Originally Posted by
K5083
If you have been converting planes with Legacy Importer, you may have noticed that your CG limits graph on the right-hand side of the w&b screen is all out of whack. The calculation of the CG as a % of mean aerodynamic chord (MAC) may be fine, but the fore and aft limits are set to crazy values so the graph is useless for knowing where the limits are. Here is the reason, and a fix.
In the aircraft.cfg of FSX models there are two optional lines in the [WEIGHT AND BALANCE] section, setting cg_forward_limit and cg_aft_limit. The values are expressed as a proportion of MAC, so cg_forward_limit = 0.15 means the forward CG limit is at 15% of MAC. For some reason, a lot of designers set these at excessively generous values of cg_forward_limit = 0.0 and cg_aft_limit = 1.0, so that the CG was allowed to be anywhere within the mean chord of the wing. These values don't effect aircraft performance at all, but such values made them pretty useless for setting your CG within limits.
In Prepar3d, the units of these values changed, and were now expressed in feet fore and aft of the airplane's arbitrary reference point (0,0,0), which the designer could place anywhere he wanted. It tended to be near the optimal CG or near the pilot's head, but didn't have to be. So a typical set of values might be cg_forward_limit = 2.5 and cg_aft_limit = -3.5. The forward limit was always a higher number than the aft limit since positive is toward the front. If you imported a plane that was set up for Prepar3d, this completely screws up MSFS because it wants the CG to be higher than the forward limit and lower than the aft limit, which is impossible. There is no CG position that will be within these limits, and the limit lines will be at absurd places on your cg limit graph. Since I convert most of my planes from P3D rather than FSX, I get this a lot.
MSFS, which has these lines in the flight_model.cfg file, reverts to the FSX proportion-of-MAC values for the parameters. So if you have converted an FSX model that had proper values for these statements, you might be fine. Otherwise, you should add statements with realistic values to your aircraft.cfg after you do your MSFS conversion. You may be able to look up what the values are. For example, I learned from the A2A Simulations forum that the limits for the P-51D are 21% to 31.5%, so your lines would be:
cg_forward_limit = 0.21
cg_aft_limit = - 0.315
That's an unusually tight range. If you can't find published figures for the aircraft in question, you could start with something like:
cg_forward_limit = 0.15
cg_aft_limit = 0.35
That will at least give you plausible limits around the sweet spot for most conventional aircraft, which is 25% MAC. And your CG limits graph will at least look sensible, and might even be of some use.
In fact, this can expose some real problems in the flight models of the planes we import. I found some payware airplanes where either the wing location (given by wing_apex_pos_lon in the [AIRPLANE GEOMETRY] section or the empty_weight_cg_position were located in physically impossible places and/or were in such a relationship that it was impossible to balance the plane with any loadout. I hesitate to "fix" these values because they affect the flight model and most likely the developer compensated for them through some artificial parameter when tuning the flight model that would now throw things off if the basic physics were corrected. However, I think all of these values would be overridden if the designer entered their equivalents directly in the .air file, in which case, changing them in aircraft.cfg would make little difference. But I have been cautiously adjusting some of the more egregiously wrong ones to see what effect they have.
Anyway, hope this helps some of you to at least get your weight and balance screens looking a little nicer.
August
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