W/B question

troopernflight

troopernflight

Well-Known Member
Someone please explain this question, and explain how to compute. Thanks.

GIVEN:
Total weight 3,037 lb CG location station 68.8 Fuel consumption 12.7 GPH Fuel CG station 68.0 After 1 hour 45 minutes of flight time, the CG would be located at station
A)69.77. B)68.77. C)68.83.
 
It can help to figure out what you are taking away first.

12.7 GPH x 1:45

45 / 60 = .75

1.75 x 12.7 = 22.23 Gallons, 22.23 x 6(lbs) = 133.38 lbs of fuel burn

The basic W&B equation is Weight x Arm = Moment, Total Moment / Total Weight = CG"
3037 x 68.8 = 208.94 (moments divided by 1000)

Subtract the fuel weight and moment from the original weight and moment.

133.38 x 68.0 = 9.06

This gives you a new total weight and moment of:

2903.62 and 199.88

Divide the new moment by the new weight

199.88 / 2903.62 x 1000 = 68.83"


Another way is to cross multiply the equation below to find what ever missing value. You just need to think about which where you are adding/subtracting/shifting things relative to the CG.


Weight Removed = Change in CG
______________ __________________

New Total Weight = Distance between weight and old CG


133.35 = x
_______ __

2903.62 = . 8"


133.35 x .8 = 106.68

106.68 / 2903.62 = .036

This gives you the variable (change in CG) of .036

The fuel station is forward of the CG so losing weight there is going cause your CG to move aft.

68.8 + .036 = 68.836


In other words.... I choose "C" :D
 
AP2B said:
It can help to figure out what you are taking away first.

12.7 GPH x 1:45

45 / 60 = .75

1.75 x 12.7 = 22.23 Gallons, 22.23 x 6(lbs) = 133.38 lbs of fuel burn

The basic W&B equation is Weight x Arm = Moment, Total Moment / Total Weight = CG"

3037 x 68.8 = 208.94 (moments divided by 1000)

Subtract the fuel weight and moment from the original weight and moment.

133.38 x 68.0 = 9.06

This gives you a new total weight and moment of:

2903.62 and 199.88

Divide the new moment by the new weight

199.88 / 2903.62 x 1000 = 68.83"


Another way is to cross multiply the equation below to find what ever missing value. You just need to think about which where you are adding/subtracting/shifting things relative to the CG.


Weight Removed = Change in CG
______________ __________________

New Total Weight = Distance between weight and old CG


133.35 = x
_______ __

2903.62 = . 8"


133.35 x .8 = 106.68

106.68 / 2903.62 = .036

This gives you the variable (change in CG) of .036

The fuel station is forward of the CG so losing weight there is going cause your CG to move aft.

68.8 + .036 = 68.836


In other words.... I choose "C" :D
Click to expand...

Thanks a lot for laying that out for me. I appreciate it. I just hope I don't get a whole lot of these type questions on the commercial. I know I can figure them out, but time limit is what I'm worried about.
 
troopernflight said:
Thanks a lot for laying that out for me. I appreciate it. I just hope I don't get a whole lot of these type questions on the commercial. I know I can figure them out, but time limit is what I'm worried about.
Click to expand...

These kind of problems are actually very easy. Learn this equation:

CG = (Total Moments - Change in Moments )/(TotalWeight - Change in Weight).

CG = (3037 * 68.8 - 133.35 * 68) / (3037 - 133.35)
= 68.84
 
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