Mixture control and fuel flow
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At my usual cruise MP of around 32 inches and 2500rpm around 20k ft altitude I'm seeing some odd behaviour with the fuel flow in the Grand duke when I pull the mixture back.
The fuel flow will climb quite sharply until the mixture lever is past the middle before starting to settle down. Once I've leaned to 50F ROP I'm usually sitting with a higher fuel flow than if I had just left the levers full forward. Is this the expected behaviour for a turbocharged plane?
My understanding is that the fuel flow should always reduce when pulling the mixture.
I recall this happening on planes using the native MSFS engine modelling but I understand the Dukes are using their own custom model so should theoretically be immune to this weirdness.
Can anyone corroborate my findings?
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Hi, I encourage you to watch one of many online videos about mixture for piston engine. While climbing, if you do not lean your mixture, you'll leave the combustion on the lean side, the rich side. Reducing mixture will bring you to the peak of EGT, peak of FF etc, reducing more your mixture you'll enter in the lean of peak side of the combustion.
Attached image maybe brings more clarificationbb
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I understand how leaning the mixture works, that isn't what my question was about.
Fuel flow should always decrease when leaning shouldn't it?
I'm seeing the fuel flow rise when leaning at typical cruise settings before coming back down and sometimes sitting higher when running rop.
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@squibby I see that, too and was wondering how a physical reduction of fuel (with closing the valves by the levers) could rise the fuel flow gauges. And vice versa, when leaning higher ROP (increasing the fuel) results in dropping fuel flow.
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Yes, fuel flow should go down when pulling mixture back. It is a long standing bug in the sim, since FSX era. There's a lot of msfs forums posts about that, not sure if any bug reports that we can vote for. If anyone finds one, please link.
Hopefully 2024 will finally make this right. -
If that's the case it's a bit disappointing that despite all the custom engine and turbo simulation we're still stuck with the flawed mixture default mixture model.
Maybe the dev can comment on this and if it's planned to be resolved with some custom modelling at all in future
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Since SU13 in MSFS, there are new options for developers in the engines.cfg file that were developed by Working Title to fix this old fuel flow bug. You can read about the new options in this MSFS dev update from last year by scrolling past the feedback snapshot and marketplace updates and looking for the Working Title update: https://www.flightsimulator.com/june-22nd-2023-development-update/
My understanding is that Black Square is still using the old config options because his custom code for turbocharger logic is built based on the old config parameters, and there is not a simple way to update it without rebuilding the custom turbocharger code from scratch.
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This seems to be one of the last significant issues with this aircraft. As a community, we kindly request your attention to this matter. Could you provide an update or solution for this problem? @Black-Square ?
The aircraft is amazing, performs exceptionally well and is enjoyable to fly, except for this one concern. -
MarcKC is absolutely correct as to the cause, solution, and why I haven't implemented it yet. Adding the new functionality from working title drastically upsets the fuel flow, which affects everything from the turbochargers, the gauges, engine performance (performance tables), tablet visualizers, and even the EDM-760. I spent several hours before releasing v1.1 to see if this could be made an easy fix, but every step I took unraveled hours worth of issues. However, hope is not lost. These changes often follow patterns for which equations can be fit to remove the guesswork from the operation, but they can be difficult to see while you're trying to preserve the balance of existing software (noticing a loss of 2-3kts of cruising speed at some altitude necessitates reexamining all the performance tables to ensure the aircraft remains within the envelope and any adjustments are made to restore the original performance). What usually happens is that I will develop a new aircraft from scratch with this technology change, and then, once I have figured out its nuances and created new equations, I will port it back to my previous aircraft when I am convinced that I have caught all the unintended consequences.
In summary, sometimes software problems are easy and affect only one thing, and sometimes they are hard and affect more things than you even know. This one is the latter. I will be using the new fuel flow technology for my future reciprocating engine aircraft, and once it works flawlessly there, I will bring it back into the Duke. Thank you for reading, and thank you for enjoying my aircraft so thoroughly that these details matter to you!
