Implementing VNAV support in FlightGear: Difference between revisions

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Implementing VNAV support in FlightGear (view source)

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== Problem ==
In comparison with other flight simulators, FlightGear has been lacking support for simulating VNAV features found on many modern aircraft.
So far, the general consensus has been that it simply isn't yet possible in FlightGear to properly implement VNAV because VNAV depends on having access to a performance database to provide the proper pitch/thrust changes to make certain altitude/speed constraints,  which boils down to a lack of support by the FDMs (JSBsim/YaSim) actually.
As an open source project, we do not have access to the corresponding  flight data or performance database, and even if we did, we probably couldn't easily use said data due to its proprietary nature. In addition, a real performance database would probably not serve us too well, because our FDM configurationos are custom, too - thus, a performance DB would ideally be provided by the FDM itself. Either in a pre-created fashion, i.e. compiled during an offline "test fligth", or preferably at runtime by running a simulation of the FDM inside the actual FDM.
It is one of those "holy grails" in FlightGear, I am not aware of any aircraft that currently support a proper FMC/CDU "progress" page - creating the page is trivial, but making it functional requires some more work - look for threads with the keywords "performance database", and you'll see that this has been talked about for over half a decade meanwhile. (computing remaining fuel vs. fuel consumption per hour is fairly trivial, but once you want to compute range, you need to be fully aware of the flight plan (speeds, altitudes) and weather/configuration (and aircraft/engine performance in particular) to make a reasonable guess at remaining range).
So, we currently do not have any aircraft with a properly working FMS and performance database support.
Overall, airliners like the 777 in FlightGear remain among the most developed aircraft/airliners in FG, especially in comparison to hundreds of other aircraft.
But compared to airliners supported by FSX or X-Plane, we are still lacking in that department and do not provide a complete solution (yet).
And it is also not foreseeable when that will change, because it will require changes in the FDMs we're using or use of some 3rd party/proprietary performance database.
Otherwise, it is not currently feasible to come up with VNAV/LNAV modes that properly work for different aircraft.
The 3rd party approach is being used for TPPs data, but in the case of FDMs, we would need 100% accurate FDMs, too - so it would be better to directly use the FDM for computing a performance DB.
Supporting VNAV/LNAV still is tricky for other reasons, and currently not on anybody's agenda - it's not just something that can/should be tackled by a single aircraft developer, it requires lower-level changes in FlightGear's architecture and the way FDMs are used.


== Objective ==
== Objective ==
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|author=Jon S. Berndt |date= Wed Nov 11, 2009 8:48 pm}}</ref>|Jon S. Berndt}}
|author=Jon S. Berndt |date= Wed Nov 11, 2009 8:48 pm}}</ref>|Jon S. Berndt}}


== Problem ==
It is the single most important reason why this isn't yet supported by FlightGear, which boils down to a lack of support by the FDMs (JSBsim/YaSim) actually.
So far, the general consensus has been that it simply isn't yet possible in FlightGear to properly implement VNAV.
It is one of those "holy grails" in FlightGear, I am not aware of any aircraft that currently support a proper FMC/CDU "progress" page - creating the page is trivial, but making it functional requires some more work - look for threads with the keywords "performance database", and you'll see that this has been talked about for over half a decade meanwhile. (computing remaining fuel vs. fuel consumption per hour is fairly trivial, but once you want to compute range, you need to be fully aware of the flight plan (speeds, altitudes) and weather/configuration (and aircraft/engine performance in particular) to make a reasonable guess at remaining range).
So, we currently do not have any aircraft with a properly working FMS and performance database support.
Overall, airliners like the 777 in FlightGear remain among the most developed aircraft/airliners in FG, especially in comparison to hundreds of other aircraft.
But compared to airliners supported by FSX or X-Plane, we are still lacking in that department and do not provide a complete solution (yet).


And it is also not foreseeable when that will change, because it will require changes in the FDMs we're using or use of some 3rd party/proprietary performance database.
Otherwise, it is not currently feasible to come up with VNAV/LNAV modes that properly work for different aircraft.
The 3rd party approach is being used for TPPs data, but in the case of FDMs, we would need 100% accurate FDMs, too - so it would be better to directly use the FDM for computing a performance DB.
Supporting VNAV/LNAV still is tricky for other reasons, and currently not on anybody's agenda - it's not just something that can/should be tackled by a single aircraft developer, it requires lower-level changes in FlightGear's architecture and the way FDMs are used.


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