01-10-2008, 06:28 PM #1
Horizontal Stabilizer Trim Setting
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?
01-11-2008, 12:25 AM #2
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.Boeing Skunk Works
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03-12-2008, 04:56 PM #3
I uploaded an excel spreadsheet I made for calculating weights and CG for the A320.
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.
03-13-2008, 07:42 AM #4
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.
03-13-2008, 03:44 PM #5
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.
03-14-2008, 02:50 AM #6
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.
03-14-2008, 10:40 AM #7
OK Peter. Good to know. Thanks for the update.
Please let me know about Robert Fischer's reply.
03-14-2008, 01:02 PM #8
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.
03-17-2008, 04:21 AM #9
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?
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
03-17-2008, 04:42 AM #10
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.
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