Since I got my hands on MFS I couldn't resist "exploring" it's ".CFG" files, following the work of many modders contributing to make them more realistic / plausible according to data on their RW counterparts and trying to glimpse what's new in the "Modern" FDM...

One of the things stumped me when looking into some of the "flight_model.cfg" files, like those of the Extra 300 and the C172 was a value of "-1" set as fuselage_length ???

Indeed, if you open those files you'll find:

fuselage_length = -1

Setting this value to realistic real world data certainly makes a difference in the flight dynamics, and laying with any values away from it, in this and other parameters in that same file, like for instance "fuselage_width" does make a difference, and can actually be easily observed in the size of that orange cylinder that represents the fuselage in the Modern flight model and can be visualized, together with other shapes used for the wings, engine(s), landing gear(s), etc... when we enable "Dev Mode" in the sim.

But I really wonder why some aircraft have this "-1" for their length ?

Any ideas ?

It's fairly common (though certainly not "good programming practice") to use an invalid value in a parameter as a flag of some sort, generally an indication not to use that value.  My guess - and this is certainly nothing more - is that this disables certain calculations related to the fuselage length, possibly replacing them with stability derivatives from the legacy flight model.  May be a useful debugging tool, either left there by mistake or because the more complex calculations aren't giving the right result. 

Thx @lzamm ! 

Very plausible hypothesis !

11 hours ago, jcomm said:

Setting this value to realistic real world data

Silly question: What would that realistic data be? Meter, Feet, CM? I'd like to try the above but have no clue what to insert.

Best regards

Edited October 5, 20205 yr by ThomseN_inc
Found it! :P

3 minutes ago, ThomseN_inc said:

Silly question: What would that realistic data be? Meter, Feet, CM? I'd like to try the above but have no clue what to insert.

Best regards

It's in feet for the corresponding flight_model.cfg entry.

I've done some of my own poking and I've found that not all parameters are used by the flight model engine. This may be one of those (so someone at Asobo decided to set it to -1 since it was unused). It would not surprise me if unused parameters are there for: legacy reasons; future consideration; 3rd party use.

Well, I did play with it and it certainly made a diffference, as well as, for instance, fuselage_diameter and fuselage_center_pos...