Some explanations on the various coefficent tables on the aircraft afm.dat file
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Hello
First of all, I am a new member of the forum seeking some information on how the flight model files work.Specifically I am trying to improve the F18C hornet’s flight model and trying some various things on the process to achieve what I am looking for.
I understand aerodynamics quite good and I am seeking some information about the various coefficient tables I see on the .dat files.
Does anybody who understands, maybe can explain me the structure as I don’t understand the layout.
For example I am trying to improve the excessive pitch down behavior of the F18C when flying inverted and therefore looking to tune the various tables.
I am looking at the Cm table looking like this :
For example,
For various delta -25, -10, 0 , 10 etc what is delta? Is this the angle of attack or pitch angle?
The table looks something like this below. What is the layout is it altitude or speed on the horizontal/vertical?
0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175 0.175
0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584 0.1584
0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159 0.159I have played with other sim’s Flight models over the years. I also consider myself quite educated when it comes to aerodynamic stuff and physics. However I have no Idea how BMS flight models system is structured.
If anyone can explain me the tables, it would be great.
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@frapes45 it won’t answer all your questions but prereq reading is Mav-JPs excellent set of articles on the development of the FM
https://www.falcon-bms.com/articles/flight-model/ (start at the bottom)
ofc aerodynamics and physics are only part of the equation, the behavior of the FLCS system (in code) is the other … realize it’s not always going to be possible to tune things like “excessive pitch down while inverted” by adjusting dat files
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@frapes45 unfortunately, it requires specific tools to properly edit and create a proper FM file…
Only a few in the team know how to do it…
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I have heard of a GUI existing to do that but apparently its not available to common mortals.
@airtex2019 Thanks, but I have already read this.
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I also have more questions for example what is LOD center position? .
Is this the center of lift or calculated drag? I tried playing with it and I noticed it affects the aircrafts pitch aoa flight characteristics meaning its possibly the center of lift force
Indeed, from a quick glance I confirm that some of the basic values in the configuration files are pretty much accurate like Wing span , Wing area , Control deflections etc. So kudos for that. Although the inertia values seem a little off like Ixx Iyy Izz and shouldn’t affect what I am trying to correct, I am looking for other sources of problem of flight behavior I want to change
I don’t want to interrogate any of the developers work and I ask with respect. Some basic explanations of the fm structure file would be appreciated.
Also, I understand in this forum there is no messaging function?
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There is a messaging function but I am not taking Private messages for support nor do I consider myself very knowledgeable on that part…
I’ll let other devs react if they want to share…
Cheers
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@MaxWaldorf No pressure.
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For explanations about AFM file structure please read NASA TP1538
As far as behavior of f18 is concerned this is FLCS related and changing Cm will like not change FLCS response
FLCS is hardcoded
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@frapes45 said in Some explanations on the various coefficent tables on the aircraft afm.dat file:
I also have more questions for example what is LOD center position? .
Is this the center of lift or calculated drag? I tried playing with it and I noticed it affects the aircrafts pitch aoa flight characteristics meaning its possibly the center of lift force
Indeed, from a quick glance I confirm that some of the basic values in the configuration files are pretty much accurate like Wing span , Wing area , Control deflections etc. So kudos for that. Although the inertia values seem a little off like Ixx Iyy Izz and shouldn’t affect what I am trying to correct, I am looking for other sources of problem of flight behavior I want to change
I don’t want to interrogate any of the developers work and I ask with respect. Some basic explanations of the fm structure file would be appreciated.
Also, I understand in this forum there is no messaging function?
LOD center position will have zero change about FM, this is just a value to correct the Visual model rotation axis in case the model LOD is not centered on the GC position
GC position will of course change FM
However as mentioned above FLCS is controlling everything you can do about FM data so you won’t see direct effects
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Thanks! you have been quite reaviling.
Another possible problem I have noticed is that on the F-18 file
the engine location relative to the noze on the .dat is a number something like -25 feet while the GC is something like -33Is this correct or is it a mistake? the engines on the F-18 are quite back
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@frapes45 said in Some explanations on the various coefficent tables on the aircraft afm.dat file:
Thanks! you have been quite reaviling.
Another possible problem I have noticed is that on the F-18 file
the engine location relative to the noze on the .dat is a number something like -25 feet while the GC is something like -33Is this correct or is it a mistake? the engines on the F-18 are quite back
Maybe it is a mistake however if the z position of the engine is the same of the GC z the X position mistake will have zero influence on the FM
Edit : Hold on
I the OFM Dat file the engine position is relative to the GC , not the noze
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Alright, that makes sense then, Thanks.
You are the man and I really appreciate it!
I would love to help the team anytime in things considering flight models. Please consider me or don’t hesitate to contact me.
Also from studying the nasa fm document I understand that the tables represent various φ , θ angles and β angles. That would be the roll pitch and yaw axes if I am correct.
Now the flcs code on the f18 might be worth investigating as I can’t pull the Blue angels maneuvers correctly especially the inverted ones, of course at empty loads.
Also one last question is Tail Ht value of +7.6000 the tail distance from GC or from what exactly?
and what is the difference of the GC value with that on the .afm GC Designed (GC of the plane stability design). From your explanations, I understand that my only option is to alter the center of gravity. Until you perhaps come with an improved FLCS version for the FA-18 C/D.Kind regards!
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@frapes45 said in Some explanations on the various coefficent tables on the aircraft afm.dat file:
Alright, that makes sense then, Thanks.
You are the man and I really appreciate it!
I would love to help the team anytime in things considering flight models. Please consider me or don’t hesitate to contact me.
Also from studying the nasa fm document I understand that the tables represent various φ , θ angles and β angles. That would be the roll pitch and yaw axes if I am correct.
Now the flcs code on the f18 might be worth investigating as I can’t pull the Blue angels maneuvers correctly especially the inverted ones, of course at empty loads.
Also one last question is Tail Ht value of +7.6000 the tail distance from GC or from what exactly?
and what is the difference of the GC value with that on the .afm GC Designed (GC of the plane stability design). From your explanations, I understand that my only option is to alter the center of gravity. Until you perhaps come with an improved FLCS version for the FA-18 C/D.Kind regards!
No you misunderstood nasa TP1538 modeling , read again thé implemented aero model from page 30 to 33
Your have no option to try to fix the issue that you are seeing
This is FLCS related and nothing you can Do about itPlaying the wizard with GC will bring you massive problems in other flight domains
Your GC must stay close to 0.35c
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Sorry but 0.35c CG is reffering to F16 which has a C.G location Aft of the center of lift. I am talking about the F18 I am not sure its the same as the information is more limited.
Nevertheless there is no point in having an accurate C.g if the Center of lift is not calculated accurately. For example, an approximation for the f16 Center of lift would be somewhere 0.27c and 0.33c. Of course this changes dynamically.
In fact the max GC aft limit for f-18 is 28% mac
From the nasa technical paper 3111
S= 400ft2 (1.440ft2)
b= 37.417ft (2.245f-t)
c= 11.517ft (0.691ft)
c.g.= 25%c -
@frapes45 said in Some explanations on the various coefficent tables on the aircraft afm.dat file:
Sorry but 0.35c CG is reffering to F16 which has a C.G location Aft of the center of lift. I am talking about the F18 I am not sure its the same as the information is more limited.
Nevertheless there is no point in having an accurate C.g if the Center of lift is not calculated accurately. For example, an approximation for the f16 Center of lift would be somewhere 0.27c and 0.33c. Of course this changes dynamically.
In fact the max GC aft limit for f-18 is 28% mac
From the nasa technical paper 3111
S= 400ft2 (1.440ft2)
b= 37.417ft (2.245f-t)
c= 11.517ft (0.691ft)
c.g.= 25%cThe GC of the BMS f18 with current data shall be close to 0.35c
This is because the f18 aero data are derived from f16 data.
All f18 data are computed from the f16 data which means they share the same center of lift which means they must share the e same GC center
If you which to change the GC you will end up with major problems in the whole domain
if you which to recalculate all data by changing the center of lift in order to change the center of gravity feel free .However as you will be unable to change the FLCS all of this would be totally useless
But feel free to do what you want
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Ha!
That explains it all.
Anyway I ended up reducing the tail ht number. No idea what this exactly does(possibly something with the horizontal tail location?) but I think I got what I was looking for.
“if you which to recalculate all data by changing the center of lift in order to change the center of gravity feel free”
Provide me the tools and I would do so anytime, free of charge…
Any chance you will give the Fa18 Fm some love?
Also does the newest f15 also use the f16 FLCS?
Thanks for all the explanations.
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@frapes45 said in Some explanations on the various coefficent tables on the aircraft afm.dat file:
Ha!
That explains it all.
Anyway I ended up reducing the tail ht number. No idea what this exactly does(possibly something with the horizontal tail location?) but I think I got what I was looking for.
“if you which to recalculate all data by changing the center of lift in order to change the center of gravity feel free”
Provide me the tools and I would do so anytime, free of charge…
Any chance you will give the Fa18 Fm some love?
Also does the newest f15 also use the f16 FLCS?
Thanks for all the explanations.
https://www.microsoft.com/en-us/microsoft-365/excel
that is the tool we use , we dont have others
i dont know what you mean by tail Ht number, what is this ?
F18 FM not a priority at all for me , however, changing the center of lift and the center of GC will have zero impact overall , except at take off because then GC and wheel contact distance will be different
F15 does not use the F16 FLCS , as a matter of fact, i am actually putting some more love in it
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@Mav-jp said in Some explanations on the various coefficent tables on the aircraft afm.dat file:
i dont know what you mean by tail Ht number, what is this ?
aircraft.dat file
Physical data
----> # Tail Ht
So there is no GUI that @MaxWaldorf is referring too?
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@frapes45
Vertical stabilizer height. -
@ccc1tw Thanks must be Ixx inertia and area2span then cause I also reduced these numbers