# Thread: CG in Takeoff Ref page of PM 737 CDU

1. ## CG in Takeoff Ref page of PM 737 CDU

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

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

3. 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.

4. 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/Pu...tmund_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.

5. 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

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

7. Thanks pete, I have a look now.

8. 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,

9. Originally Posted by 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,
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 ?

10. 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.

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