The trim bug in MSFlight Simulator

[dmaher 0]

How come the lifting component from elevator produces pitching moment (at x not zero) but the lifting components from flaps and spoilers do not ?

Note flight control surfaces have no position....only an area.Elev CL doesn’t influence moment…it only adds and subtracts from the aircrafts total lift.Elev Cm produces all ‘moment’…Cm’s include a moment arm (that's the surface’s location)The force of the moment is computed directly from surface area…true this is ‘lift’ but it's not influenced by ‘CL’.Changing CL won’t affect rotational authority of the control surface.Changing CM will.When I ran your experiment it seemed to confirmed the above.M_A = C_M qScThis is done to allow us to tune lift and stability separately.So the separation of dynamic affects allows us to make very refined flight models.Otherwise correcting a surfaces lift might harm it's moment.The elevator isn't any different - it also has no position.If you set all pitch CM's to zero you will have no pitch affect or control - no matter how large various CL's are set.The only pitch mechanism left is payload.In your experiment...Cl(alpha). x doesn't equal Cm_dt.Trim tab deflectionCM(alpha)qSc = Cm_dt.Trim tab deflectionIt's a small but important distinction...Another way to test this is to adjust CL and watch how it affects pitch....(it won't)mgh, there's a small chance 10% wingspan is used regardless of what the config says.It may explain why adjusting wing_apex (on a straight wing) has not effect - it should have a dramatic effect.

[b_kimoun 0]

One lives and learns - FSX does modify the aircraft.cfg file, although not very helpfully.In anticipation of running some tests I stripped out all the dimensional information out of the default FSX C172SP file. I fould afteroading the aircraft the file was changed. FSX added wing_root_chord=4.819945, which is wing_span / wing_area It set oswald_efficiency_factor=0.700000, which is a typical value. Both those are resonable values.FSX sets wing_pos_apex_lon, htail_pos_lon, and vtail_pos_lon all to -3.600000, which puts the wing, horizontal stabiliser, and vertical stabilser all at the same point! Not that it matters because its sets the areas of the area of the horizontal stabiliser, vertical stabilser, elevator, and rudder all to zero. But the aircraft still flies and responds normally despites having no horizontal stabiliser, vertical stabilser, elevator, or rudder!! I am now convinced that FSX doesn't really need most of most of the values in the the [airplane_geometry] section

[airplane_geometry]wing_area   			= 174.0 				//Square feetwing_span   			= 36.1              	//Feetelevator_up_limit   	= 28.0              	//Degreeselevator_down_limit 	= 21.0              	//Degreesaileron_up_limit    	= 20.0              	//Degreesaileron_down_limit  	= 15.0              	//Degreesrudder_limit        	= 24.0              	//Degreeselevator_trim_limit 	= 19.5              	//Degreesspoiler_limit   		= 0.0   				//Degreesspoilerons_available	= 0 					//Spoilerons Available?aileron_to_spoileron_gain = 0   				//Aileron to spoileron gainmin_ailerons_for_spoilerons = 0 				//Degreesmin_flaps_for_spoilerons = 0                	//Minimum flap handle position when spoilerons activateauto_spoiler_available   = 0positive_g_limit_flaps_up  =  4.0   			//Design G load tolerance (positive, flaps up)positive_g_limit_flaps_down=  2.0   			//Design G load tolerance (positive, flaps down)negative_g_limit_flaps_up  = -1.5   			//Design G load tolerance (negative, flaps up)negative_g_limit_flaps_down= -1.5   			//Design G load tolerance (negative, flaps down)load_safety_factor 		=  1.5   			//Design G load safety factor ////////////////////////////// START OF ADDITIONS  ////////////////////////////////////////////////////////wing_root_chord=4.819945wing_dihedral=0.000000wing_incidence=0.000000wing_twist=0.000000oswald_efficiency_factor=0.700000wing_winglets_flag=0wing_sweep=0.000000wing_pos_apex_lon=-3.600000wing_pos_apex_vert=0.000000htail_area=0.000000htail_span=0.000000htail_pos_lon=-3.600000htail_pos_vert=0.000000htail_incidence=0.000000htail_sweep=0.000000vtail_area=0.000000vtail_span=0.000000vtail_sweep=0.000000vtail_pos_lon=-3.600000vtail_pos_vert=0.000000elevator_area=0.000000aileron_area=0.000000rudder_area=0.000000//////////////////////////// END OF ADDITIONS  ///////////////////////////////////////////////////

This confirms that airfile and aircraft.cfg are linked and work together. The data suppressed from aircraft.cfg re-appearing when loading aircraft means that the airfile supplies back (through MSFS) to cfg files the missing and same data used for its building(the airfile). But this does not mean that the airfile builds the aircraft.cfg for use by MSFS. It means that the data suppressed is already in the airfile, and that MSFS has simply set priority to read it from the cfg file. I don't think then that MSFF rebuilds the airfile. It is however possible that some specific data may not be in both files, in other words, that are either specific to airfile or to cfg files.Also, we have to know which data ( if any) in aircarft.cfg override the data coming from airfile or vice versa.Note that for FsEdit, you have just to built your airfile using an fs9 aircraft.cfg, then after manage to copy the new coefficients and tables (computed bu FsEdit) to the fsx aircraft files.

[b_kimoun 0]

Note flight control surfaces have no position....only an area.Elev CL doesn’t influence moment…it only adds and subtracts from the aircrafts total lift.Elev Cm produces all ‘moment’…Cm’s include a moment arm (that's the surface’s location)The force of the moment is computed directly from surface area…true this is ‘lift’ but it's not influenced by ‘CL’.Changing CL won’t affect rotational authority of the control surface.Changing CM will.When I ran your experiment it seemed to confirmed the above.M_A = C_M qScThis is done to allow us to tune lift and stability separately.So the separation of dynamic affects allows us to make very refined flight models.Otherwise correcting a surfaces lift might harm it's moment.The elevator isn't any different - it also has no position.If you set all pitch CM's to zero you will have no pitch affect or control - no matter how large various CL's are set.The only pitch mechanism left is payload.In your experiment...Cl(alpha). x doesn't equal Cm_dt.Trim tab deflectionCM(alpha)qSc = Cm_dt.Trim tab deflectionIt's a small but important distinction...Another way to test this is to adjust CL and watch how it affects pitch....(it won't)mgh, there's a small chance 10% wingspan is used regardless of what the config says.It may explain why adjusting wing_apex (on a straight wing) has not effect - it should have a dramatic effect.

Hi, if you want to try the same experiment, make sure that you first delete or set to zero (in airfile) all the pitching moment sources except for Cm_dt (this means no Table 473 or values set to zero, Cm0 =0, all mach factors set to 0 etc.). You will then notice that the only pitching sources that remain are Cl(alpha). x and Cm_dt.Trim tab deflection. It is however the no effect of Cl_df and Cl_ds on the equation Cl(alpha). x = Cm_dt.Trim tab deflection that seems strange to me. Because I considere that Cl_de alone affecting (when x is not zero) the above equation makes a sense. More explicitly, the extra elevator lift Cl_de.elevator deflection adds and substracts from Cl(alpha) and the above equation turns out to be : (Cl(alpha) + Cl_de.elevator deflection).x = Cm_dt.Trim tab deflection.I expected that this equation is finally : (Cl(alpha) + Cl_de.elevator deflection+Cl_df. flaps deflection+Cl_ds. spoilers deflection).x = Cm_dt.Trim tab deflection but the experiment shows that it is not the case even though the flaps and spoilers lifts add and substract from Cl(alpha) like the elevator deflection does. Really a problem !

[mgh 89]

This confirms that airfile and aircraft.cfg are linked and work together. The data suppressed from aircraft.cfg re-appearing when loading aircraft means that the airfile supplies back (through MSFS) to cfg files the missing and same data used for its building(the airfile). But this does not mean that the airfile builds the aircraft.cfg for use by MSFS. It means that the data suppressed is already in the airfile, and that MSFS has simply set priority to read it from the cfg file. I don't think then that MSFF rebuilds the airfile.It is however possible that some specific data may not be in both files, in other words, that are either specific to airfile or to cfg filesAlso, we have to know which data ( if any) in aircarft.cfg override the data coming from airfile or vice versa.

No. The air . file doesn't supply data that is missing from the .cfg file for the simple reason that is doesn't contain any such data. The only aerodynamic data required in an FSX .air file are the following non-dimensional coefficients:

CL vs. Alpha	CL spoiler	CL flaps	CLih	CLdeCM vs. Alpha	CM adot	CMq	CMq due to propwash	CMo	CM flap	CM delta trim	CM gear	CM spoilerCDo	CD flaps	CD gear	CD spoiler	CMih	CMde	CMde due to propwash	CLqCyB	CyP	CyR	Cy Delta Rudder	ClP	ClR	Cl Delta Spoiler	Cl Delta Aileron	Cl Delta RudderCnB	CnP	CnR	CnR due to propwash	Cn Delta Aileron	Cn Delta Rudder	Cn Delta Rudder due to propwash

There is nothing there can can be used to replace missing dimensions in the .cfg file. FSX calculates the mean wing chord (area/span), and sets the Oswald facor to a typical value of 0.7. Those are the only realistic values it creates. It sets the the wing, horizontal and vertical stabilisers apex position all at the same place and sets all other missing items to zer, including areas. Those are, I suggest, the only default values that FSX uses. I refuse to believe, without hard evidence, the FSX calculates another second, hidden, set of default values for internal use.Many of the dimensions in the ,cfg file are uneccessary simply because CL(α) and CM(α) are for the comlete aircraft and are refered to a fixed origin (empy cg of visual model centre). FSX can't calxculate the effects of moving the wing apex because it doesn't have the data to do so. The main effect of moving the wing apex should be to move the wing lift forwar, but FSX doesn't know what the wing lift is - it only knows the total lift.

Note that for FsEdit, you have just to built your airfile using an fs9 aircraft.cfg, then after manage to copy the new coefficients and tables (computed bu FsEdit) to the fsx aircraft files.

.FSEdit only works with FS9 .cfg and .air files. The pitching moment coefficients in an FS9 .air file are in Table 1101, with other coefficients. FSX now permits them to be in a separate Table 1541. I doubt FSEdit can cope with that. Anyway, FSEdit is inappropriate if you are changing the .air files yourself because such changes are inconsistent with FSEdit's internal "rules" and make other unexpected changes

[dmaher 0]

I expected that this equation is finally : (Cl(alpha) + Cl_de.elevator deflection+Cl_df. flaps deflection+Cl_ds. spoilers deflection).x = Cm_dt.Trim tab deflection but the experiment shows that it is not the case even though the flaps and spoilers lifts add and substract from Cl(alpha) like the elevator deflection does. Really a problem !

:( b_kimoun, as long as you assume you can derive a CM product from a CL product there are going to be many-many problems :wink:Like the gentlemen farmers used to say, ‘a pit to a pit and a seed to a seed’#==============================================EDIT#==============================================Sorry b_kimoun, I have to eat some crow on this statement.Setting CLelev=0 and/or CLhStab=0 does not negate elevator pitch authority…as I expected.However,Setting CLwing=0, CLelev=0, and CLhStab=0 does negate elevator pitch authority.That indicates to me the elev can’t calculate a moment on area and pressure alone (my contention) it does need CL. A further test,Setting CLwing=0 and CLelevator=-0.3 CLhStab=-0.5 brought back elevator pitch authority.Further still,A reasonably stable airfile was made with empennage lift only.This seems to confirm to me empennage lift acts at the CG.#==============================================/EDIT#==============================================M=MC_MqSc=C_MqScC_M1qS₁c₁ = C_M2qS₂c₂

D=C_DqSL=C_LqSM=C_MqScD = Drag force (lbs)L = Lift force (lbs)q = dynamic pressure (lbs/ft2)S = Wing surface area (ft2)c = Wing mean geometric chord (ft)C_D = Airplane drag coefficientC_L = Airplane lift coefficientC_M = Airplane pitching moment coefficient

The moments may act on emptyCg...it's also been suggested the location is 1/4 meanChord (~by mgh).The second seems more appropriate...but leaves some unanswered questions.Still it's possible in the world of couples and pure-moments it doesn't matter.

[mgh 89]

C_L(α) and C_m(α) are for the complete aircraft and are refered to a fixed originI believe the the pitching moment coefficient in FSX about the origin for a simple aircraft with gear, flaps and spoilers all retracted and no pitching moment due to power in steady level flight is generally given byC_m = C_mo + C_m(α) + C_m_de * elevator angle + C_m_dt * trim angleThe pitching moment about a point a distance x from the origin is:C_mx = C_m + C_L(α) * xwhere typically x is the distance beween the origin and the cg, The distance x only appears when translating forces and moments from one point to another point

[audiohavoc 24]

After reading all the comments here I have to jump in.What I really don't like in FS is that in real flying a plane is trimmed for a speed, therefore if the plane is trimmed straight and level and you bring the power back, the nose will drop to maintain the speed. In reverse, if pwr is increased the nose will go up. In all versions of FS and other sims the aircraft will tend to accelerate or decelerate vice nose going up or down. It is a fondamental flaw in FS.I have been flying for 25 years (air Force and Airlines) and while I was an Air Force instructor this was always a struggle to make student pilots understand that concept. I could continue for pages on the aerodynamics lessons but I think this is enough.Happy flying in the New Year,Dan

Exactly! I took to the controls of a real aircraft for the first time yesterday and this was one of the most important differences I noticed between FSX and real flying.

[b_kimoun 0]

No. The air . file doesn't supply data that is missing from the .cfg file for the simple reason that is doesn't contain any such data. The only aerodynamic data required in an FSX .air file are the following non-dimensional coefficients:

CL vs. Alpha	CL spoiler	CL flaps	CLih	CLdeCM vs. Alpha	CM adot	CMq	CMq due to propwash	CMo	CM flap	CM delta trim	CM gear	CM spoilerCDo	CD flaps	CD gear	CD spoiler	CMih	CMde	CMde due to propwash	CLqCyB	CyP	CyR	Cy Delta Rudder	ClP	ClR	Cl Delta Spoiler	Cl Delta Aileron	Cl Delta RudderCnB	CnP	CnR	CnR due to propwash	Cn Delta Aileron	Cn Delta Rudder	Cn Delta Rudder due to propwash

There is nothing there can can be used to replace missing dimensions in the .cfg file. FSX calculates the mean wing chord (area/span), and sets the Oswald facor to a typical value of 0.7. Those are the only realistic values it creates. It sets the the wing, horizontal and vertical stabilisers apex position all at the same place and sets all other missing items to zer, including areas. Those are, I suggest, the only default values that FSX uses. I refuse to believe, without hard evidence, the FSX calculates another second, hidden, set of default values for internal use.Many of the dimensions in the ,cfg file are uneccessary simply because CL(α) and CM(α) are for the comlete aircraft and are refered to a fixed origin (empy cg of visual model centre). FSX can't calxculate the effects of moving the wing apex because it doesn't have the data to do so. The main effect of moving the wing apex should be to move the wing lift forwar, but FSX doesn't know what the wing lift is - it only knows the total lift..FSEdit only works with FS9 .cfg and .air files. The pitching moment coefficients in an FS9 .air file are in Table 1101, with other coefficients. FSX now permits them to be in a separate Table 1541. I doubt FSEdit can cope with that. Anyway, FSEdit is inappropriate if you are changing the .air files yourself because such changes are inconsistent with FSEdit's internal "rules" and make other unexpected changes

Hi, sure CL(α) ( table 404) and CM(α) (table 473) are for the comlete aircraft. But one has to know that the values that serve to build these curves are in cfg file.For example, if one tries to rebuild the aerodynamics of an aircraft using FsEdit, he has first to enter to this utility the type of data contained in the cfg file. The result will be new tables (1545 and 1546) that override tables 404 and 473 respectively. Plus an other record,1547, that serves to adjust for the centre of lift.

[b_kimoun 0]

:( b_kimoun, as long as you assume you can derive a CM product from a CL product there are going to be many-many problems :wink:Like the gentlemen farmers used to say, ‘a pit to a pit and a seed to a seed’#==============================================EDIT#==============================================Sorry b_kimoun, I have to eat some crow on this statement.Setting CLelev=0 and/or CLhStab=0 does not negate elevator pitch authority…as I expected.However,Setting CLwing=0, CLelev=0, and CLhStab=0 does negate elevator pitch authority.That indicates to me the elev can’t calculate a moment on area and pressure alone (my contention) it does need CL. A further test,Setting CLwing=0 and CLelevator=-0.3 CLhStab=-0.5 brought back elevator pitch authority.Further still,A reasonably stable airfile was made with empennage lift only.This seems to confirm to me empennage lift acts at the CG.#==============================================/EDIT#==============================================M=MC_MqSc=C_MqScC_M1qS₁c₁ = C_M2qS₂c₂The moments may act on emptyCg...it's also been suggested the location is 1/4 meanChord (~by mgh).The second seems more appropriate...but leaves some unanswered questions.Still it's possible in the world of couples and pure-moments it doesn't matter.

Hi, I am wondering of what Clwing you are talking about; this coefficient does not really exist in airfile. And in my experiment, these are the Cm's (and table 473) that I set to zero not the Cl's.The issue rised is then still pending : why ( offset x being different from zero) elevator can still produce a moment and the flaps and spoilers not, supposing that all lift components apply at the same point?

[b_kimoun 0]

C_L(α) and C_m(α) are for the complete aircraft and are refered to a fixed originI believe the the pitching moment coefficient in FSX about the origin for a simple aircraft with gear, flaps and spoilers all retracted and no pitching moment due to power in steady level flight is generally given byC_m = C_mo + C_m(α) + C_m_de * elevator angle + C_m_dt * trim angleThe pitching moment about a point a distance x from the origin is:C_mx = C_m + C_L(α) * xwhere typically x is the distance beween the origin and the cg, The distance x only appears when translating forces and moments from one point to another point

I agree even though I still find the behaviour of the couple (C_m_de * elevator angle and C_m_dt * trim angle) not really clear so far .Because all the aerodynamic litterature talks of : the elevator deflection angle to trim. The tab remains then a secondary aspect involved to overcome the elevator hinge moment and make the necessary elevator deflection possible without a necessay action from the pilot (ie hands-off). And then we can consider that the Cm expression is rather :C_m = C_mo + C_m(α) + C_m_de * elevator angle. I have said already in one of my previous posts that may be a real force feedback joystick is need to have this satisfied but still do not know if it is really the right answer to the issue of trim rised through this topic.Now concerning the equation C_mx = C_m + C_L(α) * x . We agreed that C_L(α) is the Table 404 ( wing-body-tail combination) of airfile and is with respect to a given origin. We can agree also that C_mx is simply C_m when C_L(α) does not produce any moment (that is x = 0). Now if we agree that all lift components add to C_L(α), that is, overall aircraft lift = C_L(α) +Cl_de.elevator deflection+Cl_df. flaps deflection+Cl_ds. spoilers deflection, we can agree that concerning the pitching moment [overall lift.x] (x not zero), the components Cl_de.elevator deflection, Cl_df. flaps deflection and Cl_ds. spoilers deflection contribute to create piching moment even when their Cm's are zero. However, this verifies well with the elevator deflection, but the flaps and spoilers defections affect the overall lift but do not show any effect on pitching moment. I can conclude that the elevator lift applies where C_L(α) applies, but flaps and spoiler lifts apply at the rotation point (that is at the model visual centre) whatever the value of x is. But this seems very strange to me. Because so far I have considered that since for each surface moments and lifts are separated through coefficients (Cm's and CL's), all the lift components apply at the same and unique point.

[dmaher 0]

Hi, I am wondering of what Clwing you are talking about; this coefficient does not really exist in airfile. And in my experiment, these are the Cm's (and table 473) that I set to zero not the Cl's.The issue rised is then still pending : why ( offset x being different from zero) elevator can still produce a moment and the flaps and spoilers not, supposing that all lift components apply at the same point?

“CLwing” is CL for the body (e.g. tbl 404)…a poor choice of words on my part. It’s any lift not already associated with a specific surface…obviously it’s dominated by wing lift.Actually, I wasn’t recreating your experiment…it was a variation, so to speak, looking at zeroing various CL’s. It suggest to me moments look like this…M= C_MC_LqSc“The issue rised is then still pending”When I tried your experiment, zeroing CM’s that affect pitch control, the elevator stopped producing a moment.That's just like every other surface.IMO this behavior confirms what the thread-posts already predicted.i.e. surface offsets (position) come from the airfile CM’s.

[b_kimoun 0]

“CLwing” is CL for the body (e.g. tbl 404)…a poor choice of words on my part. It’s any lift not already associated with a specific surface…obviously it’s dominated by wing lift.Actually, I wasn’t recreating your experiment…it was a variation, so to speak, looking at zeroing various CL’s. It suggest to me moments look like this…M= C_MC_LqSc“The issue rised is then still pending”When I tried your experiment, zeroing CM’s that affect pitch control, the elevator stopped producing a moment.That's just like every other surface.IMO this behavior confirms what the thread-posts already predicted.i.e. surface offsets (position) come from the airfile CM’s.

Hi dmaher !I don't think that zeroing the CL's (especially Table 404) makes your experiment possible since the aircraft is no more flyable. With the Cm's zeroed (as well as Table 473, but Cm_dt excepted), flight is however still possible. If the reference datum is (0,0,0) and aircraft cg is also (0,0,0) (in other words x = 0), I agree with you, the elevator (with its Cm=0) does not produce anymore a moment.But try a cg at, let's say, at (5,0,0). This new situation shows pitching moment when elevator is deflected. But the real problem is why the other surfaces (flaps and spoilers) do not show the same phenomena ? Because these are all identical surfaces that differ from one another only by their respective Cm's and CL's.

[dmaher 0]

I don't think that zeroing the CL's (especially Table 404) makes your experiment possible since the aircraft is no more flyable. With the Cm's zeroed (as well as Table 473, but Cm_dt excepted), flight is however still possible.

Actually there’s very good reason to play with zeroing various CL’s....even tbl404 CLalpha - lift variation due to alphaThat’s still the best way you’ll find to confirm how each influences the model.It's just like your moment experiment - Try it :) But you're right - not to expect a long flight :( To your experiment about zeroing the elevator CmDe - pitching moment variation due to elevator deflectionIn my experience zeroing all pitch control CM’s (elev+hStab+trim+any variation factors) does remove the elevator’s pitching moment...regardless of offset position.But yes, there is a small residual moment caused by CLde – lift variation due to elevator deflection.CLde affects total lift…variations in total lift affect pitch through tbl404 CLalpha and tbl473 CMalpha.Forward yoke, elevator down, increases total lift and causes a pitch up...according to tbl473Rear yoke, elevator up, decreases total lift and causes a pitch down...according to tbl473The elevator’s natural affect on total lift generates a side-effect that is opposite its intended moment.Remove the intended moment (CMde=0) and you’ll see the elevator’s side-effect only. You’ll also see CLde lift acting at the aerodynamic center of the aircraft - not at the elevator.I suspect moving the CG (load) forward and back would respectively reduce then amplify the effect .So possibly this explains why you were able to see the elevator effect appear to go away.

[b_kimoun 0]

Actually there’s very good reason to play with zeroing various CL’s....even tbl404 CLalpha - lift variation due to alphaThat’s still the best way you’ll find to confirm how each influences the model.It's just like your moment experiment - Try it :) But you're right - not to expect a long flight :( To your experiment about zeroing the elevator CmDe - pitching moment variation due to elevator deflectionIn my experience zeroing all pitch control CM’s (elev+hStab+trim+any variation factors) does remove the elevator’s pitching moment...regardless of offset position.But yes, there is a small residual moment caused by CLde – lift variation due to elevator deflection.CLde affects total lift…variations in total lift affect pitch through tbl404 CLalpha and tbl473 CMalpha.Forward yoke, elevator down, increases total lift and causes a pitch up...according to tbl473Rear yoke, elevator up, decreases total lift and causes a pitch down...according to tbl473The elevator’s natural affect on total lift generates a side-effect that is opposite its intended moment.Remove the intended moment (CMde=0) and you’ll see the elevator’s side-effect only. You’ll also see CLde lift acting at the aerodynamic center of the aircraft - not at the elevator.I suspect moving the CG (load) forward and back would respectively reduce then amplify the effect .So possibly this explains why you were able to see the elevator effect appear to go away.

Hi,then yes, you agree, there is a residual moment ( x being not zero) when Cm_de = 0.Now why the other surfaces (flaps end spoilers) do not show also a residual moment when their Cm's is zero too ?Troublesome isn't it?

[mgh 89]

I decided to investigate how FSX deals with trim extracting data from the .cfg file, the .air file (using Aircraft Airfile Manager), and from FSX itself (using FS-Interrogate _FS-I) and putting the data in a spreadsheet to correlate them. I used the default FSX C172SP because it is a simple aircraft with no need to correct for Mach number. I deslt with weight, lift and pitching moment in that order. I flew the aircraft trimmed with the autopilot altitude hold on and recorded the data from FS-I. I loered the flaps and let the aitrcraft re-trim itself. I finally raised the flaps, turned the autopilot off, changed the trim setting and then trimmed the aircraft using the elevator. Weight: FS-I returns the total weight but I also calculated the total weight using the fuel weights reported by FS-I. There was a discrepancy equivalent to ≈ 6lb (the weight of 1 gal) because FS-I reports the tank capacities as 26 gal but the .cfg file sets them to 26.5 gal. It appears FSX calculates the total weight based on the capacities in the .cfg. file FS-I returns the position of the cg as a percentage the MAC but doesn’t give a value for the MAC. I calculate the MAC in the spreadsheet and find the differences between the two cg positions are less than 0.1 inches. Lift: The spreadsheet calculates the lift coefficient using: CL = CL(α) + CL_q * q + CL_de * e + CL_dh * h CL_df * f where CL(α), CL_q, CL_de, CL_dh, and CL_df are from the .airfile α, and q are from FS-I and h is the htail_incidence and f is flap angle from the .cfg file. The spreadsheet calculates the lift using L = CL * P* A Where P is the dynamic pressure from FD-I and S is the wing_area from the .cfg file. The lifts agree with the weight sbased on 26 gal fuel capacity to better than 0.5% Pitching moment: The spreadsheet calculated the pitching moment using: CM = CM0 + CM(α) + CM_q * q + CM_de * e + C_dt * t + CM_dh * h + CM_df * f where CM0, CM(α), CM_q , CM_de, C_dt , CM_dh, and CM_df are from the .air file and t is the trim from FS-I. The spreadsheet calculates the pitching moment using M = CM * P * S * MAC (I set the coefficients relating to prop-wash and thrust to zero in the .air file because I didn’t know how to deal with them.) In both cases the resulting pitching moment was small and equivalent to the lift force acting at about an inch from the loaded cg. In principle the trimmed piching moment would be zero if the aircraft was perfectly trimmed FD-I shows that in both case there were still small linear and angular accelerations which show it isn’t. Conclusions: It seems that in FSX the lift forces from all sources effectively act at the loaded cg and that if the aircraft is trimmed the total pitching moment is zero. The lift coefficients have no direct effect on pitching moment thasn the pitching moment coefficients have on lift. Also, the distance in the .cfg seen to have no direct influence. All that's needed is wing area and a reference chord.