CG in Takeoff Ref page of PM 737 CDU

[SAS541]

I hope someone can help me here.

I have just installed the PM Boeing suite of software and everything is up and running fine.

I am a little confused however, as the CG figure is empty in the Takeoff Ref page of the CDU. I realise that i need to enter this, but how do i calculate this figure? What figure are other users entering here that are flying the FSX default 737-800?

Doesn't the PMDG software calculate this figure from FS itself depending on the way the aircraft is loaded?

How can we do the same for PM?

Looking forward to any help i can get.

Thanks.

Regards
Benja

[autocadplease]

I use 20% C of G. You can calculate it, but you need all the weights and moments to do it.

[Padraig]

Yes I was wondering that too , I remember when I was using the PMDG 737 with FS2004 it used to calculate it all the time which was great, but with FSX and default 737-800 it didn't do anything. I'l try the 20% too, but do you know the actual formula for the calculations ? because I would like to work it out, as I use loads of different payloads for displacement when flying different routes.

[autocadplease]

You could just use a load and trim sheet. I cannot find a good one for the 737-800, but there is one at the end of this accident report:

http://www.bfu-web.de/nn_53140/EN/Publications/Investigation_20Report/2002/Report__02__EX007-0__Dortmund__B737,templateId=raw,property=publicationFile.pdf/Report_02_EX007-0_Dortmund_B737.pdf

To calculated it manually, you need the moments (distance of item from datum) for each of weight - and I don't know where to get those. You could use the ones from the aircraft.cfg? Also, you need the MAC (Mean Aerodynamic Chord) in order to calculate the % MAC - again I don't have that figure either. I'm lots of help, eh!

Update:

My 737-800 aircarft.cfg says:
WEIGHT_AND_BALANCE]
max_gross_weight = 155500 // (pounds)
empty_weight = 85710 // (pounds)

reference_datum_position = 0, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)
empty_weight_CG_position = -9, 0, 0 // (feet) longitudinal, lateral, vertical distance from specified datum

max_number_of_stations = 50

station_load.0 = 170.0, 46.3, -1.5, 0.0 //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.1 = 170.0, 46.3, 1.5, 0.0 //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.2 = 510.0, 0.0, 0.0, 0.0 //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.3 = 1360.0, 33.4, 0.0, 0.0 //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.4 = 5440.0, 16.1, 0.0, 0.0 //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.5 = 5440.0, -9.7, 0.0, 0.0 //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.6 = 4760.0, -28.0, 0.0, 0.0 //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.7 = 4000.0, 24.7, 0.0, 0.0 //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.8 = 1000.0, -34.4, 0.0, 0.0 //Weight (lbs), longitudinal, lateral, vertical positions from datum (feet)

station_name.0 = "Pilot"
station_name.1 = "Co-Pilot"
station_name.2 = "Crew"
station_name.3 = "First Class"
station_name.4 = "Coach 3-10"
station_name.5 = "Coach 11-18"
station_name.6 = "Coach 19-25"
station_name.7 = "Forward Baggage"
station_name.8 = "Aft Baggage"

;Moments of Inertia
empty_weight_pitch_MOI = 1600655
empty_weight_roll_MOI = 588050
empty_weight_yaw_MOI = 2148751
empty_weight_coupled_MOI = 0

So, you can calculate the C of G based on the above, but I don't know how to get the % MAC without the MAC figure.

[Padraig]

Thanks alot for this, I will take all this into account later on tonight and try it out. I find that putting in the right CG % helps me for take off period and intial climb. I hate when its not set right, the aircraft does not feel the same. This was a ongoing problem for me since my weight is always different after I calculated the fuel needed for a certain flight I embark on.

Once again,

A big thanks :D

[Peter Dowson]

You could just use a load and trim sheet.

The current CG as a % of MAC is provided in FSUIPC offset 2EF8, as a 64-bit floating point value. You can check it in FSUIPC by going to the Logging tab, right-hand side. Enter 2EF8 as the offset, FLT64 as the type, and check the "FS Window" or "AdvDisplay" option at the bottom.

There may be a way of getting it displayed elsewhere, via pmSystems perhaps? Or if you are using any GoFlight displays you could use my GFDisplay program.

Pete

[Padraig]

Thanks pete, I have a look now.

[BillTee]

Hello Pete,

I just tried your suggestion from above and got this display; "IPC:2EF8(FLT64)=0.2826" Would that be equal to 28.3% CG of the MAC?

Best regards,
Bill Trinidad

[Padraig]

Hello Pete,

I just tried your suggestion from above and got this display; "IPC:2EF8(FLT64)=0.2826" Would that be equal to 28.3% CG of the MAC?

Best regards,
Bill Trinidad

I have done what pete said, but I was getting 23.2425 which I gather means 23.2% GG. But I might not be correct. Whats the ZFW your using and fuel on board etc ?

[BillTee]

I didn't note the weights before, so I did another;

B739 PMDG
ZFW=128000
Fuel=20006
GW=148006

"IPC:2EF8(FLT64)=0.2450"

I wonder why my numbers are different

Bill T.

[Peter Dowson]

I just tried your suggestion from above and got this display; "IPC:2EF8(FLT64)=0.2826" Would that be equal to 28.3% CG of the MAC?

No. 0.2826 means 0.2826%. Your "payload" must be spread pretty evenly, certainly much further back than mine.

Regards
Pete

[Peter Dowson]

B739 PMDG

Checking on the FS9 version of PMDG's 737-700 I have installed into FSX I get 24.3% with full tanks and a 10433kg payload at "station 1". If I move that payload to the last listed station I get a CG of -29.3%.

So, I guess you can get almost any value for CG depending on where you put your passengers and freight. Makes sense to me. What you now need to know is what are the safe limits of CG for the particular airport/runway and conditions.

BTW the same offset works on both FS9 and FSX, so that's another cross-check you can do, if you have both.

Pete