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.

To be incredibly pedantic, there always is a reference_datum_position. If it's not set explicitly in aircraft.cfg it defaults to (0,0,0) For example, the position of the empty weight cg is defined as Offset (in feet) of the center of gravity of the basic empty aircraft (no fuel, passengers, or baggage) from the Reference Datum Position.

Of course.. ANY cfg value not typed into the CFG file, will be a default value.. but I've yet to see a cfg file where it isn't defined (even if it's 0,0,0)I was pointing out that the refernce_datum, is not some sort of MSFS constant.. it can be anything you define it to be.. (now I have to find a dictionary and go look up pedantic )Edit: And the only hooking, is the focal point for the "camera" ... That will be the model origin, regardless of what you put in the cfg file.The aerodynamic "hook" is a point derived from the empty CoG, and the variables of loading, and the center-of-lift, and if on the ground, contact points.. And the center-of-lift's vector itself, is a function of; airspeed, AoA, and flaps/spoilers deployment..(is that pedantic?) :(

I can't even produce a decent dutch roll with any of the stock or add-on swept wing boeing aircraft with the yaw damper off.

And the only hooking, is the focal point for the "camera" ... That will be the model origin, regardless of what you put in the cfg file.The aerodynamic "hook" is a point derived from the empty CoG, and the variables of loading, and the center-of-lift, and if on the ground, contact points.. And the center-of-lift's vector itself, is a function of; airspeed, AoA, and flaps/spoilers deployment..(is that pedantic?) :(

That would be incorrect. Model origin is also by default the center of lift. It gets 'adjusted' outside of that based on the other factors you've mentioned. I know this because it's possible to move the center of lift via an offset value in the .air file. That offset is based on model origin.

That would be incorrect. Model origin is also by default the center of lift. It gets 'adjusted' outside of that based on the other factors you've mentioned. I know this because it's possible to move the center of lift via an offset value in the .air file. That offset is based on model origin.

Umm, what did I say that was incorrect ? EVERYthing (except the focal point), just like the empty CoG, is an offset. The offset for the center-of-lift, starts with the wing apex.. and the strength of that vector varies by the factors I mentioned.Edit: Sans a cfg entry, a parameter will go to the default value.. and that value is looked for first in the air-file, and then MSFS itself.. The logic to keep in mind, is that for ANY value or offset in the air-file, it will be over-ridden by the cfg file (if the entry exists)... ie.. MSFS will default a CoG to model origin, and the air-file can offset it, so that THAT is now the default.. and then, no matter what's in the air-file, the cfg entry for CoG is used... and of course this applies the center-of-lift too (wing-apex).... and the only thing immune to this, is that for display, the MSFS "eye" always looks at the model origin, regardless on any air-file, or cfg entry.

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.

The mean aerodynamic chord of a swept wing is a purely geometric concept and its 1/4-chord point has no general relationship to the centre of lift.Thin wing theory puts the centre of lift of a symmetric 2-dimensional wing at its 1/4 chord point, ie there's no pitching moment about that point. This theory can be extended to a 3-dimensional wing, provided the 1/4-chord line isn't swept but not if the 1/4-chord line is swept. The reason is that lift is not distributed equally across the span of a 3-dimensional wing - it's high at the centre and falls to zero at the tips. This doesn't matter if the 1/4-chord line isn't swept. It does if the wing is swept because the lift then is relatively lower towards the tips which are further aft than the root. The effect is that the centre of lift is further foward than the1/4-chord point of MAC. The 1/4- chord af the MAC for the default FSX B747 is about 38 ft aft of the leading edge of the wing root chord - the MAC itself is about. 33 ft. The centre of lift allowing for a elliptical spanwise distribution of lift is about 31 ft aft of the leading edge of the wing root chord. That's a diference of some 7 ft.Also, which MAC should be used - that of the visual model or that of FSX FDE? They are not the same unless the visual model has a simple trapezoidal wing planform which is all that FSX FDE permits.In practice, it's good enough to put the visual model centre where it looks correct.

Correct. The reference datum is just a reference and thus can take any value. The only thing to keep in mind is that this reference is with respect to the visual centre of the model. Here are enclosed a picture representing top-down views of an aircraft model. A green light is used to show a (0,0,0) reference datum and a (-5,0,0) reference datum.Note that the red cross that we can see in a top-down view is the visual centre of the model.

Correct. The reference datum is just a reference and thus can take any value. The only thing to keep in mind is that this reference is with respect to the visual centre of the model. Here are enclosed a picture representing top-down views of an aircraft model. A green light is used to show a (0,0,0) reference datum and a (-5,0,0) reference datum.Note that the red cross that we can see in a top-down view is the visual centre of the model.

The less complicated way then to choose a reference datum is to choose it at the visual centre of the model.Now how to make the aircraft model described by the aircraft.cfg file match perfectly with the visual model?-1) The ground (contact points) and lights considerations: The coordinates of these points in aircaft.cfg must comply with their corresponding locations on the visual model ( then a reference datum at visual centre helps more).- 2) Empty cg, and stations location: same as before but with help of real world data from aircrafts manufacturers.- 3) Location of wing and tail: the wing dimensions must give the correct MAC length (real world) and the coordinates of the wing apex must put the MAC at the right location (again real world). As for the tail, only the correct surface areas mind, the locations of apexes have no effect.Note however, that playing with the wing apex has no effect on aircraft aerodynamics, only the cg gauge reading is affected. The wing apex is at the right place if for example the empty cg placed at the right place (as real world) reads the correct (again real world) % MAC on the cg gauge.Note also that the centre of lift is the point corresponding to the visual centre, unless the offset is different from zero in the airfile record 1534. But anyway, the centre of lift is not of a big interest in MSFS. Because the point where a force applies is a moment issue and thus it is already dealt with in the pitching moment coefficients and table 473.

But anyway, the centre of lift is not of a big interest in MSFS.

You were looking good up to this point. Incorrect assumption... as the center of lift is of huge concern.

You were looking good up to this point. Incorrect assumption... as the center of lift is of huge concern.

And try testiing a model for smooth, realistic rotation, adjusting nothing but the wing-apex, and see if it has "no effect".. (it does)

Correct. The reference datum is just a reference and thus can take any value. The only thing to keep in mind is that this reference is with respect to the visual centre of the model. Here are enclosed a picture representing top-down views of an aircraft model. A green light is used to show a (0,0,0) reference datum and a (-5,0,0) reference datum.Note that the red cross that we can see in a top-down view is the visual centre of the model.

I'm not sure what you're trying to convey here... ? There's no disputing that the reference_datum can be offset from the model origin? Are you trying to say that the "camera" moves with the reference datum ? If so, you're mistaken.

Are you trying to say that the "camera" moves with the reference datum ? If so, you're mistaken.

Indeed it does not, as can be clearly seen from this image. I've used a SimConnect utility to display the points indicated. The "visual center" is derived directly from the .mdl file, and is used to "lock" the camera position.All other points are relative to the "reference_datum" position, as configured in the aircraft.cfg file.Note: the "Forces" reference (yellow line) is a derived value generated by FS based on the other datum configured. When stationary, it's of course co-incident to the cog.

I think the datum is starting to get a bad rap :biggrin:After all it’s there because it’s a real world aircraft feature.Sure it’s redundant in a 3D model, but I think it adds a measure of authenticity.I appreciate the good reasons not to use it, but I’d still like using the datum as specified by the designer.Like the pic above ;)

- 2) Empty cg, and stations location: same as before but with help of real world data from aircrafts manufacturers.- 3) Location of wing and tail: the wing dimensions must give the correct MAC length (real world) and the coordinates of the wing apex must put the MAC at the right location (again real world).

Information from the manufacturer is not generally available so the model will have to be sized by working from available general arrangement drawings and setting the visual centre where it looks OK.

As for the tail, only the correct surface areas mind, the locations of apexes have no effectNote however, that playing with the wing apex has no effect on aircraft aerodynamics.

The positions of the apexes do matter The longitudinal position of both of them relative to the cg is critical to both the trim and to the the stability of the aircraft.

-Note also that the centre of lift is the point corresponding to the visual centre, unless the offset is different from zero in the airfile record 1534. But anyway, the centre of lift is not of a big interest in MSFS. Because the point where a force applies is a moment issue and thus it is already dealt with in the pitching moment coefficients and table 473.

The are two relevant centres of lift neither of which are at the visual centre. The first, for the complete aircraft, is at the cg for a trimmed aircraft by definition - cg can move so the centre of lift moves. The other, for the wing/fuselage combination alone, moves andcan be at an infinite distance from the aircraft. The following curves are for the default FSX B747. The first is lift curve (Cl_ alpha from arfile Table 404) and the second is pitching moment (Cm_ alpha from airfile table 474). The second shows the effect of including pitching moment at zero angle of attack (Cm0 from airfile table 1101). The third figure shows the position of the centre of lift against lift coefficient as a percentage of the datum chord (whatever that may be). The position is given by dividing the moment by the force according to the principles of basic mechanics. In this case trhat's Cm_alpha/Cl_alpha . This position goes to minus infinity because, in this case, there is a negative pitching moment when the lift coefficient is zero.

Hi all,Responding to WarpD :Sure , the centerof lift is of huge concern, but once the airfile is computed (using FSEdit for instance) and the coefficients and tables are known, we don't have to care anymore about the center of lift location, and frankly I don't understand the use of record 1534 (lift offset centre). As I have said already, in MSFS all is a question of equations involving lift and moment coefficients and lift and moment values from curves(tables).Replying to Brett_Henderson :No, the visual centre as well as the reference have nothing to do with the view axis of the camera.I maintain then that in all situations MSFS hooks the model from an invariable point: its origin (that is the visual centre).When MSFS loads an aircraft model, it loads as well a reference point (the visual centre) that links the model and makes it coherent with the aircraft.cfg file (this is why the ref.datum is set with respect to the visual centre of model). Those who sustain that the model rotates around the varying cg point (like in real world) have not yet given any convincing proof.Replying now to n4gix :Sure that at rest , on the ground, the centre of force is at cg and, and forces are in balance( Aircraft weight + ground reaction =0).Now how about the case where the aircaft is in level flight? 2 forces have to balance the aircraft weight : the wing lift and the tail lift. But moreover, the moment of these 3 forces around the rotating point( cg in real world) must also balance and the sum be equal to zero. MSFS uses TBL 404, flaps lift, H tail lift (tail incidence not zero), elevator lift, spoilers lift(negative) to balance the aircraft weight. As for the moment balance issue, MSFS uses Cm0+TBL 473, the flaps pitching moment, the h tail pitching moment (tail incidence not zero), the elevator pitching moment, the spoilers pitching moment, the gear pitching moment and the thrust pitching moment. So, as you can deduce, the location of centre of lift does not rise because it is a concern that has been taken into account in the airfile computation. Same thing for the wing apex. It serves in calculating the airfile, but has no effect if changed after the airfile is ready. The only change I could see is the cg gauge reading (% MAC,since moving the wing apex changes the location of MAC with respect to cg location).As for the centre of lift location, I agree there are 2: The wing lift centre (at MAC/4) and the wing-body-tail lift centre(called also Neutral Point ). The airfile tables 404 (for lift) and 473 (for pitching moment) are implemented using these 2 centres. So far , I don't have a precise idea how.We have to agree finally, that the only force that changes(in location) in an aircraft in flight is the aircraft weight (cg). The aerodynamic forces are supposed to act at a fix centre of lift. These forces vary only in magnitude.