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.

Strictly speaking, I wouldn’t say Cm’s are affected by an offset…they are the offset.I’d be interested to know if the flight model compilers actually reference data in the aircraft.cfg - I don't know.But I did test the idea, by moving the wing and hstab offsets in the config I wasn’t able to change pitch behaviour after recompiling the airfile.It's possible wing_apex only concerns FS's load and balance UI.

The pitching moment curve in Table 473 gives CM against α about an arbitrary Reference Point, say for sake of argument the model visual centre. The neutral point is a different point and is defined as the point where dC_M/dα is equal to zero. The figure illustrates the point. The lift and pitching moments shown in red are those about the reference point. In the case of a wind-tunnel experiment the Reference Point is the mounting point of the model about which forces and moments are measured. If the model was mounted at the “Neutral” Point the moment about this point would be increased because of the effect of the lift acting ahead of it. Tnvention -that’s where the equation C_Mx = C_M(α) + x * C_L(α) comes from. Also remember Microsoft’s convention a positive C_M implies a nose down pitching moment. I did a further experiment moving the wing apex. Flight Simulator calculates the position of the cg as a percentage of the MAC. This is in offset 0x2EF8 and I used FSInterrogate to examine it. I found that changing the wing apex position directly changes the position of the Mean Aerodynamic chord - the two are locked together. I still don’t understand why no effect can be seen when flying. It would be interesting to know what effect were seen on the Eaglesioft Citation

The pitching moment curve in Table 473 gives CM against α about an arbitrary Reference Point, say for sake of argument the model visual centre.

Yeah it’s fair to say all CM’s are technically couples.They’re a pure rotational force (balanced so to speak) - so they could be seen as applied anywhere.But for clarity it’s still best to imagine these rotational forces are applied at the same location as the lift force.It reinforces the idea of a single body…with all forces acting about a single point.I suppose the literal offset of the ‘neutral point’ could be found by analyzing all the CM’s in the airfile.It’s a product of tuning all the various airfile moments.Again, the wing_apex in the config may very well be used to present the weight and balance to users.But not be included in the actual flight dynamics. ...where it would sort of be obfuscating Cmalpha.As far as I can tell...

I've stated several times now, it affects flight dynamics.

I've stated several times now, it affects flight dynamics.

Yes you have...how? - Let's verify this. :(

In principle the further iorward the wing apex, and hence wing lift, the further aft the CG is in relation to it and hence the stability should be reduced. My experiments with the FSX default C172 show the the MAC is moved forward by the amount of movement in the weing apex position and so the stability should be reduced. However I can't detect that effect no matter how far forward I move it. Why?

Yes you have...how? - Let's verify this. :(

Um... I've given up all the FDE information I'm going to give up. Since I make a living off of it, I'm certain you understand.

In principle the further iorward the wing apex, and hence wing lift, the further aft the CG is in relation to it and hence the stability should be reduced. My experiments with the FSX default C172 show the the MAC is moved forward by the amount of movement in the weing apex position and so the stability should be reduced. However I can't detect that effect no matter how far forward I move it. Why?

I can only offer that the default aircraft FDEs are not that accurate in design.I know with the Citation X, movement of the wing apex has a significant effect in aspects of balance as well as lift performance.

Um... I've given up all the FDE information I'm going to give up. Since I make a living off of it, I'm certain you understand.

Of course I totally understand :)Rest assured you haven’t actually said anything ;)Lol, sorry for the rub.All's well :)

I can only offer that the default aircraft FDEs are not that accurate in design.

It's not a matter of accuracy in design. I am reportting thr facts as I discovered them. Moving the wing apex forward does result in the MAC moving by the same amount to closer that 1/100 of a foot. That's accurate enough for me.Perhaps some one with an Eaglesoft's Citation X will carry out the following experiment:1 Backup the aircraft.cfg file2 Find the line in section [airplane_geometry] of that file that reads wing_pos_apex_lon = -dd.dddd where dd.dd is a number3 add, say 10, to that number to move the apex fwd by 10 ft and save the file4 Fly the aircraft and let us know what they find5 Restore the backed-up aircraft.cfg file

The pitching moment curve in Table 473 gives CM against α about an arbitrary Reference Point, say for sake of argument the model visual centre. The neutral point is a different point and is defined as the point where dC_M/dα is equal to zero. The figure illustrates the point. The lift and pitching moments shown in red are those about the reference point. In the case of a wind-tunnel experiment the Reference Point is the mounting point of the model about which forces and moments are measured. If the model was mounted at the “Neutral” Point the moment about this point would be increased because of the effect of the lift acting ahead of it. Tnvention -that’s where the equation C_Mx = C_M(α) + x * C_L(α) comes from. Also remember Microsoft’s convention a positive C_M implies a nose down pitching moment. I did a further experiment moving the wing apex. Flight Simulator calculates the position of the cg as a percentage of the MAC. This is in offset 0x2EF8 and I used FSInterrogate to examine it. I found that changing the wing apex position directly changes the position of the Mean Aerodynamic chord - the two are locked together. I still don’t understand why no effect can be seen when flying. It would be interesting to know what effect were seen on the Eaglesioft Citation

Hi mgh,I spent enough time studying the pitching moment formula C_Mx = C_M(α) + x * C_L(α) and can say now that this is what MSFS uses in dealing with the piching moment issue. Then , for sure, MSFS involves Table 473 + x * C_L(α) as a matter of longitudinal stability and neutral point location.I must say however, that the term x * C_L(α) is some how a hidden pitching moment component due to the fact that the pivot is not always the cg point. Note that when cg coincides with the model visual centre, this terme disappears (is null, obviously because x = 0).

I must say however, that the term x * CL(α) is some how a hidden pitching moment component due to the fact that the pivot is not always the cg point. Note that when cg coincides with the model visual centre, this terme disappears (is null, obviously because x = 0).

x * C_L(α) isn't really hidden. C_M(α) and C_L(α) in the .air relate to a fixed point. The expression C_M(α) + x * C_L(α) simply converts the moment about the fixed point to a moment about another point, in this particular case the aircraft's cg. The cg is the point where the inertial forces (mass and moments of inertia) act so the areodynamic forces must relate to this point. The fixed point could either be the model visual centre or the empty weight cg. I'm not aware of any definitive statement about this.

x * C_L(α) isn't really hidden. C_M(α) and C_L(α) in the .air relate to a fixed point. The expression C_M(α) + x * C_L(α) simply converts the moment about the fixed point to a moment about another point, in this particular case the aircraft's cg. The cg is the point where the inertial forces (mass and moments of inertia) act so the areodynamic forces must relate to this point. The fixed point could either be the model visual centre or the empty weight cg. I'm not aware of any definitive statement about this.

Hi mgh ,That's correct, and nobody can disagree about what you wrote about the pitching moment expression. I stick however to the evidence that the fixed point is the model visual centre. The empty cg position can not serve as a fixed reference point. It is just a particular cg position corresponding to the empty situation of the aircraft.Let 's now have a look at the point where the elevator lift acts. I performed the following experiment and noticed something a bit strange ( with AP engaged and Altitude hold on) :- I made Cm_de = 0 as well as all factors (multipliers that affect Cm_de, like mach factor etc.) = 0. I also put Cm0 =0 and removed the TBL 473 (so that the change in alpha does not complicate my experiment). In other words I modified the airfile so that the elevator cannot produce a pitching moment. But despite that, I noticed that, unless the cg is at the visual centre, both elevator up and elevator down produce pitching moment. Does this mean that the expression C_M(α) + x * C_L(α) remains however incomplete, and then the complete one is : C_M(α) + x * [C_L(α)+ CL_de*elevator deflection)] ? I don't think so however, because the aircraft weight has not changed during the experiment. In other words, [C_L(α1)+ CL_de*elevator deflection)] = [C_L(α2)+ CL_de*elevator deflection)] =[C_L(α)+ CL_de*elevator deflection) ( α for elevator at rest, α1 for elevator up, and α2 for elevator down).

Aces said the pitch moment uses the following equation(effectively it's the same as yours)...M_A = C_M qSc M_A = Aircraft pitching moment C_M = Airplane pitching moment coefficient q = dynamic pressure (lbs/ft2) S = Wing surface area (ft2) c = Wing mean geometric chord (ft)So area, chord, and dynamic pressure constitute the force (this is lift)The moment coefficient incorporates the offset - the moment arm.If you follow the trail backwards there are no offsets in the equations until you get to the CM values in the airfile.Such as CMde ~ (Distance_Elevator/MAC) * Lift-Elevatorb_kimoun that's a great experiment…I wish I had thought to try that ;)I suspect you're still getting an elevator moment because of the way elevator_trim and H-stab_incidence are incorporated.I’d continue to zero-out other moments…really you should be able to eliminate all pitch control.Also you should be able to prove CL coefficients don’t influence Pitch - I hope :) - By removing ALL moments.{The equation above also shows why CL's don't affect Pitch}Either way I’m excited to see what you find.BTW. I’ve suspected CMih is a pitch trim…it would be on a Mooney or a DC9 for instance.I’m not sure why we also have CM_dt (which I use)…it isn’t a conventional coefficient (as far as I know) but CMih is.I still don’t think the model’s visual center is the point forces act on…it’s sort of arbitrary.The fixed empty CG is a child of the ‘visual center’ and has a very specific/critical location.Because flight models are generally hand-tuned to get the desired results, a designer doesn’t actually have to know where forces act.But it does lead to the myth the flight model is a ‘black art’ – which it isn’t.The texts uses P to describe the point the forces act on – it’s the dynamic CG.Personal I thought FS might use the fixed empty weight CG…and add the dynamic weight as a vector and moment about this fixed empty-CG.It would be an approximation. That’s in the diagram above.But it’s totally just a guess ;) I’m certain the real answer is in the book I sent you guys.Also b_kimoun if you continue your experiments I think you’ll be able to infer all these relationships.But please don’t use the autopilot! :) You don’t know what’s in there that might affect your results ;)Moderators, shouldn't this thread be moved to the design forum?

Moderators, shouldn't this thread be moved to the design forum?

If it was, I'd never be reading it...

[CL(α1)+ CL_de*elevator deflection)] = [CL(α2)+ CL_de*elevator deflection)] =[CL(α)+ CL_de*elevator deflection) ( α for elevator at rest, α1 for elevator up, and α2 for elevator down).

I think we're on the same page.Just to be sure, the three conditions would 'look' the same if you happened to be looking for the wrong affect.IMO the above won't affect your pitch attitude at all - only the aircraft’s total lift (no rotation about the CG).The elevator’s contribution to total lift would be relatively small…but not insignificant.Using AFSD you could read this....maybe even the VSI would show it.But it would not show up on the attitude indicator.The aircraft will simply rise and fall with no attitude change.For your modified FDE…I’d expect the aircraft attitude to be identical for all 3 elev positionsI’d expect the aircraft total lift to be x, x--, then x++