[Flightgear-devel] FlightGear aircraft state support

[Wayne B. <kil...@co...>]

I've been working a bit with James on the state overly system. I have 
made some progress but I have created as many questions as I have answers.

One thing that seems apparent is the data that is saved to 
.fgfs/aircraft-data/c172p.xml, or Window's equivalent, by whatever 
means, is loaded after the data saved to the /sim/state/overlay tree. 
That still needs to be addressed as the data in 
.fgfs/aircraft-data/c172p.xml is loaded automatically. So to use the 
saved state right now I have to transfer the values in the 
/sim/state/overlay tree to the actual FDM or local aircraft properties 
after /sim/signals/fdm-initialized happens. I'm pretty sure this is not 
how it was intended to work. I think the overlay properties are meant to 
automatically set their corresponding local fdm and aircraft properties.

Also we have the sim presets to work with 
(http://wiki.flightgear.org/Preset_Properties), I am unclear how these 
are suppose to work with the state overlay system, or if they are? They 
can be set using a cmdline switch such as --vc=100 sets the 
/sim/presets/airspeed-kt which is used for the approach startup state. 
Again, I doubt this is meant to be used this way as it kind of conflicts 
with the --state=approach method of loading the /sim/state/overlay tree.

Then there is also the launcher which in this example of setting up a 
state called "approach" you have a GUI choice of "On final approach at 
distance of" which sets you up, paused at a predetermined distance and 
elevation, from the end of the runway. All other properties to achieve a 
stable glide slope to the runway threshold need to be initialized using 
either the state overlay method or the /sim/presets method.


By using a combination of these 3 methods I was able to get the c172p 
running, on approach to an airport. But I am pretty sure it is not how 
it is meant to be accomplished.

One of my first questions was going to be, is there a list of the 
cmdline flags which correspond to the available /sim/presets properties?

ie:
--vc=100 sets the /sim/presets/airspeed-kt property

That is the only one I know because James offered a guess as to what it was.

But thinking that the /sim/presets really shouldn't have anything to do 
with the state overlay method, this really shouldn't be of concern (even 
though I would still like to see or create a mapping).

A critical question for me moving forward then is, if you can't or 
shouldn't use the /sim/presets to set up the aircraft state because they 
require a cmdline flag to be set VS a /sim/state/overlay parameter set, 
what fdm/aircraft properties do we use to set things like airspeed or 
any of the properties that need to be set for certain conditions. I 
personally don't have a good grasp on what in the property tree is 
available, and not a tied or write protected property, to accomplish 
this. Where do I need to look to get this answer. This is all assuming 
we get past load() overwriting parameter set by /sim/state/overlay, 
keeping in mind we don't control when load() is called, only on what is 
written to .fgfs/aircraft-data/c172p.xm which we use for other things.

I suggest the possibility of leaving it up to the aircraft developers to 
set up whatever states they deemed appropriate and allow that to be 
passed to the launcher in the form of a drop down. We already have 
access to virtually everything we need to create and save a state on the 
aircraft/FDM side of things (see Space Shuttle for an example of that). 
Yes, that is done by the use of -set file entry points, but could just 
as easily be able to be accomplished by catching a, launcher set, state 
flag that can be caught at fdm-init and acted upon accordingly. That 
give you the most flexibility but I see it also might open up a can of 
worms.

I also don't have any issues with continuing down the current path of 
setting up some predefined standard states that all aircraft should 
adhere to. I'm unclear about how that is intended to be done?

I hope this all make sense. Please feel free to set me straight on 
anything I may have wrong or am not understanding. I want to continue to 
help with this effort but currently I am kind of stuck without some 
answers or a clear direction forward.

Thanks,

Wayne