• Which the release of FS2020 we see an explosition of activity on the forun and of course we are very happy to see this. But having all questions about FS2020 in one forum becomes a bit messy. So therefore we would like to ask you all to use the following guidelines when posting your questions:

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MSFS Modelling single-spool turbojets


(Asked this on the official devsupport forum but that seems fairly dead... This is a much more active and insightful forum, right?)

All (?) early jet aircraft had single-spool engines. How should the N1 and N2 related values for a such be chosen? Will making sure N1 always equals N2 work? Or is this an untested edge case and for now one should pretend that the engine is a modern one with two spools and just tweak the values and tables until the performance matches the documented performance of the real aircraft? On the cockpit instruments one would then just show values for an imaginary single spool, calculated in some suitable fashion from the simulated two spools.

I am checking two add-on aircraft from my Community folder that have single-spool engines, Sim Skunk Works’s TF-104G, and Tyler Gladman’s Bristol 188.
  1. The TF-104G has a J-79 engine. The engines.cfg makes some effort to ensure that N1 equals N2. For instance low_idle_n1=67 and low_idle_n2=67. It uses a n2_to_n1_table where for the most part N1 indeed equals N2. Except at the higher N1 (and N2) values, where N2 exceeds N1. I have no idea whether this is just an oversight or intentional to make the thing work as the developer wanted. Also some other values are different for N1 and N2, like min_n1_for_combustion=10 but min_n2_for_combustion=19.4.
  2. The Bristol 188 has a de Havilland Gyron Junior engine. In its engines.cfg there doesn’t seem to be any attempt to tell that there is no separate N1 and N2. For instance, low_idle_n1=25 and low_idle_n2=61. But the rated_N2_rpm=9200 which presumably is in the right ballpark for the RPM of the single spool of that engine, anyway it is much too low for more modern two-spool engines isn't it? (That’s the point of having separate spools, in't it, the LP one with larger fan, compressor, and turbine blades can spin slower than the HP one with small compressor and turbine blades.)

FS Thrust vs Altitude calculations Version 2​

You can find this document in the Resources area of this website. It tells you how to sort out this problem in FSXA and P3D.
MSFS uses similar tables for the engines and what they do is essentially similar.
A single spool turbine just has a N1 section and you can use the tables for N2 with the same values, ie N2 = N1 so far as the tables and calculations are concerned.
I haven't found a manual for the Bristol 188, but I have pieced together information for this aircraft from may articles and books and have also manged to track down a manual for the Gyron Junior engine that states 100% engine speed to be 9200 RPM.

Although a single spool engine, I found during tweaking the engine.cfg that a lower N1 speed provided more acceptable engine responses for acceleration and EGT. Matching the N1 and N2 speed seems to lower the indicated EGT readings and limit engine RPM. The lower N1 also simulates engine choking at low altitudes due to the static intake of the 188. This may be the wrong approach, but it seems to provide good results and I'm not sure if the sim takes into account variable inlet guide vanes, compressor and turbine stages, momentum and angular velocity etc.

I have read the FS Thrust vs Altitude calculations Version 2 and having previously worked in an engine test cell found it very interesting in helping me understand how the sim handles the real world and will study it further.

One of the main problems I have encountered is the Aircraft Editor overwriting any tweaked figures when the file is saved in the editor.

One of the main problems I have encountered is the Aircraft Editor overwriting any tweaked figures when the file is saved in the editor.
Oh yes. I don't think I will ever use the Aircraft Editor, unless it is much improved. Trolling a bit, I think one could almost say it is for people who don't know how to use a text editor...

The drawbacks are so numerous. Let me list some.
  • Whatever UI "toolkit" (if one can call it that) it is that is used for DevMode is so weird. I mean the software that handles the UI widgets of the DevMode (sub-)windows; their scrollbars, buttons, text fields etc. Small but annoying differences from how normal desktop UI widgets behave.
  • You can't add and edit comments. If you maintain your .cfg files in your favourite text editor, you can add as long and verbose comments as you like. And clearly the place where you enter some value is the right place to have documentation for it. Like pointers to the references you used to come up with some value, etc.
  • Entering and editing values in Aircraft Editor doesn't offer any additional functionality compared to just editing them in a text editor. Well, except that you can trust that when Aircraft Editor writes out a .cfg file it doesn't cause any syntax errors. But does it even have sanity checks on the values?
Now, if the Aircraft Editor offered some really neat feature that you just don't get when editing .cfg files manually, I could change my mind.

Like, what if you could simply draw *curves* graphically, instead of entering values in a 1d or 2d table in order to set up a curve (or a set of curves)? I mean, it is from curves in aircraft documentation that one in ideal cases can pick corresponding x,y values to enter into a table, right? In the lucky cases where some table in a .cfg file directly corresponds to a curve published in documentation that you have access to. It would be great to be able to draw the curve in Aircraft Editor directly (from which it would then produce a suitable table to represent the curve accurately enough, but you would never have to look at the numbers in the table).

Sorry for the rant.