I think, a modeller must have first the accurate manufacturer's data (from Boeing company, airbus company etc.). Besides the dimensions, he must know the exact MAC length and the exact position of its leading edge so that he can put the center of his model at the right position (that is MAC/4).

No he mustn't. He can simply put the model's centre where it looks right. That's good enough.Microsoft doesn't mention the MAC. The FSX SDK defines the reference_datum-position as "Offset (in feet) of the aircraft's reference datum from the standard Flight Simulator center point, which is on the centerline chord aft of the leading edge. " The FS2004 SDK defines it as "Offset (in feet) of the aircraft's reference datum from the standard Flight Simulator center point, which is on the centerline ¼ chord aft of the leading edge." Note that Microsoft doesn't specify the centerline chord, only the wing_root_chord as "Length of the wing chord (leading edge to trailing edge) at the intersection of the wing and the fuselage (feet)" in both FS2004 and FSX.

No he mustn't. He can simply put the model's centre where it looks right. That's good enough.Microsoft doesn't mention the MAC. The FSX SDK defines the reference_datum-position as "Offset (in feet) of the aircraft's reference datum from the standard Flight Simulator center point, which is on the centerline chord aft of the leading edge. " The FS2004 SDK defines it as "Offset (in feet) of the aircraft's reference datum from the standard Flight Simulator center point, which is on the centerline ¼ chord aft of the leading edge." Note that Microsoft doesn't specify the centerline chord, only the wing_root_chord as "Length of the wing chord (leading edge to trailing edge) at the intersection of the wing and the fuselage (feet)" in both FS2004 and FSX.

Correct.. the only significance of where you place the model-origin (0,0,0 from within the modeling software .. not at all affected by config entries).. is to "center" the model for the spot view "camera".Now, I personally choose to use the model center AS the reference-datum.. It's easier when setting coordinates for everything: (lights, contact/scrape-points, CoG, wing-apex, control-surface apexes, weight-stations, engines, etc.)I could go back when everything is finished, and move the reference-datum out in front of the model.. and then adust all longitudinal coordinates accordingly, and make it LOOK like a default cfg.. but I don't bother.As for accurate, manufacturer data ? It's a great staring point. Placing the empty CoG where Cessna says it should be is good practice.. and placing weight stations where Cessna says they are (per weight-n-balance arms), is a no-brainer. If your model is accurate, you can place the mouse (from within Gmax) where a passenger's butt will sit, and you'll find that it's nearly identical to the weight-n-balace sheet... same for fuel tanks, and engine thrust-points... However, the further you get, size/weight/engine/performance, from a C172, the more you'll have to tweak all this data, in order to get a functioning flight model. Most significant would be stabilzer and control-surface location, area and deflection in degrees. You might have to tell MSFS that the rudder is a few feet further back than the visual model indicates.. stuff like that.. and even customize your CoG, wing-apex, weight-stations a tad, to round things out. Last resort is to tweeak surface-areas, and deflections, and even tweak the 'Flight_Tuning' entries.

Now, I personally choose to use the model center AS the reference-datum.

So do I. After all where is the 25% MAC point for a biplane with staggered unequal size wings?

As far as I know , for MSFS the model visual centre is the 1/4 MAC of aircraft wing. This is how MSFS works, and this means that the modellers have to stick to this condition.Unfortunately, not all modellers care about the poisition of the centre of the models they built.

Ideally, the modeler will place the origin (0,0,0) at the 1/4 MAC location. But, as you've noted, they may not care or take care to be accurate. :(

Ideally, the modeler will place the origin (0,0,0) at the 1/4 MAC location. But, as you've noted, they may not care or take care to be accurate. :(

But still... the model origin plays no part in the flight-model.. and for something like one of those stretched DC-9s, where well over half the fuselage is in front the the wings.. a model origin a bit forward of the wings might be better for viewing the model in spot-view.

You guys are way too technical for mr, but I have a major problem that some of you may be able to help me with.All of a sudden my elevator trim has developed a mind of its own, making it impossible for me to maintain level flight without auto pilot. I disconnected all peripherals except for CH yoke and it still happens. to varying degee with different aircraftHELP, please.ThanksJay

Ideally, the modeler will place the origin (0,0,0) at the 1/4 MAC location. But, as you've noted, they may not care or take care to be accurate. :(

But let us talk now about the reference datum which is a reference point taken with respect to the MSFS conventional centre (MAC/4). Suppose I duplicate an aircraft that I call A. For aircraft A#1, I keep the same ref. datum ( let us say (0,0,0)) and for aircraft A#2, I change the ref. datum ( let us say (-10,0,0)) and adjust all the aircraft.cfg with respect to this new reference. Do you think that flying these 2 aircrafts (that are different only by the ref. datum location) will show a difference? Do you think that we have the freedom to place the ref. datum at (0,0,0) or we can make this choice only in the case where the visual centre of model is exactly at 1/4 MAC?

You guys are way too technical for mr, but I have a major problem that some of you may be able to help me with.All of a sudden my elevator trim has developed a mind of its own, making it impossible for me to maintain level flight without auto pilot. I disconnected all peripherals except for CH yoke and it still happens. to varying degee with different aircraftHELP, please.ThanksJay

When the autopilot is ON, does the trim modulate ALOT (as opposed to normal, small adjustments) to maintain level flight ?

Where does Microsoft state the visual model centre point is 25% of the mean aerodynamic chord?

Where does Microsoft state the visual model centre point is 25% of the mean aerodynamic chord?

This is what MSFS usually states :"reference_datum_position = 0, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)". (0,0,0) seems to refer to the reference centre of visual model.Otherwise, how does MSFS hook the visual model?

This is what MSFS usually states :"reference_datum_position = 0, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)". (0,0,0) seems to refer to the reference centre of visual model.Otherwise, how does MSFS hook the visual model?

That doesn't answer the question which is: Where does Microsoft state the visual model centre point is 25% of the mean aerodynamic chord?

<br />That doesn't answer the question which is:  <i>Where does Microsoft state the visual model centre point is 25% of the <b>mean aerodynamic </b>chord?</i><br />

<br /><br /><br />Ok I see. MSFS states 1/4 chord. But think of a rectangular wing : mean aerodynamic chord length = chord length. For a swept wing, the equivalent rectangular wing is located behind the leading edge of the swept wing( or if you want, behind the wing apex) by a distance called the leading edge distance of MAC. Try the Wrench utility to confirm that.

Unless you're building a model; this is all a non-issue. You, as a user, can't do a thing about where the model origin is set.

This is what MSFS usually states :"reference_datum_position = 0, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)". (0,0,0) seems to refer to the reference centre of visual model.Otherwise, how does MSFS hook the visual model?

I think there's confusion here.. The reference_datum is 0,0,0 , only if you state that in the cfg. The reference-datum can be set to: 11948,87.58,96633, so long as all cfg coordinates account for that. The reference_datum does not affect the model origin, in anw way.

I think there's confusion here.. The reference_datum is 0,0,0 , only if you state that in the cfg. The reference-datum can be set to: 11948,87.58,96633, so long as all cfg coordinates account for that. The reference_datum does not affect the model origin, in anw way.

Perhaps it would help everyone to visualize these relationships. The reference_datum is a child of the model origin. All other data point references are children of reference_datum, hence their absolute world coordinates are relative to their parent's absolute world position.

model origin, aka:FlightSim Reference position (0,0,0)   |...reference_datum (x,y,z)      |...all other data points (x,y,z)

In the aircraft.cfg file, the reference_datum can be co-incident with the model origin, but does not have to be!My apologies for perhaps attempting to teach religious doctrine to members of the temple singers... :(