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DOW CG


Neil Warren

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Neil Warren

Hi,

Please could one of the developers tell me what the dry operating weight centre of gravity is. I believe this is a fixed figure? and I would like to try using it with regard to planning etc. I know the cg is all calculated automatically for us. However I would really like to have a go myself :)

 

Cheers

 

Neil

 

 

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David Katona

Hi, I had the same idea, and actually you can find all the information you need in the aircraft CFG.
REFSTA, LEMAC and Length of MAC are also there, as well as the H-arms... I just put those figures in an excel, copied the take-off figures from an FCOM I have, and easily got myself my own EFB. The data I am using in there is not something I can share unfortunately, but as for the calculations, let me know if you need help.

IMG_0031.thumb.JPG.a3eef638a6d161188c870abd974b7c66.JPG

 

Regards,

David

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Andrew Hansen

I have been wanting to review and tweak where needed the weights and moment arms in PFPX and TOPCAT to align and completely match up with the bus. Ideally I would like to get the same CG and trim value from TOPCAT that comes out of the MCDU. I had started this by updating weight based on other users who post online here but like your suggestion to just look in the aircraft.cfg file, so that's what I did and I think I am seeing numbers that are off and wanted to see if folks could verify what I was seeing.

I am looking at the CFM variant of the aircraft.cfg and this is what I see for MGW and empty weight, which I think should match up to the MTOW and DOW in TOPCAT's A320 config file.

 

*weight in LBS*

Looking at the attached images...

Does that seem correct? If so I should change the TOPCAT DOW and MTOW values.

Since I don't see MZFW, MLW in the aircraft.cfg file, are the values that I have in the TOPCAT file correct?

 

Next I would like to check the CG datums. I see empty_weight_CG_position, which might line up with the empty_weight_CG_position. Does that seem right? Looks like these are way off. Would I want to change the -0.29 to -0.61.93897647?

 

What about all the stations datums, PAX, Cargo, Fuel, Crew these all seem to be different and I am not sure they should match exactly.

aircraft.cfg file

aircraft_cfg.png

 

TOPCAT file

TOPCAT 01.png

TOPCAT 02.png

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Are you sure in the value of the DOW you are using? I've been testing the weight values in MCDU and PFPX and I came to the conclusion that DOWs are 42841kg (94448lb)  and 42833kg (94431lb) for CFM and IAE engines correspondingly.

In order to calculate these values I have set the payload to 0 passengers and 0kg cargo with 6 crew members. In this case ZFW shown in MCDU is 42.8t for both CFM and IAE engines. Adding 9kg cargo to CFM and 17kg cargo to IAE A320s resulted ZFW values changing to 42.9t.

The calculated value of crew member and passenger weight was found to be 84kg.

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Andrew Hansen



Are you sure in the value of the DOW you are using? I've been testing the weight values in MCDU and PFPX and I came to the conclusion that DOWs are 42841kg (94448lb)  and 42833kg (94431lb) for CFM and IAE engines correspondingly.








In order to calculate these values I have set the payload to 0 passengers and 0kg cargo with 6 crew members. In this case ZFW shown in MCDU is 42.8t for both CFM and IAE engines. Adding 9kg cargo to CFM and 17kg cargo to IAE A320s resulted ZFW values changing to 42.9t.








The calculated value of crew member and passenger weight was found to be 84kg.




Nope I am not sure, that's one of the reasons I was asking about it here. Since I saw conflicting numbers which differed from what others had posted on the forums. It makes sense to me that the CFG file should be the source of information though and I'm inclined to trust those values over what other state but if the MCDU doesn't end up matching after updating values from the CFG file then that won't either.
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34 minutes ago, sinandgrin said:


Nope I am not sure, that's one of the reasons I was asking about it here. Since I saw conflicting numbers which differed from what others had posted on the forums. It makes sense to me that the CFG file should be the source of information though and I'm inclined to trust those values over what other state but if the MCDU doesn't end up matching after updating values from the CFG file then that won't either.

I've spent a couple of hours one day trying to figure out the values after I've noticed that if I use the values from aircraft.cfg in PFPX i get different ZFW in PFPX and in MCDU. Using the values that I've posted I get the exact match.

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David Katona

Philippe, once again thank you for the fuel arms. I could have used the index influence figures from the real loadsheet though, but had no idea that you simulated weight and balance so much spot on that it matches real life figures. May I ask, I see the pantry and water entries are out of range at the moment in the CFG, do you have plans to do something with those in the future or are they there for the PRO version? 

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Philippe Gleize

Thanks ! All is tied together in that project : CG is related to trim (you can expect really close to real number trim values in any phase of flight), maneuvrability etc. So no choice to stick as close as possible to reality to avoid further issues. For water it's not decided yet. We have to think about interesting effect we can do with it first. 

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  • 3 years later...

Brining this topic back from the archives if I may. I've been having a go at my own manual calculation of the weight & balance CG/ index values  - just as a little technical challenge for myself. I've managed to get to a stage where I can calculate the same CG numbers as the MCDU for everything excluding fuel (using index, H-arms and such like that David mentioned above). However I am stuck on how to calculate the CG including fuel.

I've compared the difference in index between ZFW & GW CG, but they don't match (or come close to) a delta "fuel correction index" i have for an actual A320 from an airline load & trim sheet. From having a quick look I believe it's to do with the "center2/trim tank". Are you able to provide some more info on how the fuel is distributed between the tanks shown in the aircraft.cfg file? Or maybe a different fuel correction index table if there is one?

Many Thanks!

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  • 2 weeks later...
Philippe Gleize
On 12/31/2020 at 1:55 AM, Rob Sawyer said:

Brining this topic back from the archives if I may. I've been having a go at my own manual calculation of the weight & balance CG/ index values  - just as a little technical challenge for myself. I've managed to get to a stage where I can calculate the same CG numbers as the MCDU for everything excluding fuel (using index, H-arms and such like that David mentioned above). However I am stuck on how to calculate the CG including fuel.

I've compared the difference in index between ZFW & GW CG, but they don't match (or come close to) a delta "fuel correction index" i have for an actual A320 from an airline load & trim sheet. From having a quick look I believe it's to do with the "center2/trim tank". Are you able to provide some more info on how the fuel is distributed between the tanks shown in the aircraft.cfg file? Or maybe a different fuel correction index table if there is one?

Many Thanks!

Fuel tank in FS/P3D is represented as a single dot with fuel weight. In real life fuel tanks are a 3d volume. As a consequence, when you empty a fuel tank its CG moves. It’s visible on a 320 W&B manual available on internet. We reproduced that Harm variation by using faked center tanks in aft and forward. 

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