• Which the release of FS2020 we see an explosition of activity on the forun and of course we are very happy to see this. But having all questions about FS2020 in one forum becomes a bit messy. So therefore we would like to ask you all to use the following guidelines when posting your questions:

    • Tag FS2020 specific questions with the MSFS2020 tag.
    • Questions about making 3D assets can be posted in the 3D asset design forum. Either post them in the subforum of the modelling tool you use or in the general forum if they are general.
    • Questions about aircraft design can be posted in the Aircraft design forum
    • Questions about airport design can be posted in the FS2020 airport design forum. Once airport development tools have been updated for FS2020 you can post tool speciifc questions in the subforums of those tools as well of course.
    • Questions about terrain design can be posted in the FS2020 terrain design forum.
    • Questions about SimConnect can be posted in the SimConnect forum.

    Any other question that is not specific to an aspect of development or tool can be posted in the General chat forum.

    By following these guidelines we make sure that the forums remain easy to read for everybody and also that the right people can find your post to answer it.

Load distribution and trim setting help needed

S

sxa1376

Messages
31
Country
greece
Hello guys,

I need desperetely some help because i have stuck and i can't understand what the error might be.
I am building an a320 for fsx using characteristics from real airbus charts. I have create both aircraft.cfg and the .air file . The problem i am facing is this. For the take off for example with gross weight c.g of 28.5% according airbus take off trim setting charts the stab trim must be "0". Ok so far with 0 trim and this gwcg i perform the take off and everething is fine. Now if for example i use a diferent gwcg lets say 25% according airbus charts i must put the stab trim for the take of to approximately 1 up. Now if i put this and perform the take of with the new gwcg of 25% and the stab trim to 1 up the nose will continue to pitch up until it stalls or if i use for the new gwcg the trim=0 as for the first i can take off normally again.
So my problem is it doenst matter which different load i use but each time i must use the same elevator trim=0.0 for the take off. Why this is happening? Any ideas? Many thanks in advance.
 
Are you using FSX default FBW with the appropriate entry or a custom FBW?
We tried taking the Default FSX FBW with our 5X/7X/8X but decided its best to design a custom FBW which is no walk in the park either.
I also recommend AirWrench *registered* if you do not have it already to help with your concern.
Best of Luck
 
You can also try to walk through the equations used by FSX' FDE and see where things start to go wrong (fs_flight_dynamics_v1.0.zip at Avsim).
 
The question is what is the relationship between trim in the a320 and trim in the simulator. What units are used in the A320 trim setting? The FSX trim setting units are degrees nose up or nose down and the trim range is whatever is given in the [airplane geometry]section of the aircraft.cfg.
I would be very surprised if there was a one to one relationship between A320 trim units and FS degrees.
Roy
 
I am using Airwrench registered and i have read the fs_flight_dynamics_v1.0. Also in the air file in section 320 into section "pitch trim variable limit(rad)" , i have put the value "0.16384" and in the section 1101 and into the pitch section
"Cm_dt Pitch Moment - Trim", i have put the value "-4.00". Now into the aircraft.cfg in [airplane_geometry] section i have put for the trim "elevator_trim_limit=9.387341". The reason i did this is i am using the fly by wire from project magenta which it has a range of "-16384 to 16384" so i did this in order to have correct trim indication into lower ecam (i am a cockpit builder using project magenta software for a320). The real airbus values for the trim is: for trim up=13.5 and for trim down=-4.5.
 
Page 52 of Fs_flight_dynamics tells you that section 320 is obsolete and since you have rightly made the same entry in the .cfg file the latter will override the air file entry. Basically any entry with an offset number less than 341 is either obsolete or unused.
Since I have no knowledge of the fbw system you are using I cannot comment further, sorry
Roy
 
Dear Roy,

I deleted all the unused entries from the air file and i kept only the same entries as the default fsx a321 has (it's about 70 entries if i remeber correct).
Please forget the fly by wire module, i just mention it. I have checked inside fsx each station's load alone with the "AJPC" and also i checked the fuel load alone again (i mean without any other load except the empty weight)
and i confirm that each result agrees with the airbus loading charts. Now the green trim limit of the a320 is from "-2.5 deggrees down to +2.5 deggrees up", so for the input into the aircraft.cfg "elevator_trim_limit=9.387341"
i have put into the air file to "Cm_dt Pitch Moment - Trim" section, the value "-3,7549364". So i assume that if this is the trim scale i have set this up to operate between the green trim limits?
Now i go for take off with gross weight of about 64000 kgs and gwcog of 29.5%. For this to working if inside air file into 1101 section, into the "Cmo Pitch Moment Coeff at AoA=0 (Trim os)" i put the value of "-0,0888671875"
then i can do the take off normaly with the elevator trim=0.0. Now if i choose for gross weght of about 68000 kgs, the gwcog will be 34.0% and the trim for the take off must be -1.1. Now if i go for a take off with the new numbers
when on air if i release the stick the nose drops down and to correct this i must again put the trim to 0.0. Any ideas? Sorry for my English are not good. Many thanks in advance.

George
 
The value of "elevator_trim_limit" is the physical limit of total possible movement in one direction for the elevators. It is in degrees. So if the elevator has a 20* trim up and a 10* trim down... "elevator_trim_limit" has to be set to 20*. If it isn't, you will never see correct trim for up. You are imposing degree limits that are based on loads and not on what the actual elevator is capable of obtaining with regards to trim.

Also the "Cm_dt Pitch Moment - Trim" value is first not a degrees value but rather a unitless value used to calculate the pitch moment induced by the trim position. It is a 'per radian' value and thus your value of -3.7549364 would impact the pitch moment by -3.7549364 per radian of trim angle. It is not doing what you think it is doing.
 
Ok so far, for the aircraft.cfg i will use the real airbus values and this will be:

[airplane_geometry]
.
.
.
elevator_up_limit=30.000
elevator_down_limit=17.000
.
.
elevator_trim_limit=13.500
.
.

So for the "Cm_dt Pitch Moment - Trim" which value i must put for this to work? Please help me to understand how this is working.
Also for the "Cmo Pitch Moment Coeff at AoA=0 (Trim os)" i think this is a number divided by 2048. What is this number? Maybe it is the mac in feet? For the sections 404 and 473 i use the default b373-400 sections.
Also for 1101 section for the "Fuselage AoA at Min Induced Drag" for the default b373-400 this is "-2,15934" and for default b373-800 is "0". What does this mean? How i can calculate a proper value?
Many thanks in advance.
 
Please forget the fly by wire module
Click to expand...
Not possible if it is working since it will/can override all the unaided dynamics.
"Cm_dt Pitch Moment - Trim
Click to expand...
Pitch effect caused by trim deflection, per radian of pitch trim.
Each 'click' of keyboard pitch trim is 1/1024 of trim range.
You seem to have a mix of "-3,7549364" and "9.387341" you should only use "." otherwise the values will be meaningless to the sim.

All of the parameters you mention vary between designs and it is not possible for any of us who do not have the model you have to come up with an answer without knowing a lot more about it.
Roy
 
Dear guys,

Thanks for your answers. I need for anyone knows to tell me about this equation as is inside setion 1101 into "Cm_dt Pitch Moment - Trim".

"Maximum Cm_t = Cm_dt * Max_Defl(rad) Sec 320"

For this example i checked the default b737-400 and i get:

Maximum Cm_t = 3 * 0.349
Maximum Cm_t = 1.047

What is this value "1.047"? What does it means? Is this a constant and each time must be the same number?
In my case now, for elevator trim i will use the following value:

elevator_trim_limit=13.500

So the above equation will be:

"Maximum Cm_t = Cm_dt * Max_Defl(rad) Sec 320"

If "Maximum Cm_t = 1.047"

Then will be:

1.047=Cm_dt*0.23562
Cm_dt=1.047/0.23562
Cm_dt=4.443596

But if the "Maximum Cm_t = 1.047" is not a constant but a dependable number, in the equation i will have two unkown factors so i can not solve it.

If "Maximum Cm_t = ?"

Then will be:

?=?*0.23562

That's why i need your help. Many thanks in advance.
George








 
First, I really recommend further study of Yves excellent explanations, which you have. That should give the answers you need.

When an airplane is in stabilized level flight and a small amount of nose-up trim is applied, the nose will rise and the airplane will slow down.
If it is stable, as speed is reduced the trim effect will diminish and the airplane will adopt level flight at the slower speed. It may try to speed up unless engine power is slightly reduced
Level flight at different speeds requires different trim settings.
The "power" of the available trim depends on its pitch moment coefficient which is Cm dt. This is expressed in pitch moment per radian of trim deflection.
The maximum "power" obviously happens when maximum trim deflection is applied. The Max_Defl_(rad) Sec 320 reference was current before the max trim deflection (degrees) appeared in the aircraft.cfg and 320 became obsolete.
That value depends on the individual airplane design. It is not a constant. Furthermore it is only an issue if you run out of nose up trim at slow speeds. You should be able to trim to just above the stall.

So there is really no need to calculate it, but if you want to, it is simply Cm_dt times the maximum trim deflection expresses in radians. 13.5 degrees is 0.2356 radians. Cm_dt is a constant for this design.
Roy
 
You must log in or register to reply here.
Back
Top