Erroneous Fuel Flow Behavior
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As shown in this chart, max fuel flow occurs when the Mixture is leaned to the max power setting (100 degrees rich of peak EGT). Power and Fuel flow can both go down if the Mixture is set too lean, or if it is set too rich... This effect is more pronounced at higher altitudes.
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@RetiredMan93231 the graph is correct and it supports what I am saying. You are misinterpreting the chart. Max fuel flow will be obtained at highest manifold pressure available, thus in a turbo, as long as you don't hit turbo critical altitude and manifold stays the same, fuel flow will not decrease unless you start leaning, at which point fuel flow can only drop, not go up! The modeling is just inaccurate.
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@mizra108 said in Erroneous Fuel Flow Behavior:
Hi there, i find the FF behavior a bit strange in the sense that it seems wrongly simulated. Anytime mixture is leaned FF should decrease based un the mixture position. It is my experience with this aircraft that fuel flow starts decreasing slowly with altitude which it shouldn't despite having the mixture set to rich. As that happens, leaning the mixture actually increases FF, which is also greatly inaccurate. FF can never increase when you lean, no matter the conditions. Is this hard to simulate? A sim limitation?
Finally someone noticing this! This is REALLY wrong, and dates back from FSX. Some developers managed to fix this, but I have no idea how. The mixture management in the Arrow (and maaany others) is completely wrong.
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Good point! Its been many many years since used FSX but now that you say that it clicked and I can remember this behavior perfectly. If its a sim limitation, im sure Just flight can find a workaround for it. Its unfortunate such a deeply simulated aircraft does not have this shortcoming taken care of @zazaboeing
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In agreement here. It seems that MSFS has maintained many of the problems with piston engine simulation from FSX (or predating it even). You can always find max power in FS by leaning to peak fuel flow. That's a bit of a cheat code that's only ever been fixed by a2a (AFAIK) with their aircraft. Plus--and this is really relevant now with the Turbo Arrow--turbocharged engines must still be leaned, regardless of altitude. I went up to 15,000 last night in the TArrow IV...TAS right on the money except I had the mixture leaned to a hair above cutoff.
I understand the situation and I'm happy with the airplane so this isn't a jab at JF but I do hope they use their status with Asobo as a respectable dev to continue to press for improvements.
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@mizra108 said in Erroneous Fuel Flow Behavior:
As that happens, leaning the mixture actually increases FF, which is also greatly inaccurate. FF can never increase when you lean, no matter the conditions. Is this hard to simulate? A sim limitation?
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I'm not a hundred percent sure of the point you're trying to make with a graph. Just to be clear, the note suggests that fuel flow will increase when you enrich
the mixture from peak egt to 100F rich of peak egt. -
@RetiredMan93231 I don’t think so. These graphs are for peak EGT and so the note is in direct relation to the chart and not to something not even pictured here.
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The chart note says the fuel flow will increase when you LEAN the mixture from full RICH
That's not what the note says. You added the words "from full RICH". The note qualifies the portion of the title "MIXTURE LEANDED TO PEAK EGT". Stated in other words, the note conveys that when you alter the mixture from peak EGT to 100 degrees ROP, you can expect an increase of 4 knots and 18% more fuel burn. The confusing part is they are using the word LEAN as a verb meaning to CHANGE THE FUEL MIXTURE. You are interpreting it to mean change the fuel flow to a leaner mixture which is not the intent.
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@RetiredMan93231 I had to read it a few times before it made sense to me! No worries mate.
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Would be interesting to see of the fuel continues to flow at the same rate when you turn the mags off with the engine windmilling...
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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.
