Cruise performance does not match POH specs



  • The current cruise performance and fuel consumption of v0.3.0 does not match the specs shown in this chart from the POH...

    Screenshot 2021-04-06 164332.jpg

    So, I recommend making the following change to the flight_model.cfg file and the engines.cfg file to come closer to meeting these cruise performance specs...

    flight_model.cfg

    [FLIGHT_TUNING]
    parasite_drag_scalar =0.8 ;0.81
    induced_drag_scalar =1.6 ;1.7

    engines.cfg

    [GENERALENGINEDATA]
    fuel_flow_scalar =0.53 ;0.65



  • @RetiredMan93231 Hi RetiredMan. There has been an ongoing discussion about this. One thing to consider is that the changes you are suggesting will impact climb performance, cruise performance, and service ceiling. I assume you are setting mixture using fuel flow rather than EGT (IMHO EGT is not modeled correctly). I would encourage you to test out time to climb and service ceiling to see how the parameters you are suggesting impact those performance characteristics. Your values are very close to the original, so I suspect they are just fine. If they are making a huge difference in cruise, then that would be a little suspicious to me.

    Just out of curiosity, what were the MP, RPM, FF, and density altitude you tested? What was the cruise speed before and after the changes?

    -bv



  • Regarding the fuel flow I noticed that since 0.3.0 I often have to lower the mixture to less than 40% to at least go down to a 75% cruise fuel flow (12.7GPH), while the mixture lever lock blocks me from going less than 40% at all.
    Doesn't feel right to me to go that low on mixture.
    I guess to get to the 55% power cruise fuel flow, I'd be close to cut-off.



  • @copper I agree with your comment on mixture. IMHO both FF and EGT modeling could be improved. The amount of travel required to do normal leaning seems excessive. Not sure if that is related to fuel_flow_scalar or controller sensitivity. Perhaps boosting the latter would help



  • @BernieV These settings were tested using the following conditions...
    Altitude: 5000
    OAT: ISA (15C at Sea Level)
    MP: 24.6 (full throttle)
    RPM: 2500
    Mixture: Peak EGT ( the EGT gauge temp readings appear to be off, so you can't set it it to +100F rich)

    Sea Level climb performance was about 800 FPM at MTOW... Final cruise speed was 135 knots true (based on GNS 530 Ground Speed)



  • @BernieV said in Cruise performance does not match POH specs:

    I assume you are setting mixture using fuel flow rather than EGT (IMHO EGT is not modeled correctly)

    I set the mixture to peak EGT, because the fuel flow is wrong... hence the new recommended fuel flow parameter change. I also agree that the EGT, or the EGT Gauge, is not modeled correctly, but the peak reading should still be correct.



  • @RetiredMan93231 Was the before or after you config file changes? What were the prior results?



  • @RetiredMan93231 I have never gotten book performance numbers using EGT. I trust Fuel Flow more. I could be wrong about that, but I have been able to reproduce book performance numbers with the 0.3 release using Fuel Flow rather than EGT to lean.

    BTW this has been discussed before. For my test results see this post.



  • Since the EGT and the Fuel Flow are both incorrect, I set the 75% power using this chart of MP & RPM... Then I adjusted the fuel flow parameter to match.

    PA28R-201 Best Power Mixture.jpg



  • @RetiredMan93231 How did you determine that both Fuel Flow gauge and EGT were wrong? The challenge is how do you calibrate one without using the other? I used cruise TAS, RPM, and MP from the power tables and then varied the mixture lever until I got the TAS I expected. I then looked at FF and the EGT gauge and determined that FF was close to the 12.1 g/h required to get 75% best power mixture.

    I tried leaning by EGT got 5-10 knots less than book. I concluded that Fuel Flow was more accurate than EGT, but in retrospect, I have only antidotal evidence that's the case.

    At the end of the day, if I can get 135 knots out of the JF Arrow at 5-7K feet, I'm satisfied. I find it a bit annoying that the Fuel Flow and EGT are a bit wonky, but not so much that I've opened a ticket. What's more important to me and how I use the simulator is that I can get in the neighborhood of 135 knots at my typical cruising altitudes. With that said, a lot of people new the the JF Arrow will try to lean by EGT and will likely be repeating this discussion.



  • @BernieV , It would appear that the only thing we can assume is accurate right now is the MP and RPM gauges. So, if you set 75% power based on those, using the POH charts, you should be in the right ballpark for measuring performance. How you should proceed from there to calibrate the EGT and FF gauges is the question?? Hopefully, the JF devs can do a better job with access to the actual sim values for these parameters...



  • @RetiredMan93231 Not to belabor the point, but power is a function of MP, RPM, FF, and Density altitude. I've been noodling the math to compute power as a function of those four variables, and can get within 10% over a broad range of those variables. The one thing that is an absolute bit*h is to try to extrapolate EGT from those 4 parameters. I can't find any data other than generic graphs that show the relationship between FF, %power, CHT, and EGT like this one.

    daff1591-2e71-41fb-a136-275e42ef5215-image.png



  • Right now there is a major flaw in the flight and engine model of this aircraft. Engine power seems to be tied to engine RPM and not actual engine output which on a constant speed prop is a function of both prop RPM and manifold pressure. You can test for yourself, leave the throttle alone and just manipulate the prop rpm at cruise, the airplane accelerates and decelerates with prop rpm like as if it was a fixed pitch prop. Until the developers fix this major issue nothing in the books will be correct because the entire basis of the simulation of the aircraft is currently flawed.



  • @pilot53 said in Cruise performance does not match POH specs:

    Right now there is a major flaw in the flight and engine model of this aircraft. Engine power seems to be tied to engine RPM and not actual engine output which on a constant speed prop is a function of both prop RPM and manifold pressure. You can test for yourself, leave the throttle alone and just manipulate the prop rpm at cruise, the airplane accelerates and decelerates with prop rpm like as if it was a fixed pitch prop. Until the developers fix this major issue nothing in the books will be correct because the entire basis of the simulation of the aircraft is currently flawed.

    First, let me (re)state that the power modeling of the JF Arrow 3 is inaccurate. Fuel flow and EGT are the weakest part of the model. Also, I have not tested a wide range of MP and RPM to see if the modeled power is accurate as compared to the engine power setting table.

    To your point, power is a function of RPM, manifold pressure, fuel flow, and density altitude. Power varies directly with RPM and MP, so the behavior you are describing is to be expected. In other words; increase MP and/or RPM and you will increase power.



  • Yes, but if you increase just rpm and let the mp drop aren't you actually reducing power slightly?



  • @pilot53 If I understand what you mean by "let the MP drop", its that MP (without touching the throttle) will decrease a little when you increase RPM and conversely MP will increase a bit as you reduce RPM.

    My point was that holding MP constant and increasing RPM increases power. I think that is true in most cases, but I can think of one where it might not be. If you are running lean of peak at a relatively low rpm and increase RPM (while holding MP constant) and fail to enrich the mixture, you could actually produce less power as you would be on the lean side of the mixture/power curve which drops very quickly.

    It all comes back to power being a function of MP, RPM, Fuel Flow, and density altitude and that power varies directly with the variables MP and RPM in that function. That statement is true but it ignores the added complexity that changes to throttle, prop rpm lever, or mixture don't necessarily impact a single variable. That's why when making a power change, you potentially need to change all 3 of the controls to obtain the right mix or MP, RPM, and FF that match your desired power setting.

    Its a lot easier to do than explain! I just came back from a flight and it took longer to write this post than it did to fly from KASH to KAQW



  • I tested economy cruise flight (55% power @ 2,200 RPM and Peak EGT) at 5,000ft and -1.5°C OAT, and the resulting performance was very close to what is shown in the corresponding level flight performance chart.





  • @Leonard-McCoy Looks like you 7-8kts too slow and burning 2 gph more than you should.


  • JF Staff

    I've posted this elsewhere in the forum, but it's probably more relevant on this thread:

    Just to be clear, we can only fix issues that are unique to our aircraft and are not as a result of limitations of the host simulation or compromises made to get around issues with the host simulation. If you see something that you think doesn't work as it should, raise a support ticket with JF and we will look into it. We are always open to suggestions (via support tickets) on how to improve the software, but please bear in mind that some changes suggested may also impact adversely in other areas of the aircraft's performance.


  • JF Staff

    This is just a quick note before the weekend to confirm that we are working on a variety of changes to the flight dynamics ahead of the next update and will provide details of those changes ASAP. As ever with flight dynamics, and particularly in MSFS at the moment, there are various limitations/inaccuracies in the flight dynamics engine itself and compromises that have to be made (i.e. tweaking one value for accuracy can often reduce the accuracy of another), so it takes time to make and test all those changes properly.