UPDATE:I've run into a curious problem. Aside from potential in-flight, aerodynamic effects... simply moving the reference_datum back should allow the model to sit on a runway (the CoG will be in the same place realtive to the contact points).We all now that moving these things within reasonable parameters yields predictable results.. but a DRAMATIC change in the reference_datum won't even allow the airplane to sit static, on a runway (aerodynamic forces on animation rendered moot).. so my test is not possible... I'll have to wait for n4gix's testing to demonstrate where the pitch axis lies.This all indicates that there is definately a tie to the model-origin for the CoG, and that cfg-based CoG adjustments are skewed (as in, even though my test CoG was still squarely inside of landing gear support, the model would not sit static))... and I'll conclude (for now), that so long as cfg-based modifications for a CoG are within reasonable distances from the model-origin, the skewing is minimized.

I carried out an experiment. In Gmax I moved the model 10m aft of the Gmax origin (0, 0, 0) and re-exported it. I left the reference_datum_position and empty_weight_CG_position at their original values of (0. 0. 0). This meant that the empty cg was still at the Gmax origin.The initial Free Flight screen showed the model rotrating about the Gmax origin which is also the reference_datum_position in this case.On flying the aircraft was semi-buried in the runway. (I can't explain this, but suspect it may be related to static_pitch and static_height which always give me problems!) Nevertheless I flew the aircraft and got the following locked spot view - it has no cameras.Again, this is related to the Gmax origin (0, 0, 0) (reference_datum_position) . When I pulled back on the stick the aircraft obviously rotated about the same pointI then set reference_datum_position to -10m (-32.8083 ft) with empty_weight_CG_position unchanged to put the empty cg in the correct relation to the visual model. I also set it to +10m. These changes made no visual difference that I could see.I conclude that the aircraft pitches about the origin of the Gmax model (0, 0, 0) and not the cg nor the reference_datum_position.

I think the aerodynamic details you are pointing to are due to some slight differences between the different litteratures on the subject.In fact, wing centre of lift at MAC/4 is considered as a good compromise and is often used. I think that this assumption holds also for MSFS (the Concorde excepted). As for Neutral Point, I often read that this refers to the centre of lift of the whole aircraft( combination of all lifting surfaces, body included).

The 1/4 MAC is no more than an approximation based on simple theory which is only valid in limited cases.I've sketched the forces for a trimmed aircraft in blue and the additional forces following a small change in incidence in red. The total lift force with both these forces is shown in yellow. They all act in different positions.

The 1/4 MAC is no more than an approximation based on simple theory which is only valid in limited cases.I've sketched the forces for a trimmed aircraft in blue and the additional forces following a small change in incidence in red. The total lift force with both these forces is shown in yellow. They all act in different positions.

The question becomes whether you’re describing a convention aerodynamic model – like a pilot might visualize.Or a Jan Roskam style simulation model - like a computer sim would use.For instance...CLih and CLde don't act through the tail.CLih and CLde don’t affect the trim moment > stabilityYour diagram is not conventionally wrong…but it’s not a great analogy for the sim. - IMOYou'll need to transform all forces to the CG point…to match the airfile coefficients."They all act in different positions."This would require a mulitbody system....in contrast to a single body system.

The question becomes whether you’re describing a convention aerodynamic model – like a pilot might visualize.Or a Jan Roskam style simulation model - like a computer sim would use...."They all act in different positions."This would require a mulitbody system....in contrast to a single body system.

I agree. I was replying to posts that were referring to centres of lift and neutral points. But individual forces can act at different positions on a single body system. Their resultant acts at a single point.I tried another experiment by setting htail_area and htail_pos_lon to zero - effectively removing the tailplane. At first sight, it doesn't seem to have made any difference!!!EDITIt's getting late. I'll try more experiments tomorrow

Hi you all,When dealing with only the forces acting on an aircraft, we don't have really to worry about where these forces act. As long as forces and their equilibrium are concerned, all we need to achieve is up forces = down forces.The point where a force acts becomes a concern only when dealing with pitching moments. For MSFS, at the stage of building the airfile, the positions (apexes) of the lifting surfaces (wing, h tail, V tail) given in the aircraft are important. They serve to set the point where each aerodynamic force acts in order to calculate the correct pitch coefficients (as well as Yaw and Roll coefficients) and table 473. Once the airfile is ready for use, many of the changes that we can make on the aircraft have no effect. This is why mgh wrote: " I tried another experiment by setting htail_area and htail_pos_lon to zero - effectively removing the tailplane. At first sight, it doesn't seem to have made any difference!!". Very true, because you didn't change anything in the airfile (the corresponding coefficients!). Try to move freely the wing_apex. No effect on aerodynamics. The only change you can notice is a change in the cg gauge reading.Back to pitching pivot. We can then now confirm, once and for all, that in MSFS , models pitch about the visual model centre.Let us now talk about pitch stability and neutral point. In MSFS, the aircrafts simulated seem to have no neutral point ( or a neutral point outside the aircraft), because the slope of pitching moment function (ie. Table 473) is always > 0. Remember, at neutral point, the slope of the curve (pitching moment vs alpha) must be zero.

Try to move freely the wing_apex. No effect on aerodynamics. The only change you can notice is a change in the cg gauge reading.

That is an incorrect statement. The value for the wing_apex does indeed affect an aircraft's aerodynamics in FS. If you are not seeing any, I suspect you have other problems in your weight/balance design for the aircraft(s).

Let us now talk about pitch stability and neutral point. In MSFS, the aircrafts simulated seem to have no neutral point ( or a neutral point outside the aircraft), because the slope of pitching moment function (ie. Table 473) is always > 0. Remember, at neutral point, the slope of the curve (pitching moment vs alpha) must be zero.

Two points. First the Microsoft conventions are that longitudinal is positive forward, vertical is positive up, and pitch is positive nose down - see the ESP SDK. This means that the Table 473 pitching moment becomes increasingly nose down with increasing angle of attack. Second the neutral point isn't determined by dC_M/dα alone.The neutral point is the point at which the change in lift arising from a change in angle of attack occurs. This means it is the point about which there is no change in pitching moment arising from a change in angle of attack. The pitching moment about a point a distance x ahead an origin is given by:C_Mx = C_M(α) + x * C_L(α)where C_M(α) and C_L(α) are measured at the origin.The change in C_Mx with α is given bydC_Mx/dα = dC_M(α)/dα + (x / c) * dC_L(α)/dα where c is the reference chordAt the neutral point (x = h) dC_Mx/dα is equal to zero so that dC_M(α)/dα + (h / c) * dC_L(α)/dα = 0or h / c = - (dC_M(α)/dα) / (dC_L(α)/dα)For the default FSX B747 over the range of angle of attack from 0 - 10 deg dC_M(α)/dα =+1.101717 and dC_L(α)/dα = +5.2008, Both are both almost constant over that range. This would put the neutral point, h/c, at -0.211865 relative to the origin, which is aft of the origin by an amount less than the reference chord. That's not outside the aircraft

I've experimented with the default FSX C272 by changing the wing_pos_apex_lon from the default value of -2.4 ft to + 1000 ft. I took off on full power hands-off with Hard realism settings. I logged time and speed to takeoff and speed and rate of climb after 4 min. I didn't see and significant differences..-2.4 ft std T/O 24 sec & 82kts: Climb 80 kts & 700 ft/min+1000 ft T/O 24 sec & 82kts: Climb 80 kts & 700 ft/min-1000 ft T/O 25 sec & 83kts: Climb 81 kts & 710 ft/minGeneral handling appeared unchanged.

I can assure you, mess with the wing apex position on the Eaglesoft Citation X Extreme 2.0 and you'll see things behave differently. To assume your conclusions are correct, you're assuming the FDE for the default aircraft are 100% accurate. Something I don't agree with. Are they flyable... sure, are they accurate... I doubt it.

Cmalpha already includes the effects of wing offset - on pitch.So, it is possible wing_pos_apex_lon is used for some other calculation...or it's just obsolete <_<But, moving the wing apex (on a straight wing aircraft at least) does not affect pitch.EDIT...These diagrams are incorrect ;)I'll fix them sometime. Forces act at 1/4 mean chord - not emptyCG.I think a good experiment to show how longitudinal offsets affect forces is to modify the elevator’s longitudinal position :((only CmDe describes this offset…it incorporates the elevator’s lon. offset distance)

I haven't got a Citation X so I can't try it. Anyone should be able to reproduce the efect I found. I shall try some other default aircraft to see if the behave in the same way as the C172.I'm surprised by not seeing any effects - I expected that moving the wing apex (hence the wing) would have a marked effect on both trim and stability and don't understand why it doesn't.

That is an incorrect statement. The value for the wing_apex does indeed affect an aircraft's aerodynamics in FS. If you are not seeing any, I suspect you have other problems in your weight/balance design for the aircraft(s).

You can not be so confident without trying the following experiment : -park your aircraft with brake on and engines off. Read your cg on the cg gauge and write it somwhere. Then save your flight and go to the aircraft cfg file and change your wing apex position.Now load the flight saved and read the new cg reading. Of course it is no more the same figure you saw and wrote somewhere previously, despite you have made no change concerning the load and balance of your aircraft. Note that mgh has tried his own experiment (take off and climb) and came out with the same conclusion. It must be clear that Wing apex has effect on Cl's Cm's and lift and moment curves only when building the airfile. After, it has no effect except ajusting the cg gauge reading.

To mgh exclusively :You wrote: "This means it is the point about which there is no change in pitching moment arising from a change in angle of attack." My way of saying the same thing : At neutral point, the pitching moment is independent of alpha.You wrote :"At the neutral point (x = h) dC_Mx/dα is equal to zero".My way of saying the same thing :. At neutral point, the slope of the pitching moment curve Cm(alpha) is zero. Then we agree so far, but I can not follow you further when you involve C_L(α) which is I guess TBL 404, a lift curve. Can you please be more explicit ?You agree that the only pitching moment curve (pitching moment vs alpha) in MSFS is TBL 473 (with positive slope) ?But since we agreed already that the pivot point is the model visual centre and not the cg location, may be this situation involves a new pitching moment component dependent of alpha.

You can not be so confident without trying the following experiment : -park your aircraft with brake on and engines off. Read your cg on the cg gauge and write it somwhere. Then save your flight and go to the aircraft cfg file and change your wing apex position.Now load the flight saved and read the new cg reading. Of course it is no more the same figure you saw and wrote somewhere previously, despite you have made no change concerning the load and balance of your aircraft. Note that mgh has tried his own experiment (take off and climb) and came out with the same conclusion. It must be clear that Wing apex has effect on Cl's Cm's and lift and moment curves only when building the airfile. After, it has no effect except ajusting the cg gauge reading.

And yet I know, from building the Eaglesoft Citation X Extreme 2.0's FDE that it does indeed have far more effect than either of you two are seeing. I've also done the FDE for the Eaglesoft Diamond Air DA-42 and wing apex had an effect there as well. However, I ensure that all weight and balance stations are accurate to real world data, as is the reference datum point.