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Exercise 5.13 help

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clarkemelia created the topic: Exercise 5.13 help

Hi,

I’m wondering if anyone can help me out with the following as it has me stumped. I’ve been able to follow so far but I can’t work out how to get the correct answer.

Exercise 5.13 Q3
An echo is loaded with the main tanks full and the auxiliary tanks empty. The gross weight is 2550kg and the total moment index at that weight is 682 index units. Calculate the greatest amount of fuel that may now be loaded into the auxiliary tanks.

My thinking; find the current CoG and use the auxiliary tank arm (2800) to place the CoG on the aft limit of 2680.

My working: 682 x 10,000 divided by 2550 = 2674.5

2680 - 2674.5 = shift of 5.5

Happy so far. But then my working to add weight:
2550 x 5.5 divided by 2800 = 5kg

But the answer is 117kg?

I feel I must be missing a step but can’t work it out. There are a few other questions like this that I can’t seem to get so I’m hoping if some one can explain this I’ll know where I’m going wrong on the others.

Thanks
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  • John.Heddles
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John.Heddles replied the topic: Exercise 5.13 help

Perhaps recheck your ballast formula and all might magically become right ....

Engineering specialist in aircraft performance and weight control.
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clarkemelia replied the topic: Exercise 5.13 help

Thanks John!!

2550 x 5.5 divided by the difference between required CoG and compartment arm ie 2800-2680 = 120

Gives me the 117kg!

It was driving me mad. Thanks again, much appreciated!
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  • John.Heddles
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John.Heddles replied the topic: Exercise 5.13 help

The reason I keep pushing the mantra not to jump willy-nilly into equations and "canned" formulae is that that is the easiest way to make simple mistakes.

Hopefully, you won't make this one again .....

Engineering specialist in aircraft performance and weight control.
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