ones shown above for the Twotter, but the problem is that, as you pointed out, constant Torque means adjustment required to the throttle.

On the real thing, if the "throttle" and of course the condition lever are left untouched, decreasing RPM will increase torque, and the way around. Thus, in order to maintain a constant torque, if you decrease Prop RPM you'll have to also adjust throttle by reducing it, and that will of course result in a lower FF. On the other hand, if you increase Prop RPM, then Torque will decrease, and you'll have to increase Throttle, and thus increase FF.

For the devs I would suggest that they simply take as a model the simulation that one of them mentiuoned in one of the posts above. It accurately displays how a free-running turboprop should work.

I understand that when using FSX or X-plane 10, we're limited by their own core limitations, but I am also sure that "plugins" can do miracles :-) After all, the original version of this aircraft, which I re-installed yesterday for testing pourposes, does have that kind of effect modelled, and everytime we change Prop RPM, it's clear that "something" is trying to make the FF stay...

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Tom, excellent answer, and one I was about to write as well.

This is exactly the same problem that was present in Flight1's B200. I remember one of the devs kindly offering the performance tables, just as the ones shown above for the Twotter, but the problem is that, as you pointed out, constant Torque means adjustment required to the throttle.

On the real thing, if the "throttle" and of course the condition lever are left untouched, decreasing RPM will increase torque, and the way around. Thus, in order to maintain a constant torque, if you decrease Prop RPM you'll have to also adjust throttle by reducing it, and that will of course result in a lower FF. On the other hand, if you increase Prop RPM, then Torque will decrease, and you'll have to increase Throttle, and thus increase FF.

For the devs I would suggest that they simply take as a model the simulation that one of them mentiuoned in one of the posts above. It accurately displays how a free-running turboprop should work.

I understand that when using FSX or X-plane 10, we're limited by their own core limitations, but I am also sure that "plugins" can do miracles :-) After all, the original version of this aircraft, which I re-installed yesterday for testing pourposes, does have that kind of effect modelled, and everytime we change Prop RPM, it's clear that "something" is trying to make the FF stay...

I´m not sure what the problem here is.

As you can see in my video, the old twin Otter does not behave correctly. Moving the propeller levers will not change Torque nor change fuel flow, which is incorrect.

The table shown, clearly tell that at the same altitude and with the torque set to the same value, fuel flow will be affected when Prop RPM is changed.

While we did use the Selkirk simulator to obtain some figures and engine behaviours, You cannot use it 1:1 since it´s a different engine, also that simulation isn´t 100% perfect either.

With the Twin Otter extended, moving the prop lever will affect both torque and fuel flow close to what You see on the real thing.

I know that Majestic has managed to simulate both flight dynamics and engines outside FSX via some dll's, but it´s sold with quite a different pricetag and is also pointed towards a different customer segment (the professional version meant to be used by airlines).

For what we got to work with, I still think that we are pretty close to the way the real PT6-27 behaves.

Finn

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Finn,

I am sure you have done your best to bring the twotter performance in the the closest to the real thing as it is possible withing FSX's limitations.

I do not own the Majestic Q-400.

Yesterday I decided to re-install the original twotter. I didn't buy this new one ( yet ).

In my original twotter, varying Prop RPM affects torque. If you increase Prop RPM, torque will decrease. If you decrease Prop RPM torque will increase. This is the correct behaviour.

In the attached tables, since the performance is given for constant torque, it means that when you choose a lower Prop RPM, the torque will increase, and so in order to adjust to the table, you will have to reduce thrust, to bring torque to the original value, hence the FF reduction. The opposite applies as well.

But! If on a free running turboprop ( like the pt-6 ) you:

A ) Reduce ** ONLY ** Prop RPM, then Torque will increase but FF will stay still

B ) Increase ** ONLY ** Prop RPM, then Torque will decrease but FF will stay still

The only simulator I own and correctly displays the behaviour of such a turboprop type is ELITE Premium with it's B200 module.

BTW: As an x-plane 10 user, I can confirm that x-plane is also wrong in modelling this type of engine, and they even include an option in plane-maker to "say" the turbine is free-running!!!

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Hi Finn,

What Jcomm write is exactly what i'm trying to say! In the older Twin Otter the Prop change and FF reaction are more like the real one because FF almost not change when you ONLY change the Prop, and in the new version the FF visible change as if it was a non free-running prop engine.

As i told, something are different and what i know from PT6 and their model in Elite Premium and Ontop simulator and even from PT6 real engine descriptions, the FF stay still when you change only the prop. This happens with the old Twin Otter but sadly not with the new one.

Regards

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Hi Finn,

When we are talking about moving the Prop is the Prop lever in the throttle quadrant and let stay still all the others (not change the others like the throttle and the condition). In this situation, the Torque and Prop RPM must move and the FF not, this happens in the original Twin Otter as also the real Twin Otter. In the extended version, no, the FF moves and alot.

The FF not change because you are only moving the angle of the propeller and as a free-running prop engine type, the link between prop and turbine is air and not a solid connection as others more conventional engines. As so, the prop RPM change and the FF stay still because FF are imposed by the turbine stage through throttle lever, not receiving any influence from the change of the angle of the prop (through the moving of the prop lever on the throttle quadrant). This is an inportant feature of all PT6A no matters the sub model.

Regards

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Hi Finn,

When we are talking about moving the Prop is the Prop lever in the throttle quadrant and let stay still all the others (not change the others like the throttle and the condition). In this situation, the Torque and Prop RPM must move and the FF not, this happens in the original Twin Otter as also the real Twin Otter. In the extended version, no, the FF moves and alot.

Regards

I got the point now and Your right...

I have forwarded this issue to our FM guru, so let´s see if what he can do about it.

Finn

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Hi Finn,

Thanks alot about your reply, I'm realy looking forward to a fix here because i saw from the original Twin Otter that it is possible to have a close to perfect PT6.

Thanks about the support and if i could be helpfull in something PM me.

Regards

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Great to know Finn!

I reeally like the original Twotter, and have been seriously considering the upgrade :-)

LAR, a portuguese regional airline, and TAP used them for many domestic routes between smaller airfields in the eighties, early nineties :-)

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Great to know Finn!

I reeally like the original Twotter, and have been seriously considering the upgrade :-)

LAR, a portuguese regional airline, and TAP used them for many domestic routes between smaller airfields in the eighties, early nineties :-)

I know that it sounds like a sales tale, but if You get the new twin Otter You really will findout how dated the graphics of the old on is.

Though the fuel flow behaviour isn´t right ;), the rest of the aircraft systems are really alot more accurate in the new Twin Otter.

End of advertisement :)

Finn

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I konw it must be a lot more complex / nice looking, at least by watching the many videos on youtube :-)

I really like the checklists, the graphics ( hd ), etc...

I'm just making space to recover from a bankruptcy September month of the PMDG 777 :-)

Thx Finn ;-)

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I konw it must be a lot more complex / nice looking, at least by watching the many videos on youtube :-)

I really like the checklists, the graphics ( hd ), etc...

I'm just making space to recover from a bankruptcy September month of the PMDG 777 :-)

Thx Finn ;-)

Clearly understoodd.

I also have bought the PMDG 777, A2A Cessna 172 and the DCS Mi-8 within a week. Now we can´t afford the butter for the bread :D

Finn

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I konw it must be a lot more complex / nice looking, at least by watching the many videos on youtube :-)

I really like the checklists, the graphics ( hd ), etc...

I'm just making space to recover from a bankruptcy September month of the PMDG 777 :-)

Thx Finn ;-)

Send us a mail on support@aerosoft.com and I'll get you a free copy. We appreciate the way you discussed this issue and it actually helps us. Just promise me to post something about your experiences with the new version. As Finn write, it really is in a whole new class.

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Well Mathijs,

what can I say? :-)

Thank you very much for your very kind offer!

I will try to report back my experience with this aircraft, specially because I was really looking forward to get it into my fleet :-)

Regards,

j.c.monteiro

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