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Shifting problem

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GeeEss created the topic: Shifting problem

Hi all,

Doing one of the practice exams and this question came up:

An Echo aeroplane is loaded at take-off as follows:

Gross weight ............................... 2659 kg
Total moment index ....................... 706.4 index units
Useable fuel in the main tanks only ........ 40 US gal

The minimum amount of weight that must be shifted from the rear compartment to the nose compartment to ensure the aircraft remains in balance at all stages of flight is closest to -
Select one:
15 kg
5 kg
nil
8 kg



If you remove the fuel, the aircraft is past it's aft limit.

So i started a shift flow

A. Present weight = 2550
B. Present moment = 687
C. Present COG = 2694
D. Required COG = 2680
E. Shift = 14
F. Compartment arm difference = 5000-500=4500
G. Weight to shift

The Weight to shift is my problem.
My calcs came to the same as the answers, but when I apply to formula A x E / D - F, I get -19, where the answer says its 8kgs.

What am I missing!!

TIA!
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  • John.Heddles
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John.Heddles replied the topic: Shifting problem

8 kg sounds fine if we presume that the aim is to make sure that the ZFW case is constrained to remain within the CG envelope (which is pretty standard fare).

I try to avoid formulae as they give me headaches. But, whence did you find your formula ? In particular, the D-F bit ? Perhaps if you revisit that bit then, should the penny not drop, we can go through the whole question to determine the error and, as day follows night, the correct answer.

I never like providing the answer straight up when you can derive much better value having to think it through yourself .... you have everything squared away but you just made an unfortunate mistake along the way.

Engineering specialist in aircraft performance and weight control.
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GeeEss replied the topic: Shifting problem

hey john,

the formula is from the performance textbook - it goes along with a flow chart that is described on shifting weight problems.

i've learned it that way and the exam isn't too far away, i don't think i have enough time to re-invent the wheel, but my wheel seems a little broken at the moment.

D - F would be taking the difference in compartment arms from the required COG.

unless I'm looking in the wrong part of the book and confused myself (which is entirely possible) i'm still lost.
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John.Heddles replied the topic: Shifting problem

Might be an idea to make sure that we are singing from the same sheet of music here.

The book section to which you refer should only be a page or two. Perhaps you can scan and post the detail and we can discuss. If you don't wish to post it, you might like to send me a copy of the scan at This email address is being protected from spambots. You need JavaScript enabled to view it.. At present, it isn't going to work with that formula, I suggest.

First, we need to establish if your book has a typo or if you have misread it. Then, it's a very simple problem to fix so we can run it quickly once I can sight the specific text details upon which you are relying.

Engineering specialist in aircraft performance and weight control.
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GeeEss replied the topic: Shifting problem

i was getting the photos ready to send to you and stumbled over the golden nugget i required.

the question and the flow in the book doesn't require you to divide by the difference between D and F, only divide A and E by F.

an oversight on my part.

I appreciate the help!!
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John.Heddles replied the topic: Shifting problem

RTFQ always helps. One trusts you now nail the exam.

I trust that you understand whence comes the formula ? It can be a slippery slope using formulae in isolation from understanding ...

Engineering specialist in aircraft performance and weight control.
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