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Talk:YASim

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With all due respect to Gary Neeley, "Contrast this with JSBSim which relies on pre-generated tabular data to build up the flight model." is not an accurate statement of JSBSim's functionality. JSBSim can use any formula the designer wants to use to specify flight, there is no constraint to pre-generate any tabular data at all. While the commonly used Aeromatic model doesn't do much in the way of physically modeling the aircraft, the framework is in place to generate models based on data similar to the data commonly used in YASim, at least aerodynamically. Jentron 21:44, 19 October 2012 (EDT)


Point acknowledged. The quote was from my old YASim guide from some years back when my understanding was weaker than I hope it is now. The quote was used without my knowledge, and though I don't object, I might have suggested that it is not the best appraisal of either YASim or JSBsim. I've taken the liberty of removing the relevant portion of the quoted text from the wiki.

The original statement, while not accurate, was actually meant to promote the flexibility and power of JSBsim rather than imply any limitations or reasons not to use JSBsim. I hope this addresses any possible misunderstanding as to the purpose of my original writing.

-Gary Neely

--Buckaroo 02:24, 26 March 2013 (UTC)

YaSim Maintenance

Cquote1.png There are some pending merge requests to add some Yasim features, but we have an issue that since none of the current C++ developers own, or are experts in Yasim, we're reluctant to be the person who merges such changes, and potentially introduces subtle regressions.

Obviously this is chicken-and-egg, since no one can become expert enough in the code to become a maintainer :)

So, I'm more than happy to apply patches *providing* I can be convinced they are sane+reasonable from a pure code perspective (happy to help with that, too, if people are new to C++), and providing we have some assurance that a representative sample of yasim aircraft are unchanged or improved by the patch. Suggestions for that means in practice, are most welcome!

Otherwise I worry, given the nature of the solver, we'll keep optimising the solver for some aircraft, and making other existing aircraft worse - until someone tests them, and announced that they're no longer working.[1]
— James Turner
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Cquote1.png I am still broadly happy to answer questions if posed (as long as I remember enough to come up with a meaningful answer). Just cc: me if you do, because my

latencies here are measured in weeks.

Bugs can always be fixed. What YASim needs is a maintainer, not really expertise per se. The latter comes from the former.[2]
— Andy Ross
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Cquote1.png Would it be a) possible, and b) a good idea, for YASim to use an 'external' engine model - i.e. the JSBSim engine model?

Please don't get me wrong in that I'm criticising the YASim engine model. The YASim solver makes it possible for me to make aircraft and I love it, but after trying to do a few YASim fdms I've come to the conclusion that there's a lot more to modelling the propulsion system than at first meets the eye, and that it could benefit from it's own subsystem.

It seems to me that a common engine model, that could be used by all of the FG solvers, would be a good idea.[3]
— Lee Elliott
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Cquote1.png It would take some work, but yes, this could definitely be possible.

The engine (and attached propeller) are used in the solver only as data sources. The engine parameters aren't "solved for", they're just computed. In the solver, YASim just asks the engine how much torque it is producing, the propeller how much thrust it is producing (and how much torque it is feeling in drag), and uses this information (along with the moment of inertia for the whole system) to "stabilize" the numbers to a steady state over a few iterations.

There's no reason a JSBSim engine/propeller model couldn't produce equivalently useful numbers. It just takes someone with the interest and time to do the code glue. Ditto for the engine bolted to the

helicopter rotor, etc...[4]
— Andy Ross
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Cquote1.png You are correct (IMHO) on both accounts:

1) Modeling a propulsion system with a high degree of fidelity (I am talking about advanced military turbines) is an art and science. I vaguely remember the effort that went into modeling the two turbojets (one by GE and one by P&W) that powered the F-16 when I worked on the F-16 trainer that Link built fifteen years ago. It was a very serious piece of engineering work, and the guys who modeled it truly impressed me.

2) A generic propulsion model would be a good idea.

With that said, though, I am not so sure how easily the JSBSim turbine model that David Culp wrote could be used outside of JSBSim - and JSBSim will always maintain the inherent ability to field its own propulsion system model (to allow standalone operation). I think it would be great if it could be used by YASim or any of the other FDMs, but due to our goals set out early in JSBSim development, the propulsion system is integrated within JSBSim based on some carefully thought out principles - one of which is that we needed to be able to run outside of FlightGear, in "batch mode", so to speak. I'm not sure if this would preclude running the SimTurbine model outside of JSBSim or not.

In any case, if someone wants to try it, I'll provide any insight I am abl to, and I'm sure if you ask nice David Culp will answer inquiries as well.[5]
— Jon S. Berndt
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  1. James Turner (Fri, 05 Oct 2012 03:54:43 -0700). YASim and documentation.
  2. Andy Ross (Fri, 05 Oct 2012 03:54:43 -0700). YASim and documentation.
  3. Lee Elliott (Mon, 20 Oct 2003 18:01:50 -0700). YASim helicopter "code review".
  4. Andy Ross (Mon, 20 Oct 2003 18:01:50 -0700). YASim helicopter "code review".
  5. Jon S. Berndt (Mon, 20 Oct 2003 18:01:50 -0700). YASim helicopter "code review".