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FS Thrust vs Altitude calculation method

Roy Holmes

Roy Holmes

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The attached file is an article I have been working on for a month or so and the time seems right to publish it on FSDeveloper.

It describes how to calculate values for tables 1503, 1504,1502, 1506 and 1507 that will result in accurate values for thrust vs altitude in FS.

The article is 19 pages in length and, in addition to the main body, it has two appendices. Appendix A contains a list and description of the tables, terms and equations used by FS when applying the corrected method of performance calculation to get actual values used in the aerodynamic calculations.

Appendix B contains a brief description of how to use the NASA applet EngineSim to verify FS flight test results.

I hope this article will help anyone who deals with .air files and I welcome discussion on its content.

Roy
 

Attachments

That's an impressive piece of work which I 'll need more time to understand fully.

On the subject of the NASA Engine Sim I have produced a spreedsheet based on this which deals with the early single-spool turbojet version.

There's a later report Interactive Educational Tool for Turbofan and Afterburning Turbojet Engines dated 1997 which I imagine deals with the current NASA Engine Sim, but I can't find a downloadable copy. I'd be grateful if anyone can provide a link - I can't find one.
 
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I welcome your comments when you have studied it. I'll look at your spread sheet and see what I can learn from it. The one I used was interactive and had about 10 sheets.

I have seen the later paper you mentioned, but I think the program described was actually earlier and turned into Engine Sim. In the article I mention that I had also modeled a low bypass turbofan (Tornado engine) and found only minimal differences in thrust with altitude. My version of EngineSim covers turbojet, turbofan and afterburning turbojet
Thanks
Roy
 
I should have mentioned that the code in EngineSim is interesting if tough to decipher
Roy
 
Looks good Roy. I still think the two-spool explanation is necessary due to the vast majority of FS users using 2 spool turbofans. I will absorb it some more and send you anything I can think of.

The piece I am working on will probably fit well with this. I am approaching it more from a step by step tutorial perspective for those who don't get the math and terminology.
 
Joe,
Thanks.
My main aim was to show a simple way to get CN1 where it should be at a given throttle/altitude/Mach flight condition. There probably are other ways to do this but this method is easier to understand if CN1=CN2. Of course that just means the percent CN1= percent CN2 and the actual RPM's can be different which is invariably the case. The turbofan I mentioned as an aside was a RB199 which in real life has three spools.

However, given the limits of FS and the program flow I think the only feasible way to get CN1 where it should be is to link CN1 to CN2. It is a bit like TSFC, a program limitation.

I deliberately simplified the corrected method process. I just put in enough information, I hope, to make it understandable in the context of getting CN1 right.

I believe an expanded description would fit well with my paper since we seem to have a lot of interest in equations at this time.

I look forward to your comments and all comments. For this to be a useful document it should have a high degree of acceptance by the forum.

Roy
 
Roy,

Good work. I think you should post this document in the Wiki or download section when able.

Regarding your comment about RR engines having 3 spools, in this case FS's N2 must be tied to engine's N3 which is the core spool (then CN2 would be kinda CN3) and the real N2 can be modeled by using polinomials from real tables or interpolated N1-N3 values.

Tom
 
Roy,

Yes, it definitely needed to be done as one spool first for explanation purposes.

Tom,

Yes that would work, but the problem is you have to calibrate n1 vs CN1 after it flows through CN2, where CN2 has it's own curve. Both are affected by normalization.

I suppose we'll explain it one day.
 
Tom,
I think you should post this document in the Wiki
Click to expand...

There is no flight dynamics section in the Wiki and I do not know how to create one.

It was always my intention that it should be permanent and the Wiki is the place.

I posted it here first to see if it merited inclusion in the Wiki and to take suggestions on how it might be improved

Roy
 
Roy,

Maybe we can ask Bill (n4gix) if he could create a FLIGHT DYNAMICS section, or eventually put your article in one of the already existents -ie Panel and gauges, etc-

Tom
 
Tom
I sent Bill an email asking if it was possible to create a flight dynamics section
Thanks for the pointer
Roy
 
Bill added a Flight dynamics section in the Wiki for me to edit.

The process is not easy and will take some time for me to understand. I tried pasting text, it must have gone somewhere??? I saw the text for about a millisecond with no images included

As an alternative approach I looked at adding it as a download.

That process is not easy either and will take some time for me to understand. I zipped the file and uploaded it, it must have gone somewhere???

Is there an article in the Wiki that tells you how to put an article in the Wiki? If so I hope it is easy to understand.

Will try again tomorrow
Roy
 
Tom,
The article was unsuitable for posting in the Wiki, so I put a note there to the effect that it was available as a .zip in the Aircraft Design Downloads section.
Roy
 
For the record, I have had feedback from two people on the paper so far

One concerned the fact that despite incorporating my tables, Mach did not increase like I said when compared to sea level and the tropopause. Turns out the reason for this was an unrealistic high value for CD0 in 1101, combined with the fact that 1524a had Mach drag rise commencing below 0.7 mach as opposed to somewhere above 0.8 mach. Or, put another way, Mcrit was way to low. Fixing those fixed the issue.

The second comment was that although the relationship between 1503 and 1504 values was correct, the description of how they were arrived at was in error.
I said "In 1504 the values for each IAP column are the corresponding 1503 values times 1/ (1+.2*(M)^2)."
I should have said
" In 1504 the values for each IAP column are the corresponding 1503 values times sqrt(1/ (1+.2*(M)^2))"

I'll wait another month or so to see if any one else spots errors and then both correct the statement above and add some caveats about how the initial sea level performance has to be realistic for the method to work.

Roy
 
Roy, thanks for this excellent explanation, i'm still learning and i'm reading it second time now.
I have a question - is there a way to tweak NASA EngineSim to accept larger values? For example, i need a larger value for turbojet inlet area, EngineSim accepts up to 50
Sorry for bumping this old thread :)
 
So far as Engine sim and a turbojet is concerned the inlet area dictates how much thrust is modeled. Inlet for a turbojet is the area within the engine through which air passes for combustion.
If you have an "inlet" area bigger than 50 square feet you are probably talking about the intake area of a turbofan nacelle. Not the same thing.
Even with a turbojet the inlet area is less than the intake area because there is always something solid in the center.
In the sim the inlet area simply dictates the amount of ram drag (when multiplied by the factors in 1507). It does not affect gross thrust in the sim, just net thrust.
Roy
 
Yes, that's right, it's a big turbofan engine. I don't know why i tried to set turbojet inlet area, probably becouse of FSX config. file. I checked it and now everything is working just fine :)

Finally, i'll be able to model my engine...

Thanks
 
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