Cruise RPM setting



  • Using the POH values for 65% cruise power why is it that the speed is so much slower when using the 2200 rpm setting vs the 2500 rpm setting? The MP book value is higher for the 2200 RPM setting vs the 2500 and power is determined by torque on a constant speed prop airplane, so shouldn't the 65% power setting deliver the same TAS regardless of whether or not you are using the 2200 or 2500 rpm setting as long as you are using the correct MP setting? It feels like the simulation is using just the prop RPM to determine aircraft performance, and If that is the case I will be very disappointed that I spent $45 on this otherwise nice aircraft.



  • I sent a ticket about this days ago and have not gotten a response there or on this forum. Are the developers aware of this issue? The entire flight/engine performance model is programmed incorrectly for the simulation of a constant speed prop and its a pretty big issue for the money this addon costs.



  • Changing the RPM (prop pitch) on a constant speed prop aircraft is like changing gears on a car... With a lower prop pitch (low gear) the engine RPM will be higher, and the power output and climb performance will increase, but the top speed will be reduced. In level flight, increasing the prop pitch (high gear) will reduce the RPM and increase top speed (TAS) for any given engine power setting (MP). The engine efficiency and max power output is also different at different RPM's, with max power being produced at 2700 RPM, as stated in the POH and engine specs. Also, as the air density is reduced at higher altitudes the prop pitch will automatically increase to maintain the constant RPM, and the top speed (TAS) will increase... So, TAS will always vary with different RPM settings. But, how closely this aircraft matches the POH performance specs is another question entirely...



  • @RetiredMan93231 said in Cruise RPM setting:

    Changing the RPM (prop pitch) on a constant speed prop aircraft is like changing gears on a car... With a lower prop pitch (low gear) the engine RPM will be higher, and the power output and climb performance will increase, but the top speed will be reduced. In level flight, increasing the prop pitch (high gear) will reduce the RPM and increase top speed (TAS) for any given engine power setting (MP). The engine efficiency and max power output is also different at different RPM's, with max power being produced at 2700 RPM, as stated in the POH and engine specs. Also, as the air density is reduced at higher altitudes the prop pitch will automatically increase to maintain the constant RPM, and the top speed (TAS) will increase... So, TAS will always vary with different RPM settings. But, how closely this aircraft matches the POH performance specs is another question entirely...

    Yes that's very good analogy, right now the behavior is backwards in cruise though. If you decrease prop RPM during cruise and raise mp by the correct amount you get a drastic decline in TAS. Its like they modeled it as a fixed pitch prop and tied power to rpm only.



  • @pilot53 Agreed. I was experimenting with 65% power settings at 1,000 ft this morning. The high MP/Low RPM setting for 65% best power produced an airspeed about 6 knots slower than the low MP/high RPM 65% best power setting (from the POH delivered with the model). I concluded that power is not being modeled correctly (as I cant measure power, I used TAS as a proxy under no wind constant altitude conditions and using the alt and heading hold features of the autopilot).

    I am curious if see if a setting of 24 inches MP + 2400 RPM results in a TAS greater than that of 21 inches of MP + 2400 RPM. The TAS should drop (obviously) when you lower MP. This test would tell us if the model is ignoring MP al together. I kind of doubt it, but it will be an interesting data point.

    Putting this in context, you can set 24 inches by 2400 rpm and 12ish GPH and enjoy all most Arrows can give you in terms of TAS. Its great to strive for perfection, but this shouldn't stand in the way of enjoying or getting the most out of using the model for instrument approach practice. At 24X24 for cruise, and 20 inches X 2400 RPM for approach work, it behaves very much like a real Arrow.


  • 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.


  • JF Staff

    And just to clarify, the Arrow III is configured with a constant speed rather than fixed pitch prop. We are currently trying to figure out why the prop/RPM behaviour might differ in MSFS.

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  • @Martyn said in Cruise RPM setting:

    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.

    Thank you for the update, I do hope these issues can be fixed. The main reason I purchased this aircraft is because I thought it had custom code to get around these annoying and very basic fundamental flaws with the MSFS flight dynamics.



  • @pilot53 I may be wrong, but i don't think there is any aircraft available for MSFS right now that uses a custom flight model. I don't think the SDK is quite there yet.