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## Weight Shift Formulae (p.173 & p.190) & Concept

• Posts: 20

### dontgiveup replied the topic: Weight Shift Formulae (p.173 & p.190)

Edit: attached the wrong file

I guess I am starting to understand where my confusion is. I have probably mixed up the two graphs, concept-wise...
This graph (see highlight) has an aft limit shifting aft as weight increases.

I am confused - I have always thought the index (which represents moment) is a reflection of the the position of CG somehow...

The other graph that I am talking about (book p.183):
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• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 843

### John.Heddles replied the topic: Weight Shift Formulae (p.173 & p.190)

I guess I am starting to understand where my confusion is.

You sure are .. it takes time to get your head around this stuff.

This graph (see highlight) has an aft limit shifting aft as weight increases.

Absolutely NOT the case. This is a very common misunderstanding. This graph shows the moment (IU, if you prefer) moving rearwards and that is quite correct. However, the CG is stationary at 2680 mm. The weight by CG graph gives you the same information as the weight by moment (or IU) graph, just recast in the units.

I have always thought the index (which represents moment) is a reflection of the the position of CG somehow...

Sure is.

Moment = weight x CG
IU = moment /constant

But IU does not equal CG. This causes a lot of confusion.

The other graph that I am talking about

All Bob is doing there is figuring an equation for the upper forward CG limit with weight as an input and CG as the output. No more, no less. The equation lets us figure a more precise CG location for the exam. You would never use this technique in the real world but it is useful for getting over the hurdles in the exams

Engineering specialist in aircraft performance and weight control.

• Posts: 2441

### bobtait replied the topic: Weight Shift Formulae (p.173 & p.190) & Concept

To find the weight to remove on the envelope, you need to start at 2852kg/770.02 moment index and move down the line you have drawn until you hit the aft limit on the envelope. That represents a weight reduction of a bit over 20kg. You have started at the other end of the line (2698/704) and moved within the envelope to hit the aft limit. So you have actually found the weight of parcels that could be added. That gives you your answer of about 100kg.
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