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lykon

ifdg weight and balance problem

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Hello guys i have a problem with ifdg's a319, a320 and 321 weight and balance. When i load an a320 for example i check the the center of gravity with an external program ( in this case with the istation ) and it is -15% which is out of the limits. If then i perform a take off with elevator trim at zero position the aircraft is performing take off nice but this is out of how the aircraft has to perform in reallity. Bellow i have upload a chart just for reference. So as you understand if CG is 15% and i put to the elevator trim a value of +3.4 for example the aircraft is starting to pull up the nose from 80 knots and then as you understand is stalling. Does anyone can help me configure the weight and fuel sections inside aircraft.cfg so this can perform well according reall charts and inside limitslike the image i have post uploads? I have try other aircraft like pa a320 which is balanced good but its flight dynamics are not good as the ifdg's aircrafts. Many thanks in advance.

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In case you are not aware, the CG is measured along the Mean Aerodynamic Chord of the wing. That's pretty basic knowledge but I wanted to put that out there first. FS9 calculates the MAC from the information in the geometry section of the .CFG: Also pretty straight forward. There are also two entries in the .AIR file that will move the leading edge of the MAC: 1204 and 1515. I love these two entries because you can tweak the MAC to where it is supposed to be.First, you'll need to go to the TCDS for the FAA and figure out what the MAC should be. I'm pretty sure that all of the aircraft in the A319, A320, A321 family use the same wing. I'm guessing this because the data given in the TCDS has that information in the :Data pertinent to all models section. It reads: Reference Datum (where everything on the aircraft is measured from) is 100 inches in front of the fuselage nose. The MAC is located 700.85 inches aft of that point and it is 165.1 inches long.Next you can get a top-down three view drawing from the Airbus site: just look up the Airport Planning Documents for the aircraft. After you scale the photo (it's a little different after you print it out versus what the scale is depicted on the page), you can then measure out the MAC and find it on the wing.Fill out entries 1204 and 1515. The MAC leading edge position entry in 1515 is measured from the reference_datum_position in the .cfg. (positive aft of the reference point and negative ahead of it). I usually always set this reference_datum_position point at the nose unless I have hard data such as this case. It's a lot of work converting every other position in the .cfg but I like it. The aerodynamic center entry in 1204 is a very hard entry to find and compute. But as a general rule, most airfoils have this point between 25% and 28% of the chord. The true data is proprietary for Airbus and you'll never find out legally. It is important not to have it too far back or else the aircraft won't rotate properly without over-doing the elevator and horizontal tail entries in the 1101 section of the .air file. Those trim settings you have are great, but only will work if you have everything located in the correct place. Taper ratio, in case you don' tknow is simply the tip chord divided by the root chord. The .cfg entry that locates the main wing apex is also critical.I'm going into all of this because a lot of freeware I see isn't even close when it comes to the most important aspect: weight and balance. It seems some people just go for the trial-and-error to make the plane fly approach. There are a lot of good freeware out there, don't get me wrong, but this aspect of the aircraft is critical.Also critical, after locating the MAC and the wing apex in the correct place, is the empty weight CG location. This is the CG of the aircraft itself as it sits without passengers, bags, cargo and fuel. How you find this is darn near impossible as far as hard data goes. I personally research through Just Flight videos (sometimes you can catch a glimpse of the W/B charts. If you know what the loaded aircraft CG is, you can reverse engineer from there), Google, and even the friendly guys over at PPRUNE tech forums.Once you put everything in the proper place of where it goes, you should be able to get a good CG reading that makes sense. I once saw a freeware aircraft whose loaded CG was -55% but it flew stable. That should fly stable in real world ops. LOL.But please understand that having all the weight and balance correct in the .cfg and the .air entries will not ensure a well-behaved airplane. You must set the coefficients in the 1101 section correctly... specifically the lift and pitch sections. The pitch moment coefficient (the first entry) in the pitch section is the main wing pitching moment coefficient. This, along with the H. stabilizer, elevator, and even the pitch trim moment all come into play as far as what trim is necessary for each phase of flight. Typically, cruise trim is just a very minute positive trim which, in real life, would minimize drag. On take-off you need some help to rotate the aircraft; which is why you need a lot of positive trim.Personally, I set the main wing pitching moment first, then the h. stabilizer lift and moment, then the elevator lift and moment, then if I need to, tweak with the trim moment entry in the 1101 pitching section. To set the H. tail moment, I guess at a lift number and use the following equation from Dynamics of Flight by Etkin and Reid: Vh= ("l sub t" * "S sub t") / (MAC * S) Where "l sub t" is the distance between the main wing aerodynamic center and the H. tail aerodynamic center. "S sub t" is the H. tail area. MAC is the main wing aerodynamic chord. and S is the main wing area. Cm(tail) = -Vh*Clt+Clt*(St/S)*(h-hn) Where: Clt is the H. Tail lift (remember to divide what you put into 1101 by 2048). St is the h. tail area. S is the main wing area. (h-hn) is the distance between the empty weight CG and the Aerodynamic Center of the main wing. That's a good example of the research you need. It's a large undertaking to say the least. I"ve been at this for over 5 years now and I'm just getting a good understanding of how all of this works together. Any questions, just let me know.Scott B.

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Oh, and once you start moving CG's around, I've never run into a aircraft that didn't become very unstable on the landing gear. So be ready for that! :(

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Just a note of caution. Air file entries 1204 and 1515 are now obsolete with FSX and aren't used in default aircraft.

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Ah! Thanks! Most of my work is in FS9 for now. So how does FSX calculate the MAC and it's position?

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Just a note of caution. Air file entries 1204 and 1515 are now obsolete with FSX and aren't used in default aircraft.
Both records are still recognized and supported by FSX despite being marked as obsolete.

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In case you are not aware, the CG is measured along the Mean Aerodynamic Chord of the wing. That's pretty basic knowledge but I wanted to put that out there first. FS9 calculates the MAC from the information in the geometry section of the .CFG: Also pretty straight forward. There are also two entries in the .AIR file that will move the leading edge of the MAC: 1204 and 1515. I love these two entries because you can tweak the MAC to where it is supposed to be.First, you'll need to go to the TCDS for the FAA and figure out what the MAC should be. I'm pretty sure that all of the aircraft in the A319, A320, A321 family use the same wing. I'm guessing this because the data given in the TCDS has that information in the :Data pertinent to all models section. It reads: Reference Datum (where everything on the aircraft is measured from) is 100 inches in front of the fuselage nose. The MAC is located 700.85 inches aft of that point and it is 165.1 inches long.Next you can get a top-down three view drawing from the Airbus site: just look up the Airport Planning Documents for the aircraft. After you scale the photo (it's a little different after you print it out versus what the scale is depicted on the page), you can then measure out the MAC and find it on the wing.Fill out entries 1204 and 1515. The MAC leading edge position entry in 1515 is measured from the reference_datum_position in the .cfg. (positive aft of the reference point and negative ahead of it). I usually always set this reference_datum_position point at the nose unless I have hard data such as this case. It's a lot of work converting every other position in the .cfg but I like it. The aerodynamic center entry in 1204 is a very hard entry to find and compute. But as a general rule, most airfoils have this point between 25% and 28% of the chord. The true data is proprietary for Airbus and you'll never find out legally. It is important not to have it too far back or else the aircraft won't rotate properly without over-doing the elevator and horizontal tail entries in the 1101 section of the .air file. Those trim settings you have are great, but only will work if you have everything located in the correct place. Taper ratio, in case you don' tknow is simply the tip chord divided by the root chord. The .cfg entry that locates the main wing apex is also critical.I'm going into all of this because a lot of freeware I see isn't even close when it comes to the most important aspect: weight and balance. It seems some people just go for the trial-and-error to make the plane fly approach. There are a lot of good freeware out there, don't get me wrong, but this aspect of the aircraft is critical.Also critical, after locating the MAC and the wing apex in the correct place, is the empty weight CG location. This is the CG of the aircraft itself as it sits without passengers, bags, cargo and fuel. How you find this is darn near impossible as far as hard data goes. I personally research through Just Flight videos (sometimes you can catch a glimpse of the W/B charts. If you know what the loaded aircraft CG is, you can reverse engineer from there), Google, and even the friendly guys over at PPRUNE tech forums.Once you put everything in the proper place of where it goes, you should be able to get a good CG reading that makes sense. I once saw a freeware aircraft whose loaded CG was -55% but it flew stable. That should fly stable in real world ops. LOL.But please understand that having all the weight and balance correct in the .cfg and the .air entries will not ensure a well-behaved airplane. You must set the coefficients in the 1101 section correctly... specifically the lift and pitch sections. The pitch moment coefficient (the first entry) in the pitch section is the main wing pitching moment coefficient. This, along with the H. stabilizer, elevator, and even the pitch trim moment all come into play as far as what trim is necessary for each phase of flight. Typically, cruise trim is just a very minute positive trim which, in real life, would minimize drag. On take-off you need some help to rotate the aircraft; which is why you need a lot of positive trim.Personally, I set the main wing pitching moment first, then the h. stabilizer lift and moment, then the elevator lift and moment, then if I need to, tweak with the trim moment entry in the 1101 pitching section. To set the H. tail moment, I guess at a lift number and use the following equation from Dynamics of Flight by Etkin and Reid: Vh= ("l sub t" * "S sub t") / (MAC * S) Where "l sub t" is the distance between the main wing aerodynamic center and the H. tail aerodynamic center. "S sub t" is the H. tail area. MAC is the main wing aerodynamic chord. and S is the main wing area. Cm(tail) = -Vh*Clt+Clt*(St/S)*(h-hn) Where: Clt is the H. Tail lift (remember to divide what you put into 1101 by 2048). St is the h. tail area. S is the main wing area. (h-hn) is the distance between the empty weight CG and the Aerodynamic Center of the main wing. That's a good example of the research you need. It's a large undertaking to say the least. I"ve been at this for over 5 years now and I'm just getting a good understanding of how all of this works together. Any questions, just let me know.Scott B.
In other words, messing with the CoG will just give you a world of headaches :)Nice explanation though Scott.

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Thanks Anthony. It's difficult to try to briefly explain. :) And thanks to WarpD as well. That's good to know. A good deal of airliners are not simply trapezoidal wings, but double delta wings (the inboard wing section has a different taper ratio than the outboard portion) and the MAC just can't be placed using the simple formula for trapezoidal wings. Those two entries in the .air file really help to set the plane up correctly.

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FSX seems to calculate MAC according tohttp://www.nasascale.org/howtos/mac-calculator.htm & cmEI.exeAccording to Aircraft Airfile Manager V2.2:Record 1204: All of this data are overwritten by AIRCRAFT.CFG, section [airplane_geometry] or AIR file, record 1534 (Wing Center of Lift Offset). Probably meaningless in FS9FS9: Beech King Air, Boeing 747, Cessna 172 and all historic a/c don't use this record.CFS3: not usedRecord 1515: Sets FS variables for CoG gauge, etc.FS2002: Sweep, Root Chord, TaperThis data are overwritten by AIRCRAFT.CFG, section [airplane_geometry].FS9: used only by Cessna 182 and 208, Extra 300, Mooney Bravo, Learjet, DH Dash-8, ConcordeCFS3: not usedI have an ATR-42-500 which has both records. However the entries in 1515 are all set to zero and it still flies realistically. I think there is still uncertainty about the actual effects of these records in FSX.

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Hello guys and thanks for the support. I will start with the steps described above and maybe be able to create a good weight and balance section for the a320.

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For FSX, the FSUIPC offset 0x2EF8 returns the position of the calculated Centre of Gravity (CG) as a fraction of the Mean Aerodynamic Chord (MAC). FSUIPC values can be read using FSInterogate for a running FSXAs an experiment , I took the ATR42-500 (the only aircraft I can find with records 1204 and 1515). With the default value of 106 in MAC the CG position was -0.15244. I used Aircraft Airfile Manager to change this to 0 in. I then, loaded another aircraft before reloading the ATR42-500. The CG position was unchanged at -0.15244.This demonstates that changing the MAC in the .air file does not change the MAC calculated by FSX.I suspect the reason is the the CL and CM curves in records 404 and 473 are now for the complete aircraft - not just the wing (as I believe was the case pre-FS9). In this case MAC no longer has any significance other then as an arbitrary reference datums. The position of the Centre of Lift (CoL) of the complete aircraft is given as fraction of this reference chord by CM/CL. If the MAC is changed then CM should be changed to reflect this.

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I use 1534 in the ESDG Citation X v2.0 because of it's supercritical wing design. It definitely has an effect.Also, I don't believe setting values to zero is a valid test as there's no reason to believe that FS doesn't have code built in to deal with zeroed (missing) values.

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I use 1534 in the ESDG Citation X v2.0 because of it's supercritical wing design. It definitely has an effect.Also, I don't believe setting values to zero is a valid test as there's no reason to believe that FS doesn't have code built in to deal with zeroed (missing) values.
We are discussing the effects of records 1204 and 1515, not 1534.I changed the MAC in record 1204 from 106 in to 212 in. The position of the CG as calculated by FSX is still unchanged ar -0.15244. Based on this evidence, the conclusion is that changing the MAC has no effect in FSX. As I suggested earlier, there are reasons because the way FSX deals with lift and pitching moment. MAC is defined as the chord of a rectangular wing, which has the same area, aerodynamic force and position of the center of pressure at a given angle of attack as the given wing has. However, FSX no longer includes CL(α) or CM(α) for the wing - only for the complete aircraft..

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We are discussing the effects of records 1204 and 1515, not 1534.
No, I was discussing the fact that despite things being marked as 'obsolute'... that doesn't mean they don't work. Which is your conclusion.Having discussed this with people who are more knowledgeable regarding the 'insides' of FSX than any of us in these forums... I can tell you that you are right... and wrong. Some things that shouldn't work, do and vice versa... so the accurate conclusion is that things aren't as expected nor can things be dismissed 'carte blanche'. I am currently working to get a more accurate list regarding what does and does not work. It will take some time.

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What I originally said was and is correct.

Just a note of caution. Air file entries 1204 and 1515 are now obsolete with FSX and aren't used in default aircraft.
I've subsequently shown that changing MAC in the .air file does not affect the calculated position of the CG on the MAC in FSX.

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Hello guys and thanks for your support. So if i want to change the mac of an aircraft which values i must change in the .air file and in the aircraft.cfg? Many thanks in advance.

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