centre of gravity in cdu

[telly53]

How does one get a figure to enter in the cdu page for centre of gravity?

[dodiano]

That depends on the plane actually! But for example on the A-320 25.0 is like a mean for the C.G.... But actually that is determined when they load the plane!

Regards,

Roberto

[telly53]

yeah i was wondering if the figure is diplayed in fs, or does one need an aftermarket payload/fuel loader program to show it.

i just often wonder how much it affects the performance of the aircraft in flight. i also wonder if the top of climb temp and toc wind speed/direction should be worried about either.

anything to help the pm cdu from going bannanas !!

sean.

[David Rogers]

It's a good point, I too am a little uncomfortable about never touching this setting...

Certainly not something you could ignore on Concorde, or even Mike's 727 ;-)

.......but in the scheme of issues with the Boeing PM CDU, it's not too high on the list!

[dodiano]

Ok on the real plane at least on the Airbus and I must think it is the same on the Boeing all performance calculations made by the plane which affect fuel consumption, estimates, Optimum Levels and all that, are taken from the Loading and performance Data you insert into the FMGS so it will deffinatelly affect the performance on the plane... But again in real life is a bit more crittic as well as the data is made by a dispatch center... I would advise to get a dispatch proggram that helps you with that stuff!!

Regards,

Roberto

[JonathanRichardson]

Hi

Actually, the CofG and weight and balance is a very important factor for all accurate simulated flight. Whilst it is not important for the PM CDU itself, loading the a/c correctly, entering the Empty Weight and payload weight into the CDU type.txt file is very important for correct perfromance data. But aside from that, if the CofG is wrong or too far out of limits it makes the life of the MCP far more difficult to keep accurate control of the a/c especially with regard to the vertical modes with the AP engaged and without the FD indications - it will try and compensate for errors - but usually this results in much greater trim changes than should normally be used giving an impression of instability. If it is really bad, it can result in overshoots and undershoots of target altitudes. Also, for accurate T/O if the W&B is done correctly and you have correct elevator trim for your a/c model type (usually you have to run tests) but once you have determined a correct W&B / CofG with given payload and fuel, it makes Take-offs much more accurate and the FD will give you very accurate information to fly without having to make big trim changes or corrections with the elevator directly after T/O. It is an area often not paid enough attention. It is however all flight model based - it will only effect PM if you are out of limits - or degrade performance the further you stray from those limits.... I added a section about this to our docs which will be released in the next version.

Regards
Jonathan Richardson

[Michael Carter]

In the 727 it's all in the weight and balance tables. There is a summer table and a winter table. Weight is figured for each and every passenger in every seat and row.

After the weight is figured and the CG determined as a % of MAC, it in turn is used for the trim setting.

Unfortunately, not many people have a 727 POH available to them (not that they'd spend upwards of an hour to use them if they did), so Dreamfleet included a small MAC guage on the engineer's lower panel.

This can be used directly to figure a trim setting based on flap setting.

Performance for FS aircraft, or at least the DF727 can and does change with a shift of CG and/or weight. After installing the DF727 on the new system, I had forgotten that the default pax load was set to full with a full forward and aft cargo load. I couldn't figure out why my canned flight plan settings were giving me problems on approach even with the correctly calculated fuel load.

The aircraft felt 'dumpy' and my flying technique for the previous load and CG wasn't cutting it. Stabilizing an ILS approach was an exercise in futility. after resetting the previous pax and cargo load, the aircrat once again handles as it did before.

No matter how much fuel is loaded aboad the 727, the CG change is negligable, but those pax and cargo have a huge impact.

[JWS]

Hi

Also, for accurate T/O if the W&B is done correctly and you have correct elevator trim for your a/c model type (usually you have to run tests) but once you have determined a correct W&B / CofG with given payload and fuel, it makes Take-offs much more accurate and the FD will give you very accurate information to fly without having to make big trim changes or corrections with the elevator directly after T/O. It is an area often not paid enough attention. It is however all flight model based - it will only effect PM if you are out of limits - or degrade performance the further you stray from those limits.... I added a section about this to our docs which will be released in the next version.



Jonathan, that's good to hear. Hope you fill us in on how to enter correctly the right data. I usually press the line select key and hope it presents the weight & fuel numbers that are in FS itself, but I'm not sure about this.
Does the weight changes automatically when I select in FS another payload?

Grtz,

JWS

[JonathanRichardson]

>Does the weight changes automatically when I select in FS another payload?

Hi

No, the CDU needs to know two bits of information that must be added to the type.txt manually by the user. These are the Netweight (emptywieght) of the a/c model you are using and the payload. The CDU will read the fuel load as in the real a/c automatically. It is very important to get this info from the FS airfile and add it to the CDU type.txt. If you change a/c models, then you must update your type.txt file weights otherwise all your perfromance data is not going to be correct. Note that this data must be entered in LBS in the type.txt.

Regards
Jonathan Richardson

[JWS]

Thank you, Jonathan.

BTW, I read another thread about trimsettings and the difference between the CDU and the GC trim scale. Maybe I've overlooked something but where do I find in the (737) CDU the trimsetting?

JWS

[JonathanRichardson]

Hi

You need to put in the CofG position into the CDU and it will give you the take-off trim. Typical values are around 25% but it depends on the way the a/c is loaded. The CDU does not read the MSFS CofG position. Typical take-off trims for a normal weight 737 are between 3.5 and 4degrees of up trim. Again, testing has to be done re your flight model.

The trim scale indication on the GC has to be set as per the txt info in the type.txt for these a/c types for it to match. Note that the trim setting in MSFS is totally different to real a/c due to the fact that it goes into negative values. Where as on a real a/c (just an example for 737) the scale goes from 0 to +15. So, some careful work has to be done to get all these things to match-up if you want to also use the EICAS GC trim indication.

Regards
Jonathan Richardson

[JWS]

Hi

You need to put in the CofG position into the CDU and it will give you the take-off trim. (...) So, some careful work has to be done to get all these things to match-up if you want to also use the EICAS GC trim indication.

Regards
Jonathan Richardson


And how all this has to be done will be revealed in the new documents that will be released in the near future, I guess. I'll wait and see.

Thanks,

JWS

[JonathanRichardson]

Hi

Not really, I only discuss weight and balance that needs to be observed. You put in the CofG in terms of a percentage in the CDU take-off page... you can do this without new docs, the doc part is only to draw attention to this, because I visisted a few sims where people were complaining about bad FD and AP control and it turned out they were flying around without paying any attention to weight and balance at all - things were very badly balanced and they were pointing blame in the wrong place. This goes for pro level sims as well as hobby level sims. So, I just added a section. You have been able to add the CofG percentage in the CDU for a long time, arriving at the percentage though is another matter, for normal a/c it is 25% but swept wing a/c it is different. If the load is more or less correct, you should have a take-off trim of 3.5 to 4.5. You can get an idea of whether this correct by simply performing a few tests, after rotation, if the a/c climbs with a good pitch to FD indication (i.e. you are not having to make large [or perhaps any] initial trim changes with a specific take-off power setting in TOGA, then you know the a/c was basically in balance and you had a good take-off trim setting for this payload and fuel weight. The idea is to make notes with constant factors, and then keep the payloads and fuel the same - if you want to fly with different payloads and fuel, then you have to make your own tables based on your tests re the trim settings etc. For the sim here, we have one set of weights that we fly with most of the time.

Right now, the CDU does not automatically read the CofG position and so you need to enter it if you want the trim indication *but* it is really best to arrives at your own actual values through testing because the CDU system is only a simulation and not really knowing the actual CofG, we might add it automatically reading the payload for the future which has already been discussed, perhaps the next step will be to try and read the CofG and to see if it does provide accurate information.

Regards
Jonathan Richardson

[JWS]

Hi

You have been able to add the CofG percentage in the CDU for a long time, arriving at the percentage though is another matter, for normal a/c it is 25% but swept wing a/c it is different. If the load is more or less correct, you should have a take-off trim of 3.5 to 4.5. Regards
Jonathan Richardson


Jonathan,

:o Didn't know that this was possible. Always thought it was something for future developments. A 3.5 to 4.5 % that is (?). OK, thx. See what can do about it.

JWS

[michelmvd]

Hi Jonathan,
Thanks a lot for all the info regarding de CoG. I really didn't knew this was implimented already in the PM cdu to that level. Will check this out and compare in the next days.
Certainly in the B744 with his huge weight differences trim settings are extremly important.
B. rgds
Michel

[JonathanRichardson]

Hi

Just to confirm, it has been in the CDU software for over 1.5yrs now I think, but if you read my posts, it is a simulation, you need to accurately test your trim settings with reference to your weight and balance because the CDU does not read the actual MSFS CoG (yet).

Regards
Jonathan Richardson

[michelmvd]

Okidoki, thanks for further clarification.
B Rgds
Michel

[JWS]

Hi Jonathan,
Will check this out and compare in the next days.
Michel


Michel,

can you keep us posted about the results, please?

Grtz,

JWS

[michelmvd]

Sure I will, but looks not so easy as it is als airfile/ model related.
B Rgds
Michel

[Jan Pemöller]

Here are my results:
PMDG 737-800 Winglets 27% is ok on my SIM