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Weight and balance : how to determine CG position

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dewami View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dewami Quote  Post ReplyReply Direct Link To This Post Topic: Weight and balance : how to determine CG position
    Posted: March-07-2007 at 5:24am
The first step before ensuring the aircraft loading is correct is to determine  the empty aircraft weight and corresponding CG position.

Then, it is easy to determine CG position for the aircraft with  payload and fuel.

Here is the Flight1 ATR72 aircraft.cfg (ref : 

http://www.microsoft.com/games/flightsimulator/inc/fs2000/sdk/fs2000_aircraft_container_sdk.pdf):

[weight_and_balance]
empty_weight =29762
reference_datum_position =0, 0, 0   
empty_weight_CG_position =-1.3, 0, 2
[fuel]
//Longitudinal (feet), Lateral (feet), Vertical (feet), Usable(gallons), Unusable (gallons)
LeftMain = -4.13, -14.9, 5.1, 836, 5
RightMain = -4.13, 14.9, 5.1, 836, 5

* reference_datum_position = offset (in feet) of the aircraft's reference datum from the standard Flight Simulator center point,which is on the centerline chord aft of the  eading edge
* empty_weight_CG_position = offset (in feet) of the center of gravity of the basic empty aircraft (no fuel, passengers, or baggage) from the Reference Datum Position
* the sign convention for positions is positive equals longitudinally forward (with this convention, MAC is negative)

Conversion from %MAC to H-arm :
 %MAC=100*((H-arm_CG - H-arm_lem)/len_mac)  (ref : http://www.wingfiles.com/files/performance/weightandbalance.pdf)

          with
             len_mac = length of MAC = - 2.303m = - 90.67 " = -7.556 ft
             25 = 25% MAC
             lem = leading edge of MAC

Here, we have :
       H-arm_25 = 0 (standard Flight Simulator center point)
       H-arm_lem = + 0.25*|len_mac| = 1.89 ft
       H-arm_CG_empty = - 1.3 ft -> H-arm_CG_empty - H_arm_lem = -1.3 - 1.89 = - 3.19 ft

Dry operating :

                CG_empty = 100*(H-arm_CG_empty - H_arm_lem)/MAC
                CG = 100*(-3.19)/(-7.556)
                CG = 42.2% Question

If we add fuel (2000 lb) :
       moment_TOW = moment_empty_weight + moment_fuel
                  = H-arm_CG_empty * weight_empty + H-arm_CG_fuel * weight_fuel
                  = (-1.3) * 29762 + (-4.13) * 2000
                  = -49951 lb.ft
                  = H_arm_CG_TOW * weight_TOW
                  = H_arm_CG_TOW * (weight_empty + weight_fuel)
       H_arm_CG_TOW = moment_TOW / (weight_empty + weight_fuel)
                    = -49951 / 31762
                    = -1.478 ft
       CG_TOW = 44.6% Question

But CG max = 37%

Where is the mistake?

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VSG975 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote VSG975 Quote  Post ReplyReply Direct Link To This Post Posted: March-09-2007 at 3:26pm
waouh you are blowing my head with all these calculations. Seems quite advanced, and I am highly interested by the results so I hope that some pros can correct this !
Benoit - VSG975 (VSA Group and Fly IVAO)
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dewami View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dewami Quote  Post ReplyReply Direct Link To This Post Posted: March-10-2007 at 12:28pm
Only one data is missing : CG for DOW (13500 kg). I think it is possible to calculate it with aircraft.cfg. But I don't know how.
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Graeme View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graeme Quote  Post ReplyReply Direct Link To This Post Posted: March-10-2007 at 5:23pm

I use the configuration software provided with the F1 ATR and set random loads. I notice on the current load that FS9 shows the COG being right on the forward limit. I have my trim normally set at 1 of the green band range 0-2.5 and dont have to alter it much after take off.

What we need is the realtionship between the value of %MAC or the position of the COG as shown in the FS9FSX load diagram and the trim setting value to set in our ATR.

Does anyone have that co-relation?

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dewami View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dewami Quote  Post ReplyReply Direct Link To This Post Posted: March-12-2007 at 3:41am

I use this one :

TRIM = (-2,5/23)*(CG-37)

 

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Graeme View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graeme Quote  Post ReplyReply Direct Link To This Post Posted: March-12-2007 at 4:35am

Thanks  but Im getting a dozen different answers.

Can you work and example  through using  TOW  18482 kg and FS9 shows the cog at the fwd limit.

What value are you using for the COG?.

My chart that came with the F1 software shows on the COG envelope, that for 18.4t the fwd limit to be 15% MAC. Do you use that or the actual 0%?. Seems to me that in FS9 I would have to move things around a little first.

Graeme   

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dewami View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dewami Quote  Post ReplyReply Direct Link To This Post Posted: March-12-2007 at 7:20am

Here are the necessary data for CG determination :

DOW : 13500 kg

CG for DOW : it seems that nobody knows it for Flight1 ATR72/500

FWD CARGO WEIGHT : 376 kg 

AFT CARGO WEIGHT : 245 kg

PILOTS : 2

FWD STEWARD : 1

AFT STEWARD : 1

CABIN A PASSENGERS NUMBER (seats 1A->5D): 14

CABIN B PASSENGERS NUMBER (seats 6A -> 12D) : 12

CABIN C PASSENGERS NUMBER (seats 14A-> 18D) : 10

FUEL : 1260 kg

-----------------------------------------------------------------------

TOW = 13500+376+245+2*78+68+68+14*71+12*71+10*71+1260=18489 kg

If empty CG=25% (why not), moment TOW is :

13500*0+376*(-7.07136)+245*10.02792+156*(-8.5344)+68*(-6.03504)+68*8.56488+980*(-4.09651)+840*0.631371+700*5.358384+1260*0.65532=-98.6028 kg*m

And then , CG=-98.6028/13489=-0.00534m=24.8%

And TRIM = (-2.5/23)*(24.8-37)=1.3

 

 

 

 

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dewami View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dewami Quote  Post ReplyReply Direct Link To This Post Posted: March-12-2007 at 7:50am

In aircraft.cgf, there is the line "empty_weight_CG_position =-1.3,0,2".

Is it possible that -1.3 is the TRIM?

And then, the empty CG=25%?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graeme Quote  Post ReplyReply Direct Link To This Post Posted: March-12-2007 at 3:09pm

Thanks, Ive followed that through on a spreadsheet - the calculator doesn't like multiplying minus's at all.

On page 1-01 of the Aircraft General pages the Operating empty weight  specifies 25% as the COG. Alongside that are max ramp weights with a COG at 14% and 37%  and the consequent ground  measurements.

Looks like I will have to use the FS9 load configuration window to balance out the load better than the ATR config software does and keep as close to  a 25% MAC as possible.

More fun to add to the equation.

 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graeme Quote  Post ReplyReply Direct Link To This Post Posted: March-12-2007 at 3:29pm

in your example posted earlier - how do you get 19% MAC?. Shouldnt that be 0%?.

I can certainly see 32%.

The ATR pilot in the same thread stated their setting was normally 1 to 1.8 so if we work that backward then that represents 20.42% through to 27.79 %.

 

 

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dewami View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dewami Quote  Post ReplyReply Direct Link To This Post Posted: March-13-2007 at 12:47pm
Finally, I doubt that my interpretation of that diagram is correct.CG 32% and CG 19% are erroneous, I think. In practice, with ATR random load, trim is always approximately 1.2, because the aircraft is good balanced.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graeme Quote  Post ReplyReply Direct Link To This Post Posted: March-13-2007 at 3:01pm

I agree there as I have only required one click of ANU trim occasionally with the random load settings. Its very well balanced.

However I have since made up a print out of the trim setting against the MAC% within the take-off and inflight range. I will try it in the next few days after adjusting my freight load to match a 1-1.8 setting.  

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dewami View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dewami Quote  Post ReplyReply Direct Link To This Post Posted: March-14-2007 at 3:47am

Min CG for TOW=18000kg is 14 %.

Max CG for all TOWs is 37%.

Source : Flight1 ATR 72-500 Aircraft Flight Manual (Limitations, Weight and Loading, 2.01.02 P1)

With TRIM = (-2,5/23)*(CG-37) :

     CG=14% <-> TRIM=2.5 (CG is forward center of pressure).

     CG=37% <-> TRIM=0 (CG = center of pressure).

 

 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote DLH1032 Quote  Post ReplyReply Direct Link To This Post Posted: November-06-2007 at 3:20pm
Hi folks,

the link mentioned in the first post is dead, i think: http://www.wingfiles.com/files/performance/weightandbalance.pdf

Could anyone make this pdf document available again (i only have it printed out and used, but not the pdf-file)?

Thanks in advance,
Tobias
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dewami Quote  Post ReplyReply Direct Link To This Post Posted: November-07-2007 at 5:22am

Originally posted by DLH1032 DLH1032 wrote:

Hi folks,

the link mentioned in the first post is dead, i think: http://www.wingfiles.com/files/performance/weightandbalance.pdf

Could anyone make this pdf document available again (i only have it printed out and used, but not the pdf-file)?

Thanks in advance,
Tobias

Here is another url :

http://www.kmtrainingteam.com/_pdf%20files/Weight_and_Balance.pdf

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