There is a new parameter in the [AIRPLANE GEOMETRY] section of the flight_model.cfg file called wing_cg_refchord. It is described as the length of the wing reference chord in feet and is used only for mass and balance calculations. The default value is given as 0.
FS2020 airplanes ported over to FS2024 will not have this parameter, so the value will be zero. How can the wing reference chord length be zero? Or does a value of 0 actually mean that the value will be computed from the wing root chord and the geometry of the wing?
What is the use case for adding this parameter?
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Hello @AwarePlot117729
I’ll get this clarified, as from what I can see in the code, the default value when the parameter is not specified is not 0 but rather some mean chord computed by the sim.
Regarding the use case, I need to have that clarified as well but my guess would be that it’s used in the %MAC calculations shown in the EFB empty page.
Regards,
Sylvain
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Thank you Sylvain. To clarify, my question about use case meant the use case for specifying a different wing_cg_refchord than would be computed by the sim. I assume this would be for a planform different than rectangular or trapezoidal, for example, a delta wing or something?
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This parameter was introduced in FS2000. Originally, it was in the .air file under Rec1515. In ESP it was called OBSOLETE_AIR_70_WING_SPECS_2. In FS2004 and FSX the parameter could be added to aircraft.cfg as “wing_cg_ref_chord” and was overwriting the one in the .air-file.
Like Sylvain said, it is used to specify a different MAC length than the one calculated by the sim. It is just used for the CG% indication in SimVars for EFB etc. It has no effect on the flight behaviour.
Why is it needed? Normally, the designer adjusts wing_root_chord until the sim calculates the correct MAC length. But wing_root_chord cannot be smaller than (wing_area / wing_span) which is the MGC length (mean geometric chord). This can become an issue for some airplanes like the Boeing 747-200 where the real world MAC length is 27.33 ft., but MGC length is 28.11 ft. (= 5500 sq. ft. / 195.67 ft.).
In this case we would need to set wing_cg_refchord = 27.33 ft.
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I’m with you right up until your example of why it would be needed. The Boeing 747-200 wing is not rectangular, that is, the tip chord length is significantly shorter than the root chord length. There should not be a need to use a root chord length smaller than the mean geometric chord length to obtain an accurate mean aerodynamic chord length. So, shouldn’t you be able to determine a wing root chord length that would provide the correct MAC length without having to use the wing_cg_refchord length?
In F2000, I guess you could not have an MAC that is shorter than the MGC, rather than the wing_root_chord not being shorter than the MGC. Is that still true for MSFS 2020/2024, or is it, as you stated, that the wing_root_chord cannot be smaller than the MGC? If it is the former, then this parameter would be needed for nearly all tapered wings. If it is the latter, it should not be needed for normally tapered wings.
Well, in all MSFS versions the wing has a linear trapezoidal shape, defined bywing_area wing_span and wing_root_chord.The latter is actually measured at the fuselage centerline and indirectly defines tip chord, taper ratio and MAC.
But since the real-world wing shapes are more complex, we need to find a value for wing_root_chord that represents an equivalent trapezoidal wing. Not always easy. All MSFS versions calculate the MAC based on the three input parameters above. But the result may not be the same as the targeted officially published MAC which all CG calculations and indications are based on.
The reason is that the aircraft manufacturers have each their own way for defining wing area and MAC. Boeing has at least three methods and the ones applied on the classic 727, 737, 747 resulted in an official MAC that was lower than MGC. It is normally the other way around.
The MSFS MAC can never be smaller than MGC. That is just how the internal formula works.
Regarding the example of the 747-200 I was referring to FSX or older versions. Back then, the wing geometry did not matter so we could set wing_root_chord such that the offical MAC came out as result of the internal formula. wing_cg_refchord was only required if offical MAC was smaller than MGC (e.g. 747-200). For the 747 I set wing_cg_refchord to MGC to be as close as possible to the real-world MAC (the pitch moment coefficents Cm are based on wing area and MAC).
BTW wing_cg_refchord also works in MSFS2020.