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luisgordo
01-10-2008, 07:28 PM
Hi folks,

I am trying to figure how the Trim is set for take off. I know it depends on where the Center of Gravity is located once the a/c is loaded, but I'd like to know:

1.- How is the trim setting calculated based on CG?
2.- How is it inputed (via MCDU or via the Trim wheels?)
3.- How is this done with PM software?

Many thanks!

Michael Carter
01-11-2008, 01:25 AM
In the old days, your trim setting was based on a percentage of Mean Aerodynamic Chord and then figured to an amount of trim measured in units based on a chart in the aircraft's POH.

Quite often was pre-figured by the flight dispatcher and signed off by the crew.

It is included in the POH for the 727. Not sure about newer aircraft. I'm pretty sure it just involves pushing the right buttons.

luisgordo
03-12-2008, 04:56 PM
Hi folks!

I uploaded an excel spreadsheet I made for calculating weights and CG for the A320.

http://www.mycockpit.org/forums/downloads.php?do=file&id=179

You only have to input the values in BLUE. Everything else is automatically calculated.

The method is exactly the same as the official Airbus Load and Trim Sheet, but I managed to avoid the use of the graphs by adjusting to ecuations instead.

I compared results obtained with the official (graphical) method and all is OK. If you use it, please let me know if you find anything wrong or if you have any improvement suggestions.

As for the use in our simulators, if you use Project Magenta, I am not quite sure what the MCDU does to this respect, because independently of the payload, the ZFWCG is always the same, and the Trim position for take off is always the same too (DN1.5). Can anyone shed some light on this?

Hope the file is useful.

pdpo
03-13-2008, 07:42 AM
Hi there,

I used the load sheet proposed above and it worked ... wow...

Concerning the PM behavior. I had too that PM sets the trim to 2.1 UP until I passed the
thrust reduction altitude. Put this in my issue list which I communicated to Enrico. He replied
me that I should set a parameter in the INI file : togatrim to 0. This results now in the fact
that the trim isnt set by PM anymore, I think.
So before using the load sheet, I had loaded the plane more forward to get the plane stable during takeoff.
This sheet of course let me to load the plane normally and suggested DN0.6 for the takeoff
trim. During taxi I saw the trim going from DN0.6 (manually set) to 0 but I had the fscommunicator set also the trim to 0 on ground. Now I have switched this off again and need
to observe. I'm not sure it isnt PM setting it still to 0.0.
Anyway, I added now in my checklist just prior to takeoff to check the trim setting again.

Greetz Peter

luisgordo
03-13-2008, 03:44 PM
Hi Peter,

I have the same issue with the Trim setting, only that I always get DN1.5 as the setting PM always fixes. Not sure if this is commanded by the MCDU or by the FCU. Will try to set the TOGAtrim to 0, and see what happens. I too have FSCommunicator setting the trim to 0 on the ground, so I guess that will have to be turned off.

I will post back once I get the chance to test.

Glad you liked the speadsheet. As I said, if you find any glitches, please let me know.

pdpo
03-14-2008, 02:50 AM
Hi Louis,

now that I have unchecked the trim setting to zero on ground I did not notice anymore a change in the trim setting during taxi so PM does not reset it anymore with the togatrim in the ini file.
I'll contact robert fischer from fscommunicator to check out which are the triggers to reset the trim to 0 on ground. The trigger should be something which is unique for the situation were the plane just landed.

greetz Peter

luisgordo
03-14-2008, 10:40 AM
OK Peter. Good to know. Thanks for the update.

Please let me know about Robert Fischer's reply.

luisgordo
03-14-2008, 01:02 PM
I got the chance today to test the Trim setting (cancelling any input from PM and FS COMM), with satisfactory results. However, there is the issue about setting the Trim to 0 once the aircraft lands. If this is not done, the aircraft will tend to get back up in the air upon touchdown. I can't remember now if there is another setting within FSCOMM that can handle this situation.

I guess this could also be done with pmSystems some way (we would have to find a suitable FSUIPC offset to identify "touchdown"). If not, we could always set the Trim to 0 when the aircraft is on the ground (only if the aircraft has been in the air first).

As I said, let me know if you get some feedback from Robert.

pdpo
03-17-2008, 04:21 AM
Louis,

I made some other flights and loaded my project airbus A320 in some more realistic ways.
This made the CG move more aft then before. The sheet suggested DN0.9 so I set this
but on takeoff, at Vr, I had to pull the stick fully backwards to get the nose starting to
move up. After lift off FS communicator trimmed out the plane for steady climb and the trim was then around 0.2 DN... so some difference ....
I looked at the sheet and there is one value i dont know how to fill in. On the upper left side there is the dry weight of the plane and the % CG? What value do we put there?
In AST ECAM I see when I fule only 500 kg in the plane and no payload at all a CG of 6%
... in the air file I found something like ZFWCG 31%...
Any light on this?

Greetz Peter

PS : I had the FS communicator trim set to 0 back on an I had just the opposite effect...
during approach still trim DN and at touchdown trim set to 0 and plane wanted to
leave the runway again....
still need to ask Robert about this

with the CG more to aft I have some issues with project magenta which seems to have issues
keeping the aircraft stable at higher altitudes. It tends to oscillate in cruise level.
with the weight more forward PM has no problems at all

luisgordo
03-17-2008, 04:42 AM
Hi Peter,

I still keep testing too. What flight dynamics and plane model are you using? In order to have the LOAD and TRIM sheet give accurate calculations, the loading of the plane have to match the settings in FS. Probably, the most difficult value to know is as you say the empty plane %CG shown in the upper left part, next to the dry weight.

The dry weight is the weight of the aircraft with no fuel or payload. This is shown directly in FS. The %CG shown beside this value on the sheet is the corresponding position of the CG with the aircraft empty. My findings is that this value is about 24-25%. However, we should be able to find this somewhere in FS for the plane selected (theoretically, if you remove all payload and fuel, the graphical position of the CG shown at the right side -talking about FS9- would be that %CG value). Not sure how to get this one.

Right now, I am using a combination of Project Airbus + Pedro Oliveira's flight dynamics (as this is the recommended FDE to be used with FS Communicator). This has given me the best results in terms of performance. Though I still have some issues.

You are right about the problem when landing, and I have it too: since the Trim is not set to 0 automatically, the plane tends to want to lift again...

(One important thing here, which I have not paid attention to in the past, is not to land above the MLW, which is around 64 tons for the A320, in order to keep things balanced)

At the same time, I am not sure that PM handles all these parameters correctly. For instance, when I load the plane in FS to have a ZFW of 56.7 tons, the MCDU INIT B page show up a ZFW of 58.7, hence not matching the value of FS9. Also, the ZFWCG is not calculated, I think.

So yes, I still have issues too. I guess we need to find the best combination of model and flight dynamics to suite our needs. Also some PM issues may need to be addressed to this respect.

luisgordo
03-17-2008, 04:56 AM
By the way, I just found this in the "Getting to Grips with Weight and Balance" Airbus official documents:

http://img.photobucket.com/albums/v289/luisgordo/homecockpit/wandbsummary.jpg

pdpo
03-17-2008, 09:33 AM
Hi Louis,

I found on avsim the FDE by Pedro version 12. This is the one?
If yes, this is made for PSS visual model . Anything needed for integrating this with project airbus? (example : in the LIGHT section, refering to PSS effect files,...)
I see also in the aircraft.cfg that this FDE has 10 load stations. The project airbus has the same load stations as in your load sheet. How to map these two different load stations then?
(the laod stations of the sheet - the load stations in the aircraft.cfg file)

Greetz Peter

luisgordo
03-17-2008, 12:19 PM
Hi Peter,

That is one, but as you say it is optimized for the PSS model. The other FDE from Pedro Oliveira is found in the IFDG Airbus models. It is optimized for use with them. However, I do not get good performance when landing with those models (the plane tends to float and climb again instead of flaring). Have you tested with iFDG?

I am using now a simple merge of the FDE with the Project Airbus model (I substitued the Project Airbus file with this one, and changed reference to it in the .CFG file.) It works quite well taking into account the boldness of the solution, but it lacks some power on climbs.

Still looking for the best solution. It looks like we have to stick with Pedro Oliveira's FDE since FS Communicator is optimized for it. The iFDG model would be the best solution, but as I say, landing performance is far from good.

As you say, when you encounter a model that does not stick to the A320 real compartments, it gets tricky to load the aircraft and configure with the LOAD and TRIM sheet. The Project Airbus and the IFDG models are configured in FS quite similar to the real thing.

I am thinking about another option, which would be to use the PSS model (removing the advanced panel, as it will interfere with PM) and test it. This has the inconvenient that you mention about the payload compartments not being modeled as the real ones. Also the visual model is not so appealing as the Project Airbus and iFDG ones (personally I prefer good performance than nice visuals anyway, but also something to consider).

I will keep testing. Let me know about your progress.

pdpo
03-19-2008, 03:54 PM
Hi Louis,

this trim setting has kept me thinking and finally I cam up with the following

We can actually get from FSUIPC offset 2FE8 the %CG directly and this is calculated by FS
for whatever model and aircraft.cfg file used.
Then I have looked at your trim sheet and printed the TakeoffCG/TrimPos scale on paper,
measured a little and came up with following calculation (seems rather linear) :
trimsetting = -0.220264 * %CG + 6,268722
I have put this in my SIOC code and show the %CG and the calculated trim setting on my
radio panel when I set a hidden switch in the cockpit in his setup mode. (setup mode means
I can steer the refueling and now also the trim/CG via the radio panel)

The only thing I havent done yet is finding out the ZFWCG to fill in to the MCDU.
I can off course empty the plane of fuel, note the CG that fsuipc gives and then fill up the plane with fuel again....

I'll keep you posted .....

Greetz Peter

luisgordo
03-19-2008, 08:22 PM
Hi Peter,

Thanks for your update. I've realized your adjustment of the trimsetting vs. %CG will give a negative value for DOWN, and a positive for UP (according to the scale). I think it is the other way around (at least that is what is shown in the PM lower ECAM). Hence, it would be trimsetting=0.220264 * %CG - 6.268722.
Maybe I am wrong, but we should check this out.

I have used FS Interrogate to check the FSUIPC offset you mention. It seems to give the %CG for the conditions of the plane (payload and fuel loading). However, this value (per my findings) never matches any of the values calculated on the official A320 LOAD and TRIM sheet. Nevertheless, the trim setting obtained seems to give good results (haven't tested much though).

Good idea that of implementing the calculation based on that offset in SIOC. I could do the same thing in pmSystems if this proves to work. By the way, the aircraft can be refueled via the MCDU INIT B page (setting the block value will change the fuel of the aircraft).

I am not quite sure about what PM uses the ZFWCG in the MCDU for (if it is used at all).

What model and flight dynamics do you normally use in your setup?

pdpo
03-20-2008, 04:02 AM
Well,

since the load sheet corresponds with the project airbus 320 .. I started using that one...
however, if this sytem seems to prove working fine I might change again to my ifdg model of the A319 since this is the plane for brussels airlines VA and the A330 of posky.

Also I'll check out that latest FDE V12 by pedro of the A320 with the visual model as suggested by PSS (got PSS). I think that this FDE should be closest to reality in matter of fuel use and handling....

Greetz Peter

luisgordo
03-21-2008, 06:58 PM
I have tested the use of the latest FDE v12 by Pedro Oliveira with the visual PSS model (got PSS too). It works great with PM and FS Communicator (works better for me than the ifdg models). The PSS IAE version however, does not work nearly as well as the CFM.

Now I have to dig in to the CG aspect of it.

Will report back.

I see you talk about the A330. I have thought about doing this as well, but have never really got to try. Any particular recommendation for the A330 model / FDE ?

luisgordo
05-13-2008, 02:33 PM
Hi Peter,

have you gone further into investigating if the CG given by FS gives good results using the Trim formula you stated?

I tried implementing this in pmSystems, but it seems it does not handle Floating point variables, so couldn't automate the calculations...

If this is finally working well, I may program a little code in Visual Basic to get the calculations done by interfacing with FSUIPC. Otherwise, I may wait until I buy some Opencockpit cards (which I probably will in the future) and program it in SIOC code as you have done...