Flight Model Files
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This is my first post. I’m really loving Falcon BMS and have been playing for the last 6 months or so since I got a machine that can run it. So first off is a big thank you to all those who have obviously put in a lot of time to make this sim what it is today. I love the depth of this sim, and I love that I can look under the hood, see how a lot of it works, and make personal tweaks to things like key mapping, snap views, and painting my name under the cockpit. I’ve found I spend more time fiddling with files than I do flying, and that is fun for me too. I’ve recently started diving into the flight model files (e.g. f16bk50.dat and f16bk50_afm.dat) to see how the dynamics work. So I have a question about these files and I promise I have done due diligence in trying to find the answers in the forum, and reading manuals (including “HFFM”), but perhaps the answer is there and I did not see it… apologies if that is the case. How do these two files work together? There seems to be overlap between them (e.g. Cd and Cl vs. mach and alpha tables) then some things are unique to each (e.g. thrust is not in _afm.dat). So is there a precedence between them (e.g. _afm.dat supercedes anything in .dat)? Or is there a hardcoded selection of which sections to use from each file based on user selection of advanced vs. basic flight model? Any insight into how this all works would be greatly appreciated. Thanks.
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From my understanding in doing the same research as you have, the AFM is used only by human pilots, with some fallbacks to the non-afm flight model (eg. thrust ratings).
The AI ONLY uses the non-afm flight model. -
Ah, that makes sense and explains a lot. Thanks for the info.
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Correct
AI use standard dat file OFM , players AFM
Both share the same engine model that is in OFM file including thrust , fuel flow and spooling data and nozzle behavior data
The engine model has been changed from original to this new one during the OF era
Copying the same data in the AFM dat file would have been a waste of memory.
Both model share also the auxaerodata values in the OFM file
Apart from that OFM and AFM use am entirely different physics engine.
In AFM you have two aero modeling possible : global which is the one of the NASA Tp 1538 and local that you can find in the a10 AFM file for instance
Bms AFM has also several FLCS usable
- the Lockheed Martin f16 one
- the NASA one which is a loockeed Martin simplified and tweakable one (used by m2k for instance)
- independant modules for NFBW aircraft that allow to add flcs stuff like yaw SAS for instance
- f18 flcs (under construction in 4.33
)
You will also find a module for thrust vectoring aircraft , read av8 AFM file
The whole FM stuff is quite complex in BMS with huge potential that remains very poorly used by Data devs at the moment but that opens a lot of possibilities for the 20 years to come
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Thanks Mav-jp for the insight, this is good info.
So I’ve been playing with the thrust, drag, and lift vs mach, alpha, and throttle data in Excel to simulate different conditions and discovered something interesting (to me, at least). For a given altitude, weight, drag factor, and throttle setting, it is possible to have two different steady-state combinations of airspeed and AOA. There is both a high AOA/low airspeed, and a low AOA/high airspeed state that satisfies both the lift/weight and drag/thrust balances. I guess I just assumed that if you set your throttle to the same value, you would eventually end up at the same airspeed, but this indicates that your final airspeed is path dependent. I guess this makes sense, but I’m curious if this is true to real life. If so, is this characteristic unique to aircraft with high thrust to weight ratios? I would think this may not be possible in a passenger jet, but I could be wrong.
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There is both a high AOA/low airspeed, and a low AOA/high airspeed state that satisfies both the lift/weight and drag/thrust balances.
Are you familiar with wing & drag polar curves
https://en.wikipedia.org/wiki/Polar_curve_(aerodynamics)
https://en.wikipedia.org/wiki/Drag_polar
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Thanks Mav-jp for the insight, this is good info.
So I’ve been playing with the thrust, drag, and lift vs mach, alpha, and throttle data in Excel to simulate different conditions and discovered something interesting (to me, at least). For a given altitude, weight, drag factor, and throttle setting, it is possible to have two different steady-state combinations of airspeed and AOA. There is both a high AOA/low airspeed, and a low AOA/high airspeed state that satisfies both the lift/weight and drag/thrust balances. I guess I just assumed that if you set your throttle to the same value, you would eventually end up at the same airspeed, but this indicates that your final airspeed is path dependent. I guess this makes sense, but I’m curious if this is true to real life. If so, is this characteristic unique to aircraft with high thrust to weight ratios? I would think this may not be possible in a passenger jet, but I could be wrong.
Broadly speaking, all aircraft. Its one reason you have to fly specific flight profiles to get to the high speed and altitude corner.
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This is my first post. I’m really loving Falcon BMS and have been playing for the last 6 months or so since I got a machine that can run it.
6 months into the BMS and you are concern about afm data?
Go shoot missiles and drops bombs and enjoy the sim for now. -
6 months into the BMS and you are concern about afm data?
Go shoot missiles and drops bombs and enjoy the sim for now.He just said he is enjoying the sim… plenty of ways to have fun.
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Are you familiar with wing & drag polar curves
I’ve seen plots with induced drag and form drag that create a drag “bucket” but I have never heard of the “drag polar.” Thanks for the links. I have to admit, it is not intuitive to me that the low speed/high drag config is more stable than high speed/low drag… will have to ponder that some more.
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6 months into the BMS and you are concern about afm data?
Go shoot missiles and drops bombs and enjoy the sim for now.You’re right, I’m probably getting “in the weeds” too much, but not to worry, I have dropped lots of bombs and fired a few missiles. Right now I’m learning MITL, trying to remember to maintain SA and not get shot down while I focus on the MFD, flying the bomb! But aside from the fun of flying a jet and blowing stuff up, I invest time in these types of sims to learn stuff, and there is a lot to learn in BMS. Today, I learned about the drag polar (thanks Dee-Jay).
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Broadly speaking, all aircraft. Its one reason you have to fly specific flight profiles to get to the high speed and altitude corner.
Oh, now that is interesting. I have heard of stepped climb profiles, is that why they are used? I’ve wondered why I can’t seem to get high and fast in the F-16 like I thought I should be able to. I Googled this and found something called the “coffin corner,” is this what you are referring to?
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Oh, now that is interesting. I have heard of stepped climb profiles, is that why they are used? I’ve wondered why I can’t seem to get high and fast in the F-16 like I thought I should be able to. I Googled this and found something called the “coffin corner,” is this what you are referring to?
The F-16 doesnt really have this corner in its flight envelope as such, because of the alpha limiter preventing you from getting to it. In short, before you slow down enough, the nose automatically pitches down from FLCS commands, which makes it rather difficult to get slow enough.
How high and fast did you think you should be able to get? And the other question is, how heavy and draggy were you when you tried? Drag slows you down a lot… and your gross weight will do its bit, too. Mach 2.05 is basically reserved for zero drag index, medium altitude, afterburning flight.
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How high and fast did you think you should be able to get?
I think it was more about getting high than fast. I though I would be able to cruise close to FL500 without AB, not that you would want to do that normally. But I wonder if I couldn’t get there because I just tried to fly straight there at ~320kts CAS and ended up in a high drag/high attack state and the FLCS prevented me from going further, as you describe. But maybe I was just too heavy/draggy when I tried. I will try again.
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I think it was more about getting high than fast. I though I would be able to cruise close to FL500 without AB, not that you would want to do that normally. But I wonder if I couldn’t get there because I just tried to fly straight there at ~320kts CAS and ended up in a high drag/high attack state and the FLCS prevented me from going further, as you describe. But maybe I was just too heavy/draggy when I tried. I will try again.
Rule of thumb : reach mach 0.8 before climbing and maintain mach 0.8 during climb
Of course block 50 climbs much much better than the shitty block 42 ….
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You can… if you are clean (not draggy) and not too heavy. You can cruise AT FL500 without afterburner, in that condition. You cant really do steady state above that much, though. You can get to FL700 without too much effort, but you wont stay up there for long. Unsure how much higher than that you can get in BMS. Its all just zoom climbs though - not a great deal of use for it. Other than knowing the limits of the aircraft I guess.
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He just said he is enjoying the sim… plenty of ways to have fun.
Sure
Just saying that AFM Data analysis was not the first thing that came in my mind when i start flying falcon back in those days.
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Rule of thumb : reach mach 0.8 before climbing and maintain mach 0.8 during climb
Of course block 50 climbs much much better than the shitty block 42 ….
Much less to say about MLU ones…:D
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Thanks Blu3wolf and Mav-JP, this gives me something else to experiment with.
A related topic: I remember as a kid growing up next to Scott AFB in Illinois, that when the Thunderbirds came to do a show, my dad would take me on base to watch. The solo always did a low speed, high attack pass and I was always amazed that it looked like he was barely moving… great times. I wasn’t thinking about the drag polar when I was 10, HA!
But this makes me think of another question: At high attack, the thrust vector is significantly skewed from the flight vector, so that more of the thrust pushes against the weight and less against the drag. Is this accounted for in some way in the BMS flight model? I can imagine that the effect could be hardcoded, or it could be just accounted for in the lift and drag coefficient tables, or maybe its too insignificant of an effect to bother with at typical AOAs. I noticed there is a small “thrust alpha factor” table, but I assume this has more to do with getting air into the engine than with the thrust vector.
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Mav can explain it far better… but ‘in some way’? Well, yes, it is accounted for
Put it this way, the physics that act on the real jet end up giving pretty damn similar results to the simulated physics that BMS applies - to the point where you can just use the real flight manuals for performance calculation for the simulated aircraft.
