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Can I set any JSBsim FDM property with a FDM function ?

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

Can I set any JSBsim FDM property with a FDM function ?

Postby TheEagle » Thu Oct 14, 2021 1:25 am

I recently saw a function that determines milthrust based on altitude and other props in a turbine engine. Does that mean that I can control any FDM property with a function ? Does such a function have to be in a special place depending on the context (engine properties in the engine file, propeller properties in the propeller file, gear properties in the main FDM file, etc.) or can I put them wherever I want ?
Last edited by TheEagle on Thu Oct 14, 2021 5:49 pm, edited 1 time in total.
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Re: Can I add a thrust / RPM function to JSBsim piston engin

Postby Octal450 » Thu Oct 14, 2021 3:45 am

That is not a good way to fix the problem. Its better to fix the tables in the engine so that they behave correct.

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Josh
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Re: Can I add a thrust / RPM function to JSBsim piston engin

Postby S&J » Thu Oct 14, 2021 6:58 am

Complete the static engine test at various simulated altitudes and you'd understand more.
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Re: Can I add a thrust / RPM function to JSBsim piston engin

Postby TheEagle » Thu Oct 14, 2021 12:51 pm

Octal450 wrote in Thu Oct 14, 2021 3:45 am:That is not a good way to fix the problem. Its better to fix the tables in the engine so that they behave correct.

Kind Regards,
Josh

I know, I just wanted to know if it is possible.
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Re: Can I add a thrust / RPM function to JSBsim piston engin

Postby S&J » Thu Oct 14, 2021 3:14 pm

Think of it as being like a Lilo...

You press down on a problem in one area and a new problem pops up in another.

You end up forever solving and creating problems with seemingly no end to it.
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Re: Can I add a thrust / RPM function to JSBsim piston engin

Postby TheEagle » Thu Oct 14, 2021 3:37 pm

S&J wrote in Thu Oct 14, 2021 3:14 pm:Think of it as being like a Lilo...

You press down on a problem in one area and a new problem pops up in another.

You end up forever solving and creating problems with seemingly no end to it.

Hmm ? :? I don't get the point …
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Re: Can I add a thrust / RPM function to JSBsim piston engin

Postby benih » Thu Oct 14, 2021 4:36 pm

I would also like to know. That might be a better Doktrin for my roughness implementation at the spark plug fouling code i just completed.
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Re: Can I add a thrust / RPM function to JSBsim piston engin

Postby S&J » Thu Oct 14, 2021 5:26 pm

You have both a faulty engine and faulty prop both of which I could fix in under two hours.

I've spent considerably more time trying to show and explain to you and anyone else interested in being able to resolve these issues themselves.

Kinda like teach a man to fish.

Solving one problem that results in another that then needs resolving which creates another is just not my idea of a hobby.
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Re: Can I add a thrust / RPM function to JSBsim piston engin

Postby TheEagle » Thu Oct 14, 2021 5:46 pm

S&J wrote in Thu Oct 14, 2021 5:26 pm:You have both a faulty engine and faulty prop both of which I could fix in under two hours.

I've spent considerably more time trying to show and explain to you and anyone else interested in being able to resolve these issues themselves.

Kinda like teach a man to fish.

Solving one problem that results in another that then needs resolving which creates another is just not my idea of a hobby.

I understand you do not want to waste your time - I wouln't want to either. Also, I do not want to make you waste your time. I understand my engine and prop are faulty and that I have to do the static tests. I think I am going to edit this question.
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Re: Can I set any JSBsim FDM property with a FDM function ?

Postby Thorsten » Fri Oct 15, 2021 4:56 am

Well, basically any simulation of a piston or jet engine inside JSBSim is a function - it has input parameters, output parameters and there's a unique relation between them.

Also, JSBSIm being JSBSim, you're not constrained to use pre-packaged functions, you can replace them by your own.

The question then is - do you have the knowledge to come up with a better function for something than the pre-defined JSBSim does? If you have a degree in engineering and work with, say, jet engines, changes are that you are indeed able to code an engine simulation that is better than what comes from setting parameters in the JSBSim one. If, on the other hand, you have a table of numbers of engine performance from somewhere, don't really know what they mean in detail - then you're not likely to end up creating something better.

Generally, the JSBSim pre-defined stuff is quite good, and unless you can specifically and in detail discuss a problem you see with it, you're not in a good position to replace it.
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Re: Can I set any JSBsim FDM property with a FDM function ?

Postby benih » Fri Oct 15, 2021 6:20 am

Hello Thorsten,
on the mailing list I asked a question maybe related to this one here: https://sourceforge.net/p/flightgear/ma ... sg37364830

Hi there, i currently work on the spark plug fouling simulation for the c182s and for the effects i need a way to reduce the power at runtime, ideally with an rpm drop (to simulate rough engine running i want to also oscillate that later).

I definitely lack insights into jsbsims internals, and I don't want to mess around with it's functions; yet i need some way to alter the power output without a change in input MP or fuel flow.
Can you maybe help out here?
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Re: Can I set any JSBsim FDM property with a FDM function ?

Postby Thorsten » Fri Oct 15, 2021 9:06 am

I've never done anything other than a rocket engine in JSBSim, but can't you simply take the output result of the JSBSIm engine simulation and post-process it with a filter that reduces power/RPM/whatever in the way you need?
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Re: Can I set any JSBsim FDM property with a FDM function ?

Postby benih » Fri Oct 15, 2021 9:58 am

Thorsten wrote in Fri Oct 15, 2021 9:06 am:but can't you simply take the output result of the JSBSIm engine simulation and post-process it with a filter that reduces power/RPM/whatever in the way you need?


Probably - but I don't know the property in question and playing around with the props available (for hours!) did not yield much so far.
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Re: Can I set any JSBsim FDM property with a FDM function ?

Postby S&J » Fri Oct 15, 2021 11:50 am

BSFC
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Re: Can I set any JSBsim FDM property with a FDM function ?

Postby benih » Fri Oct 15, 2021 1:17 pm

Thanks will try it.
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