[Piston Duke]Possible Reasons for Discrepancies between Charts and In-Sim High Altitude Performance
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The Duke had two version of the Lycoming TIO-541. The E1A4 and the E1C4. Basically the difference being the Turbochargers. The E1A4 has the T-1823 supercharger and the E1C4 has the T-1879 Turbocharger. I've seen references to the Duke having a critical altitude of 17000' PA and the Blacksquare Duke manual states a 25,000' critical altitude. Now I haven't been able to find a specific reference if these turbochargers have different critical altitude but it could be a possible explanation between the difference of what we see in the Blacksquare manual and the sim.
From an actual POH below:
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@meh1951 I found this B60 performance chart for comparison to the Blacksquare manual
Is our Duke aircraft based on the B60 or the A60 because the Blacksquare manual performance charts seem to more align with the A60 manual yet the Blacksquare manual references the B60 in places? In either case the B60 manual looks pretty comparable to the A60 KTAS at FL280 with ~1 knot difference.
One thing that does stand out is that the Blacksquare manual shows open throttle at FL240, and both A60 and B60 manuals show open throttle at FL280. Now in the sim, it does seem like you’ll get around 37” of MP at full open throttle at those temps and altitudes, so still a decent amount above what any of the manuals are stating.
Edit: I did find this reference for the Duke having a critical altitude of 15,000’ vs the 25,000’ in the Blacksquare manual. And just to complicate matters further, the reference document mentions the 15,000’ critical altitude is for 380 BHP at 2900 RPM and 300BHP @ 2750 RPMs is at 21,000’, so my conclusion is further testing required. Yes I know I’m obesssing over this, it’s what I do. I can’t help but pull things apart.
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Well guys, sadly I've dove a little too deep into how the sausage is made and discovered there isn't a custom turbocharger simulation going on. Simply put, in the aircraft engine.cfg files, the Duke is using the basic MSFS turbocharger simulation, and not using the custom boost tables (density_to_boost_table) that you can do with the SDK. This is probably the reason for the unrealistic performance numbers as using MSFS's basic simulation won't produce the exact realistic curves to match the performance charts. Hats off to @Black-Square for creating a wonderful aircraft still, but my hunt for the perfect turbocharged aircraft continues.I was wrong, please disregard.
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@Hawkeye Hopefully you will pardon me for jumping in a little more forcefully than I usually do, but this is absolutely not true.
I do not use the new turbocharger tables available after one of the recent sim updates precisely because I have my own turbocharger simulation.
If you want to see how the sausage is made, you can look in the behaviors xml and template xml where my turbocharger simulation lies. There, you will find that I even use the inertia of the turbocharger's rotor in my calculations to affect my fuel-to-air ratio output and wastegate position. Below that code, you will find the equivalent of my "boost tables" in 2,500ft increments.
The reason I have refrained from joining the conversation on turbocharger performance yet is because I like to research and test on my own before answering the community when a more nuanced issue arises, like this one. Rest assured that I will be examining turbocharger performance and critical altitudes before the next update, and will consult the Duke owner who has contributed the most to this project to get his opinions on the performance tables included in the manuals.
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@Black-Square ah, my apologies for my assumptions. I assumed it was using the basic MSFS simulation because when I would edit the critical altitude value within the engine.cfd file from 25,000’ to 15,000’ it actually did change the critical altitude and MP began to drop after climbing above FL150.
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There is an interplay between the default turbocharger value, which is required to maintain the possibility of maximum power output at any fuel-to-air ratio, and my internal critical altitude value, which you can find in the behaviors xml within the first 50 lines, "TurbochargerCriticalAltitude".
As some of you have pointed out, the 25,000ft number would not appear to match the numbers presented in various engine publications of between 15,000 and 17,000ft. While the requires a revisit, I recall the crux of the issue at the time being that much of aircraft turbocharger design knowledge has been lost to the sands of time, at least as the internet is concerned. For example, the definition of "critical altitude" has been eroded, and cannot be taken to mean "the altitude at which the turbocharger can no longer maintain maximum rated boost pressure" versus, "the altitude at which the turbocharger can no longer maintain sea level pressure".
You're more than welcome to tweak the two turbocharger values, but more adjustments may be required to most closely match the real world behavior, depending on what I rediscover. I will conclude by saying that, if nothing else, the enthusiasm you are all showing for a couple knots near the tropopause lets me know how seriously you are all taking my aircraft, which is a delight to see.
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@Black-Square Through my digging I found this NASA paper on turbochargers and one of the engines referenced is the Lycoming TIO-540. I can’t say which variant, but perhaps it would have something useful.
https://ntrs.nasa.gov/api/citations/19840013811/downloads/19840013811.pdf
