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?



  • I found the same but also few different issues with fuel flow in the turbo versions. Not only the FF is rising when leaning the engine, the complete mixture setting seems off.

    Tested lots of settings and found that the most accurate fuel flow values above 3000ft are retrieved when you simulate 100% mixture equals 40% lever position.

    So to recreate you pull the lever all the way back to 40% and then start leaning from there. At 8000ft according to some IRL videos of Turbo Arrows, the lever should be at approximately 60%.
    Calculation: 40 (our new 100%) - 40% (to be at 60% mixture) = 24% (shown lever position) and this is exactly the moment where the fuel flow comes close to the books again.



  • 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.

    70de6b1e-cb42-456c-90e4-6fc01ae1bdf9-Screenshot 2021-05-31 102319.jpg



  • @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.



  • @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.



  • 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



  • @mizra108 @zazaboeing

    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.



  • @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?

    cf105523-839c-4815-b634-49f8df48bba1-Screenshot 2021-06-04 074107.jpg



  • @RetiredMan93231

    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.



  • @BernieV, The chart note says the fuel flow will increase when you LEAN the mixture from full RICH to 100 ROP EGT... Is that not the behavior you see in the real aircraft?



  • @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.



  • @frulx241 You are correct... I failed to note that the chart was based on PEAK EGT... I stand corrected!



  • @RetiredMan93231

    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.



  • @RetiredMan93231 I had to read it a few times before it made sense to me! No worries mate.



  • 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...



  • 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.



  • @frulx241
    The fuel flow at max power (200 hp) should actually be about 20 gals/hr. based on this chart from the engine POH. The term "Best Power" means a mixture setting of 100 ROP EGT, not full power...
    52fee4b1-aec1-42a7-a1fe-d00bd09d2f12-Screenshot 2021-06-01 154954.jpg



  • @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...



  • @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.



  • @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.