FF vs Prop changes

[STAVKA 4]

Hi Eric,

Thanks alot, no problem. As i realy like this addon, is one my first choice to fly. For me is a great pleasure to help and be useful to turn this addon even better.

Best regards

[jcomm 8]

And reading accross this thread I do think that indeed Stavka, the OP, and actually Tom (aka taguilo), have given the most precious contributions.

I am still thankful to Mathijs for his kind offer, and to Finn who helped identifying this interesting detail and forward it to the devs for a possible update in future releases.

Honestly, I do not think any offer should be directed towards me, and actually, the best offer, for Stavka and Tom who I assume already own it, and for me who am about to buy it in just a few minutes, is to be able to profit from yet another excellent add-on for the great flight simulator that, after all, FSX still is!

This add-on comes, IMO, in the line of products that Aerosoft started with the new Katana, have recently mastered with the excellent AXE, of which I am a proud owner, and looking forward for the next versions, being still the best Airbus simulation available ( again IMO ) for the FSX / P3D platforms, and all have in common a remarkable attention to detail, but above all a rather didactic approach to the use of a flight simulator, allowing the hardcore simmer to really engage in the most technical details of operating a particular aircraft and it's systems.

To summarize, it's clearly Aerosoft, and it's dedication to creating some of the best products for us simmers who should get the prizes!!!!

All of the best from an about to be, proud owner of this DHC6 :-)

[taguilo 12]

FF vs Prop RPM issue is an FSX's bug. It is present in all standard-modeled FSX turboprops, including this Extended version and also the original X version, not matter it could seem that works fine on this one.

The error is related to a table included in .air file (1505), which is user configurable in turbo jets but hard coded in turboprops, so there is not much that can be done here to fix it.

The simplest way would be to fake FF gauge via polynomials, but this could really be a tough job considering the pool of variables that affect FF (power, temperature,etc).

The hardest way would be to manage FF through Simconnect, another difficult job indeed -maybe tougher than the first solution.

Tom

[STAVKA 4]

Hi Taguilo,

Yes, its a fsx core engine problem, but on the twin otter original it is almost perfect, when you move the Prop lever only, the FF are almost fixed. I don't know the trick that devs used, but it will be great to see it again on the extended version.

I have sure that Finn and his team will do a great solution for the extended version because they have the experience and know-how from the original version.

Without mention this relation of prop vs ff, the extended version are a much better version in all aspects, improving this engine model using what they did on the original version, turns the extended version into a even more superb addon and even more a must-to-have turbo-prop for fsx.

Regards

[Snave 466]

Are you sure you're right? Can you cite some sources for this?

Only it seems to me that power is the ability to perform work or function.

I must admit I am at a loss to understand how there can be no increase in power when there is a change in force? torque necessarily changes force, and force necessarily changes energy.

I can see how your argument could hold for GEARED turbines - and in fact have my own evidence of such on Allison engined turboshaft examples,but I don't see how this can apply to FREE turbines.

Please elaborate with a technical model.

A most excellent technical discussion that has left the morons far behind..

[STAVKA 4]

Hi Snave,

power, torque, force, is not the subject that matters here. We are talking about Fuel Flow of PT6A versus the movement of Prop lever in throttle quadrant. The working was already explained and the reasons of it in the previous posts. About a tecnical description of this engine i give you an example:
http://www.pwc.ca/files/en/Know_your_PT6A.pdf

In this engine you not have any solid "physical feedback" (as in a conventional engine) from changing the prop RPM and the fuel flow. The FF are inposed by the condition lever and the throttle to mantain a constant compressor turbine RPM. The only situation that prop control change the fuel flow is the protection system implemented on the FCU (Fuel Control Unit) to reduces the FF when the prop RPM reaches overspeeds of 105% or greater to protects the engine.

So, when you change the prop lever it changes the prop RPM (because change the pitch of the propeller blades), free turbine RPM and his torque, but not change the compressor turbine RPM and his FF because not exist any link or relation from free turbine to compressor turbine.

Anyway as mentioned before you can check it in the PT6A simulation (link and instructions above) as also in professional simulators as the Elite Premium that have a well modeled PT6A for real pilots instructions and training.

Regards

[obie311 20]

Being more of a wrench twister than an engineer, I must truly say I am one of the morons cited by Snave, that is left far behind in the discussion.

However... I am highly impressed by the forthright manner that Finn, Mathijs, and Eric have reacted to the explanation of the problem. Rather than being dismissive and making excuses, they are open minded and clearly interested in making an excellent product even better. This company culture sets Aerosoft apart from many lesser developers.

Obie

[Snave 466]

Then how does changing the prop rpm NOT change the torque?

The basic premise of the rpm change is to alter the pitch of the blades, and with it the torque. If you look at the assembly of the engine while there is no physical link between the two sections, for the torque to remain the same following rpm change, does it not automatically imply a change in the Power lever if the FF is to remain the same? You understand that the prop rpm is not some arbitrary figure but is actually a definition of blade angle, and therefore thrust?

Your reference document is familiar to me, but it does not explain the relationship between rpm, torque and power sufficiently to explain how fuel flow does NOT change with a change in rpm/torque? The PT6 parameter for power output is torque. That torque is derived from the front end not the Hot Section/Compressor

http://www.unitedturbine.com/pdfs/PT6%20Training%20Manual.pdf

Has a dedicated diagram on page 23 to affirm this is correct.

Therefore as the fuel provides the energy to drive the compressor that fires the engine, which in turn drives the prop to create the torque, I don't see how commanding a change to torque (`work`) can be implemented without a change to the amount of fuel required to support that increased or decreased workload, even if the compressor speed stays constant. I can see how one might change the torque setting then modify the power lever setting to re-establish the previous fuel flow (exactly the process seen in the TwotterX), but I don't see how fuel flow stays the same just with a change in prop rpm...

Could you perhaps explain in a more technical manner, and perhaps cite a source that is useful to the discussion?

[jcomm 8]

@Snave:

First off I would like to point out that, given the many many details and features modelled in this add-on, this quest for correct FF display is just a detail, one that hardcore simmers may ask for, but I am sure that STAVKA can perfectly live without it, although I agree that, for the sake of completeness, if possible some sort of "mask" on the unwanted FF variation would be great to have.

As Tom pointed out, a deep and realistic modelling of such a turbine would be very complex, and should be performed outside of the FS core. Even that way, many problems would still be present when, for instance through SIMCONNECT we tried to inject the output of the system to FSX...

We have to accept the limitations of FSX, which, IMO, is still, after all these years, the best civil flight simulator available for the PC.

There are so many other problems/limitations/details, even with conventional engines. To give just another example involving exactly FF we shouldn't forget about the "Leaning bug" that plagues all asphirated / injected prop aircraft in FSX, where the higher you fly te bigger the inconsistency will be in the variation of FF with leaning. At sea level when you lean your engine, you see FF decreasing, as it should, but above, say, 6000', there will be an initial rise in FF, and only after a while will it start to decrease. The higher you fly, the bigger this discrepancy between the sim and RL will be...

Well, with this said, I now ask you to look at this FF in a free-running turbine subject in the following way:

1) A free-runing turbine has 2 independent co-axial shafts, one driving the compressor, the other one connected to the prop gearing. There is no connection between these two other than the air produced by the gas producer, that flows accross an intricate set of fixed blades and moving blades causing the prop shaft to turn. It's just as if you had a jet engine, and put in front of it a prop, that will turn because of the flow of accelerated air exiting the jet.

2) When you use the condition lever, you set the amount of fuel injected in the gas producer ( well, sort of... ) and of course that changes FF.

3) When you use yor "throttle" you also change the power produced by the turbine, and the compressed air leaving it will exert more / less force over the "prop shaft blades", thus providing more / less power to it.

4) The Prop RPM lever changes, as you perfectly pointed out, what you want the CS prop to set in terms of prop RPM, and it will adjust the prop blades pitch in order to achieve that required RPM.

Thus, when you vary Prop RPM, of course you also vary the torque, in such a way that, just like in any prop aircraft even with a conventional engine but a CS prop, increasing prop rpm will lower torque, and decreasing it will increase torque.

Now, this changes in prop rpm and torque are not felt by the gas turbine behind it.

Theoretically, by plasticity of the air inside of the turbine, their could be some return into the compressor blades of the turbine, but, it would be minimal and would certainly not alter the FF.

To sum up: Condition levers and throttle alter power of the gas producer / main turbine, and with it also FF, but the air leaving that turbine, will turn the blade system attached to the prop shaft, which is completely independent, thus being (that air) te only physical connection between the two systems.

On a Garret, like those used for instance on the Jetstream 41, you have a single shaft, so, the turbine directly drives the prop shaft, and in this case, being non-free, changes in RPM will necessarily change FF as well.

Hope this helps

;-)

[Snave 466]

That is clearer, thank you. There is a change in fuel flow, (and presumably ITT if the engine is working harder in order to generate the power-to-torque translation) but it is simply not visible within the confines of the limitation of the actual gauges.

Seems that this `looseness` should factorable within the gauge, not the flight model.

[Finn 873]

The easiest would be to fake the fuel flow readings to always indicate the same as long as power isn´t changed, so only affected by altitude and temperature.

But it would be a fake value and FSX fuel consumption will not change due to this.

Finn

[jcomm 8]

The easiest would be to fake the fuel flow readings to always indicate the same as long as power isn´t changed, so only affected by altitude and temperature.

But it would be a fake value and FSX fuel consumption will not change due to this.

Finn

Yes, been thinking about that too Finn... Know what? Better leave it as it is, I believe.

[STAVKA 4]

Hi Finn,

I trust on your opinion and judgement what will be better, sometimes trying to repair one tiny issue creates bigger ones. The actual extended version, i love it. Any improvement of course will be great but only if not add new issues after.

If in future you get a way to bypass the fsx core engine problem and not have side issues, great. If not, no worries. Still be my loved aircraft and the best DHC-6 ever made for PC.

Thanks about the support.