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JSBSim or YASim?

Good sims require good FDMs (the "thing" that makes an aircraft behave like an aircraft).

JSBSim or YASim?

Postby helijah » Fri Jan 10, 2014 2:03 am

Moderator note: Posts split from viewtopic.php?f=4&t=19584&start=45

flug wrote in Sun Mar 31, 2013 2:19 pm:Aha--let's rephrase that to say that I first tried to implement this via YaSim and couldn't figure out how to do so. So very possibly a reflection on me rather than YaSim. In fact it would be an interesting and useful addition to the Camel YaSim model if it can be done.

Do you know how or can you point me to any examples or tutorials about how to do this using YaSim?

Thanks!


Unfortunately, whenever I had the right to criticisms YASim, it was always, then but ALWAYS , by people who did not understand its operation. Without understanding they prefer to criticize. It is sad and domageable.

If you want to learn and understand YASim, one address : http://www.buckarooshangar.com/flightgear/yasimtut.html

Regards Emmanuel

P.S. Stop believing that the FDM (even the best) are "realistic". Physics around us is millions of times more complex than can simulate the most powerful computer. So imagine our simple PC desktop ! We must be very ignorant or very pretentious to believe that we can create FDM "realistic". At best it may be closer to reality with the simulation and code simplifications required.
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Re: New historically realistic Sopwith Camel JSBSim FDM rele

Postby Thorsten » Fri Jan 10, 2014 7:57 am

Stop believing that the FDM (even the best) are "realistic". Physics around us is millions of times more complex than can simulate the most powerful computer.


Sure physics around us is very complicated, but most of it cancels out or is averaged over. I don't really need to know what electrons in orbitals of atoms in the livery paint do, it's enough to see that the net result of their interaction with light comes out green.

Unfortunately, whenever I had the right to criticisms YASim, it was always, then but ALWAYS , by people who did not understand its operation


Well, do you? I mean, do you actually understand how the equations of motion for an aircraft tie together with what you enter in YaSim? Because if you would, you wouldn't make such a statement.

JSBSim inputs a natural set of parameters/parametrized functions which you get when you derive aircraft equations of motion from first principles, making a few assumptions (approximately steady-state flow, limited dynamical deformation of the airplane,...) on the way. So for decent input data, it won't be perfectly realistic, but realistic in the sense that it reproduces to high fidelity the performance of an aircraft inside the known envelope.

YaSim does not input a natural set of aerodynamically relevant parameters, it inputs aircraft outline and performance limits and tries to guess the relevant parameters from there. Internally it is like JSBSim a coefficient scheme, but the coefficient tables aren't actually based on performance tables but guessed by the YaSim solver, so they're simpler and more generic. So the best it can do is to get the rough performance characteristics right, but it can't conceptually be as realistic as JSBSim for the simple reason that even if you have better data, YaSim doesn't allow you to use it. And you're kidding yourself if you think aircraft can be described accurately by specifying things like wingspan, cruise speed or stall speed. The reality is multidimensional tables of lift and drag as a function of AOA, airspeed, ...

The fact that no simulation can be perfectly realistic doesn't imply that there are still vast differences in the potential degree of realism possible. And JSBSim has the higher potential degree of realism.

Having said that, there's garbage in, garbage out, so if you give JSBSim the wrong tables, you get a bad FDM.
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Re: New historically realistic Sopwith Camel JSBSim FDM rele

Postby DFaber » Fri Jan 10, 2014 10:25 am

It seems that YaSim vs JSBSim has become a religious Issue within the community. I've chosen my side too, but I doubt that flame wars like these are of any use to anybody. After all YaSim and JSBSim have different aims and different concepts to get there.

JsBSim surely is the more "engineering" Approach, with it's strict mathematical, table based approach while YaSim is better suited to fine-tune the Pilot Experience. Ground Interaction never was a big theme in JSBSim, while YaSim gets a lot of appeal with it's sophisticated Ground/Water Effects. I chose YaSim for exactly that Reason. The Aircraft needs to fly and feel (at least visually) like a real one. That means I need to adjust lots of "emotional" Parameters to get specific Feedback from the FDM. This is what YaSim is good at.

Thorsten wrote in Fri Jan 10, 2014 7:57 am:YaSim does not input a natural set of aerodynamically relevant parameters, it inputs aircraft outline and performance limits and tries to guess the relevant parameters from there. Internally it is like JSBSim a coefficient scheme, but the coefficient tables aren't actually based on performance tables but guessed by the YaSim solver, so they're simpler and more generic.


This may be true, but looking at reality, how exact need these values to be? The Data is gathered in Wind Tunnels providing absolute laminar Airflow, a constant Pressure, Temperature and an absolute clean Quality of the Wings Surface. It needs to be this way to be comparable and it's nothing wrong with that.
But in reality the Airflow isn't laminar and the usual Aircraft Finish has Rivets, Bumps, Filler Caps and Doors on it. The Windtunnel Model has sometimes hundreds of small holes to measure the pressure distribution, which would limit the usability on a real Aircraft Wing. Finally no Aircraft flies like the other. Two Aircraft produced at the same day in the same factory can have totally different flight characteristics and performance figures. And using them makes it even worse. A bump on the wing, a bend aileron, a patch on the fuselage changes the way the Aircraft flies. This is even worse with fabric covered Aircraft, since the pressure in the wing itself changes the Profile form of the wing. Even in flight Aerodynamics alter by the amount of dead flys, birds or squirrels sticking on the wing. On Ground, the B-52s wingtip bends 2m downwards only by filling the tanks and in flight even more.

You see, there is a large dead zone between the Windtunnel Data and the real Aircraft, and I doubt that at the End JSBSim has a real edge over YaSim.

Anyway I don't want to speak in favour of JSBSim or YaSim, just say that it doesn't matter which FDM one chooses. If the Aircraft is enjoyable, the whole Community wins.

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Re: New historically realistic Sopwith Camel JSBSim FDM rele

Postby Thorsten » Fri Jan 10, 2014 11:11 am

This may be true, but looking at reality, how exact need these values to be? The Data is gathered in Wind Tunnels providing absolute laminar Airflow, a constant Pressure, Temperature and an absolute clean Quality of the Wings Surface. It needs to be this way to be comparable and it's nothing wrong with that.


Well, evidence would suggest that wind tunnel data is good enough for all practical purposes. After all, if it weren't, people would not be doing wind tunnel studies. And why would a wind tunnel not test turbulent airflow? I've seen plenty of images of a turbulent wake of objects in a wind tunnel.

But in reality the Airflow isn't laminar and the usual Aircraft Finish has Rivets, Bumps, Filler Caps and Doors on it.(...) And using them makes it even worse. A bump on the wing, a bend aileron, a patch on the fuselage changes the way the Aircraft flies. This is even worse with fabric covered Aircraft, since the pressure in the wing itself changes the Profile form of the wing.


That's not a limitation of JSBSim though. If you like, you can run a computational fluid dynamics code on very detailed models of your airfoils for few thousand hours and use this for input. As long as the pressure changes the wing profile in a reversible manner (i.e. doesn't damage the plane), the deformation of the profile can just be absorbed into the coefficient table.

You can even change the coefficient tables runtime to simulate damage of the aircraft. If you look at e.g. the way the Vostok-1 is done, the flight dynamics changes completely from the full launch vehicle to the final entry capsule.

But, I mean, conceptually the fact that JSBSim can't ever get fully realistic doesn't say that it can't still be much better than YaSim.

You see, there is a large dead zone between the Windtunnel Data and the real Aircraft, and I doubt that at the End JSBSim has a real edge over YaSim.


I think conceptually the question is clearly settled - JSBSim can be almost as realistic as you can make your tables, YaSim can not.

In practice, I agree that the answer is less clear as in many cases we do not have an arbitrarily large body of data to use. So the following is my personal experience:

I didn't initially pay attention to what FDM underlies a particular plane, and at some point looked all the planes where I really have a realistic feel to the flight dynamics vs. where I do not. I found that 90% of what I fly were JSBSim planes. Compared to my real flight experience, JSBSim delivers something which YaSim does not - for me, YaSim planes often have a generic feel. Aircraft developers who fly the real thing also tend to gravitate to JSBSim to get closest to their experience - TorstenD's Seneca II springs to mind, I recall Patrice Poly's dissatisfaction with YaSim getting realistic glider performance at high bank angles just as well., Vitos immediately went to JSBSim after making a YaSim FDM for the MiG-15b and was dissatisfied with the inability to tune it properly.

It's difficult to really nail this down in a fool-proof way, as we all have limited experiences in the real thing, and we would need a test case of optimally tuned YaSim vs. JSBSim for the same plane to really see where this gets us as compared to published performance data. For the P-51d, the JSBSim version wins flat out, but I don't know how well tuned the YaSim version is and what more would be possible. YaSim seems to have unconditionally trouble in the transonic and supersonic regime (wave drag missing?), with rocket engines and I have yet to see mean stall behaviour in a YaSim plane, so I will put these out as fairly generic.
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Re: New historically realistic Sopwith Camel JSBSim FDM rele

Postby Thorsten » Fri Jan 10, 2014 11:57 am

After some thinking - the 'dipsy' maneuver of the SR-71 Blackbird is something which I find a rather impressive bit of JSBSim modelling. I've looked into the JSBSim version of the plane a while ago, and I could not get it supersonic without the short dive phase. And this is based on the subtleties of wavedrag and transonic vs. supersonic regime.

Another example is the way the aileron effectiveness lowers and reverses with Mach number in the MiG-15bis.

So what I would like to see is anything comparable for YaSim - some maneuver which is necessary / effect which is visble both in the real world and in FG which relies on some rather subtle property of aerodynamics.

I may be wrong, but I have the impression that this could not be implemented in YaSim.
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Re: New historically realistic Sopwith Camel JSBSim FDM rele

Postby DFaber » Fri Jan 10, 2014 1:45 pm

Thorsten wrote in Fri Jan 10, 2014 11:11 am:
But, I mean, conceptually the fact that JSBSim can't ever get fully realistic doesn't say that it can't still be much better than YaSim.

I guess "better" or "worse" in this case boils down to personal preference. You have a strong appeal to the scientific nature of JSBSim (nothing wrong with that), I prefer the "Feel" of YaSim.

Thorsten wrote in Fri Jan 10, 2014 11:11 am:That's not a limitation of JSBSim though. If you like, you can run a computational fluid dynamics code on very detailed models of your airfoils for few thousand hours and use this for input. As long as the pressure changes the wing profile in a reversible manner (i.e. doesn't damage the plane), the deformation of the profile can just be absorbed into the coefficient table.


That is true, but this data is not available for the Aircraft I am interested in, and if I had the funds to gather them I'd rather invest it in a real Aircraft and lots of Fuel :-)

Thorsten wrote in Fri Jan 10, 2014 11:11 am:I think conceptually the question is clearly settled - JSBSim can be almost as realistic as you can make your tables, YaSim can not.


... but if you don't have this data, you have to guess.

Thorsten wrote in Fri Jan 10, 2014 11:11 am:Aircraft developers who fly the real thing also tend to gravitate to JSBSim to get closest to their experience - TorstenD's Seneca II springs to mind, ...


The Seneca is fantastic, that's true, but in the line of Real Pilots with real Planes, you need to add David Meggison, the Author of the pa28 and Cub (both wonderful Birds), Dave Perry Pa 24-250, both YaSim.

Thorsten wrote in Fri Jan 10, 2014 11:11 am:It's difficult to really nail this down in a fool-proof way, as we all have limited experiences in the real thing, and we would need a test case of optimally tuned YaSim vs. JSBSim for the same plane to really see where this gets us as compared to published performance data. For the P-51d, the JSBSim version wins flat out, but I don't know how well tuned the YaSim version is and what more would be possible.


I wouldn't say it wins hands down, especially in Landing configuration I miss the good glide angle the Mustang was famed for.
I have worked on the YaSim p51d FDM Years ago, together with Jim Wilson, but stopped when Hal took over. It does nearly hit the Performance Numbers specified (real life is always slower), but the Handling could be done better.

Thorsten wrote in Fri Jan 10, 2014 11:11 am:Vitos immediately went to JSBSim after making a YaSim FDM for the MiG-15b


The original MiG-15 Yasim FDM was made by Lee Elliott. I doubt that any attempt was made to improve it.

Thorsten wrote in Fri Jan 10, 2014 11:11 am: YaSim seems to have unconditionally trouble in the transonic and supersonic regime (wave drag missing?),


It isn't modelled. This is up to the developer to add something. The transsonic area is not well with JSBSim too, I have yet to see any FG Aircraft with transsonic shudder up to the point of breaking the Airframe.

Thorsten wrote in Fri Jan 10, 2014 11:11 am: I have yet to see mean stall behaviour in a YaSim plane, so I will put these out as fairly generic.


Stall Behaviour was not very priorized with some Developers, which doesn't mean it cannot be tuned. A rather unpleasant Stall can be found in the fw190 which tends to snap roll in tight turns.

Performance numbers is not everything. There is more about flying a plane than hitting numbers. It's that hard to describe sense of Control Harmony, as Eric Brown puts it (Eric Brown holds the world record for flying the greatest number of different aircraft, 487, sub-types not counted).

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Re: New historically realistic Sopwith Camel JSBSim FDM rele

Postby DFaber » Fri Jan 10, 2014 2:05 pm

Thorsten wrote in Fri Jan 10, 2014 11:57 am:So what I would like to see is anything comparable for YaSim - some maneuver which is necessary / effect which is visble both in the real world and in FG which relies on some rather subtle property of aerodynamics.

That is not seldom, even without modelling it. I always wondered when I made the F4U FDM, why it always tended to stomp one Mainwheel onto the Deck, even with the exactest approach and while having a gentle stall. Back then I wondered wether it needed to be corrected, but later I read in the Manual that Pilots are warned exactly about this.
The F-86 was known for it's high angle of attack during takeoff. The YaSim FDM does too. It's Airbrakes induce a nosedrop like the original and the engine has an enormous spooltime. You have to approach at 80% RPM to retain enough power in case of an Emergency.

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Re: New historically realistic Sopwith Camel JSBSim FDM rele

Postby Thorsten » Fri Jan 10, 2014 2:14 pm

I guess "better" or "worse" in this case boils down to personal preference. You have a strong appeal to the scientific nature of JSBSim (nothing wrong with that), I prefer the "Feel" of YaSim.


Well, performance data is testable and an objective measure. An FDM which does not meet published performance figures is in all likelihood wrong.

The Seneca is fantastic, that's true, but in the line of Real Pilots with real Planes, you need to add David Meggison, the Author of the pa28 and Cub (both wonderful Birds), Dave Perry Pa 24-250, both YaSim.


It so happens that I've tested both the Seneca and the Pa 24-250 for the same thing - high altitude flying in mountain ranges, careful mixture management, shallow climbs, just exploring technically how high I can get them (I've tried to get to the summit of McKinley/Alaska). The Seneca's performance deteriorates with altitude as expected and I barely eased it over the summit, the Comanche will drop EGT to the bottom but the engine performance stays good and you can't actually choke the engine. So I have a clear opinion here what's more realistic, the Seneca wins this flat out.

The original MiG-15 Yasim FDM was made by Lee Elliott. I doubt that any attempt was made to improve it.


Vitos actually made a YaSIm model first (I still have it somewhere) and then turned to JSBSim because he couldn't get it to do what he wanted from it.


I in general agree that there's more than performance benchmarks - but here it's very subjective. To me JSBSim feels in general better, to you apparently not. I guess neither of us has ever flown an F-16, so I wouldn't really know how it should feel properly. So we might discuss 'feel' of an aircraft in terms of our personal enjoyment, but it's difficult to argue the degree of realism here.

I think we're in agreement than when we don't have good data tables, JSBSim buys you nothing. In that situation, YaSim is likely to result in the better FDM as you adjust properties in a more straightforward way.

I don't know - I expect to see some effects on physical grounds, and if they're not there I miss something. Like in the JSBSim P-51D, when I spin up the engine, I have to counter the torque with ailerons.

I know some effects to be there in reality, and when they're not there I miss something. Like the dipsy maneuver for the SR-71 - the JSBSim version can largely be flown by the real manual, the YaSim version can not.

And I know some things just can't be true in reality - like the IAR-80 YaSim with flaps and gear out and engine off had a glide ratio exceeding 1:70 - such incredible gliders too often come out of the YaSim solver which concludes that the given speeds can only be reached with a fantastic lift/drag ratio.

Hm, I guess if we want to continue the discussion, it'd really be better to focus on actual examples and side by side comparison of a test case.
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Re: New historically realistic Sopwith Camel JSBSim FDM rele

Postby helijah » Fri Jan 10, 2014 2:47 pm

It's distressing. I have absolutly no criticism JSBSim but only people who cricize witjout knowing YASim and now the war resumed !

I've always written that JSBSim was a fantastic job. But as Detlef points out, it's sorely lacking in "feeling" in piloting airplanes. To want the perfect values is a mistake.

JSBSim is absolutely perfect for creating games. It simulates the flight of aircraft perfectly. But only the flight. This is not a simulation engine but a flying engine. And FG is a global simulator, not just for airplanes in the sky.

JSBSim is absolutely perfect for creating games. It simulates the flight of aircraft perfectly. But only the flight. This is not a simulation engine but a motor of aiplanes in flying. This is the basic understanding that is bad in JSBSim for FG.

This does not affect the great work done on JSBSim.

JSBSim can, with work, generate FDM almost perfect.
YASim can, with work, generate FDM almost perfect.

Just the plane JSBSim be sad. the same in YASim be fun. Just the plane JSBSim be sad. the same in YASim be fun. And I'm not talking about bounce the aircraft on the ground or water as hard as concrete with JSBSim (totally unrealistic and unacceptable in a simulator).

Regards Emmanuel

P.S. And not to be too rude, I speak not of the quality of code that generates the warning line for each line of compilation or disrespect of properties FG (JSBSim is external, it is for him to adapt to FG and not vice versa).
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Re: New historically realistic Sopwith Camel JSBSim FDM rele

Postby adrian » Fri Jan 10, 2014 2:52 pm

Thorsten wrote in Fri Jan 10, 2014 2:14 pm:
And I know some things just can't be true in reality - like the IAR-80 YaSim with flaps and gear out and engine off had a glide ratio exceeding 1:70 - such incredible gliders too often come out of the YaSim solver which concludes that the given speeds can only be reached with a fantastic lift/drag ratio.


That's actually a math bug in the way drag is calculated by Yasim. I discovered it when I was testing the IAR with Emilian. I attempted to solve it, but gave up because Andy was not available, and nobody else is maintaining the code. Then there's also the gear friction bug, same problem.
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Re: New historically realistic Sopwith Camel JSBSim FDM rele

Postby Thorsten » Fri Jan 10, 2014 3:52 pm

I've always written that JSBSim was a fantastic job. But as Detlef points out, it's sorely lacking in "feeling" in piloting airplanes. To want the perfect values is a mistake.


With all due respect, I think you simply have no experience to tell you how the 200+ aircraft you've modeled should be feeling. I seriously doubt you've flown all of them.

Feeling is a purely subjective category, we usually do not have references to check against. And of course for a simulation to miss the real value is a mistake - a gross one at that.


JSBSim can, with work, generate FDM almost perfect.
YASim can, with work, generate FDM almost perfect.


Did you read any of what I wrote? YaSim conceptually can't do that - you simply can't enter enough parameters. YaSim, with work, can get you really decent and enjoyable planes. It can't give you realistic behaviour across the full envelope, since it doesn't allow you enough input data. You can't argue with the math of the problem, and especially you can't invoke a category like 'feeling' where a physically realistic solution to equations of motion is concerned. Aerodynamics is primarily a physics problem. Whether an FDM is close to reality or not is a measurable question, not a matter of taste.

There's no shame in acknowledging that YaSIm can't do certain things. But to claim that it has to potential to be as realistic as JSBSim is demonstrably wrong. If you think otherwise, show me how to derive realistic equations of motion from the YaSim input parameters. Or if you can't do the math, don't argue math problems like fidelity of a simulation with me.
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Re: New historically realistic Sopwith Camel JSBSim FDM rele

Postby KL-666 » Fri Jan 10, 2014 4:50 pm

As i understood it, yasim can be considered a windtunnel itself. It should calculate the windtunnel results based on the form of the aircraft. If that is the case you would not need any windtunnel data. Just in case of imprecision in yasim you would have to testfly and tweak a little. Hence the less options to put in compared to jsbsim.

In jsbsim you must put in all the windtunnel results yourself. Also here there may be imprecision in the engine, so you still have to testfly and tweak a little. It is of no use to say: "I put all the numbers in correct, so the plane is perfect", while if someone tries to take of with it, he needs a 10 mi runway. I think this is part of the "feel" that was talked about.

Theoretically you should be able to make a perfect plane in jsbsim. But practically i notice not many people put in many numbers. Certain things that should happen, like decrease in climb potential at great altitude are often forgotten. A commercial plane really does not climb 3000 ft/min above fl 300. Or if you pull flaps, something should happen to how the plane behaves in the air, not nothing or the opposite of well known change in behaviour. Here is the "feel" again.

It is true that one can not know to what extent something should happen. But one can know that something should happen and in what direction. In that sense i think it is legitimate to test planes by "feel".

All in all i personally came to the conclusion that i prefer for high flying planes yasim, so i get at least some modelling over the full envelope. I noticed that in light low flying planes some developers tend to put in enough parameters in jsbsim, that they are actually better then their yasim counterparts.

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Re: JSBSim or YASim?

Postby Thorsten » Fri Jan 10, 2014 6:09 pm

As i understood it, yasim can be considered a windtunnel itself. It should calculate the windtunnel results based on the form of the aircraft.


No. Really not. If you do something like a realistic finite element fluid dynamics calculation, this runs for hours minimum, open ended. Which is why that part is factored out in JSBSim - it makes no sense to try to do that in real time. YaSim guesses the results based on the form of the aircraft and supplemental parameters. That's rather different from actually calculating them.

Here you can read on actual realistic computational fluid dynamics for JSBSim. To quote:

In the case of the parachutist simulation: the amount of computation time required to generate 2 scenarios (with a back pack and without) was approximately 25,000 cpu-hours — or around 2 years of compute time on a single processor PC.

That's very different from what YaSim does.
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Re: JSBSim or YASim?

Postby helijah » Fri Jan 10, 2014 9:08 pm

I'm tired of being confronted with people who do not want heard and read. So I leave here.

Most users prefer it for fun YASim furniture provided. If the authors of JSBSim pretend to better understand and better know, much the better for them. The fact is that currently, YASim remains a great value and JSBSim an error FG. I am saying and maintenance. JSBSim remains a fantastic assembly of any (almost) work what has been done for 30 years. It represents the past of the simulation is (for me) has nothing to do in FG (except see the planes rebound on the ground or water.).

I will continue to create models and forget once and for all JSBSim. Since the authors are so blind I will use the same attitude now.
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Re: JSBSim or YASim?

Postby Bomber » Fri Jan 10, 2014 10:06 pm

Helijah... I think you need to grow a pair.

Personaly I think the biggest idiots here are those that divided the development with 2 flight models....

And also there's not a single person here that can say there FDM is correct or feels correct as they offer no testing data... And saying it hits all the correct values is simply bollocks, unless you provide documented evidence..

A point in case are any jsbsim models including the P51D as there's no documented proof that the jsbsim piston engine works correctly with changing alt.... So how can it hit the numbers ?
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