Erroneous Fuel Flow Behavior
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So I tweaked the engine.cfg a little because I had two problems. First the FF increase when leaning. I found lots of reports of this behavior in other forums discussing different aircrafts. This seems to be a sim limitation and probably needs lots of effort to eradicate with the given .cfg files only.
Second was the FF not matching the POH tables. SFC is set to 0.45 and fuel_flow_scalar to 1.0 in 0.1.1. The calculation for SFC should be FF/hp so in this case it was calculated for 15gph (x6 to get lbs), so we get 90, divided by 200 max hp equals 0.45 SFC. After some testing I found out that this is exactly the FF for 75% power at peak EGT (best economy) and not best power despite the naming of this variable.
The books state a FF of 12gph for peak EGT. So I recalculated the SFC. 12x6 = 72lbs/h divided by 200hp equals a SFC of 0.36
With this value I reach the numbers in the books very closely. 100° RoP gives me the corresponding 18% additional FF and peak EGT shows the expected FF as shown in the tables already posted in this thread.
Maybe someone could have a look into this and check if I‘m totally wrong and my calculations were not accurate or if the engine.cfg delivered with the latest release is off.
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@frulx241 said in Erroneous Fuel Flow Behavior:
So I tweaked the engine.cfg a little because I had two problems. First the FF increase when leaning. I found lots of reports of this behavior in other forums discussing different aircrafts. This seems to be a sim limitation and probably needs lots of effort to eradicate with the given .cfg files only.
I agree that the reduction in fuel flow seen when you set the mixture from 100 ROP EGT to full RICH looks like a MSFS bug that can't be easily fixed with .cfg file settings...
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@RetiredMan93231 yes, that‘s what I meant. If you try out my value of 0.36 you can see that the FF of 200hp is around 20+ gph. So either there is some conversion going on inside the sim or the wrong conversion from gal to lbs was used (that’s dependent on different liquids, in this case AVGAS so 6.0 instead of 6.7). As you can see in your table, the FF for BP (75%) is about 75lbs/h which is 12.5gph. Set the SFC to 0.36 and you will get those numbers.
The point I tried to make is that there is an issue with the SFC calculation and you can see that the default value gives FF way above the standard.
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@frulx241
I would hold off trying to tweak the FF until after JF has fixed the current problems with the turbocharger behavior, which will probably affect these settings... Also, the 12 gals/hr. you used in your formula to arrive at .36 sfc is the fuel flow for 75% power (150 hp), not full power. -
@RetiredMan93231 I don’t want to sound unfriendly but I don‘t know if you‘re part of the JF Staff or somehow involved in the development of this aircraft and that’s the reason you are defending the current state and posting tables of the POH to make a point. I read the PA-28RT-201 POH (every single page) for this very modeled engine and I have the real world values of FF and power settings and I have knowledge in software development to know, something is not right here and it’s not only the turbocharged settings. Sadly some parts of MSFS are an absolute blackbox and we just don’t know what’s in there. I was not talking about full power, I always wrote precisely what I was referring to.
Back to the SFC formula: Yes, I agree that the 12gph are for 75% at peak EGT (not best power 100° RoP, there FF is 12gph + 18% - you posted the table yourself). And that’s just my point. Obviously the SFC formula should not be used to calculate the FF at best power, instead it should be used to calculate FF at peak EGT and 75% power settings. Only then you get the correct values.
Maybe the other settings determine that the max continuous allowed hp are 150 and not 200hp (so 75%). And the corresponding FF to use in the calc is the minimal possible value that is allowed for fuel consumption at the point where every drop of fuel is burned (peak EGT). That would mean that the name of the variable does not match what is actual behind it from a mathematics perspective.
The formula:
Used here: SFC = FF at 100° RoP / max hp
0.45 = 90/200
New: SFC = FF at max continuous power at peak EGT / max hp
0.36 = 72/200Before arguing about not changing values or posting tables from the POH, try it out yourself! You can always revert this one single line to the default. Maybe then we can discuss the issues in formulas and variables together.
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@frulx241 Yes, RetiredMan is part of the beta team and is being particularly helpful with the engine work. The reason he has suggested not bothering to adjust the fuel flow specifically is because there is a much bigger situation runing behind the scenes at the moment and every adjustment we make is having an impact on the fuel flow.
What we are dealing with is a core sim which defines the turbocharger as having a variable wastegate. That is the core coding of the sim, there is no option. The Turbo Arrows have a fixed wastegate (unless an aftermarket upgrade is fitted). As such, we tried to replicate the fixed wastegate behaviour through a combination of the core sim engine and coding. It appears that, whilst not far off, this is throwing up a few minor problems, so we are looking at it again and trying to emulate the behaviour of the fixed wastegate in other ways.
There have been very few issues with the Turbo Arrow, but we are not happy and want to make it as good as we possibly can. Until we have an engine which is behaving in the very basic level as it should to our satisfaction, things like fuel flow and egt will not be looked at as they will need to be tuned to the engine.
I hope that helps clarify things a bit.
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I've read quite a few pilots state the standard fixed wastegate is complete rubbish and an aftermarket variable upgrade is one of the best things they have done...
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@FrontSideBus Yes, that's what we heard too but the aim of this is to replicate a specific aircraft and that has the factory setup. After all, who'd want the easy option... 🤣
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@Delta558 Thank you for your explanation! I already thought so.
@RetiredMan93231 As I wrote, I didn’t want to sound unfriendly or rude. If that was the case with my post then I am sincerely sorry!
The Turbo Arrows are both absolutely great aircrafts and together with the NA my absolute favorites. I am aware of the limitations and that these are a big impediment for the development of a „perfect“ aircraft. I just wondered why I get a FF so much higher than the values should be and especially since this was not the case with the NA Arrow. For now I am assured that this issue is known by JF and will be tweaked in future releases.
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@frulx241 said in Erroneous Fuel Flow Behavior:
@Delta558 Thank you for your explanation! I already thought so.
@RetiredMan93231 As I wrote, I didn’t want to sound unfriendly or rude. If that was the case with my post then I am sincerely sorry!
The Turbo Arrows are both absolutely great aircrafts and together with the NA my absolute favorites. I am aware of the limitations and that these are a big impediment for the development of a „perfect“ aircraft. I just wondered why I get a FF so much higher than the values should be and especially since this was not the case with the NA Arrow. For now I am assured that this issue is known by JF and will be tweaked in future releases.
I am also really hoping for that! The fixed wastegate seems a less necessary fix in my opinion, when compared to such erroneous FF readings and mixture behavior. If you don't lean aggressively when passing, say 6000, the plane engine cripples horribly.
Thanks for the Just Flight team in looking and noticing this.
