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• Mario
• Topic Author

Hi Bob and team and all out there.

In your answer to determine minimum weight of cargo removed to obtain safe centre of gravity, you do this by obtaining another point on the Load vs Moment index graph. You picked an arbitrary 100lbs to be removed and then obtained a moment index of 15.1, then plotted this point.
My question is how you obtained this moment index? If I plot 100 lbs weight on the loading graph for cargo I get a lower moment index of about 11. why did you use a moment arm of 151 inches?
Thanks
Mario

• Posts: 2469

To get the moment index for 100lbs in the cargo compartment you have two choices. You can use the conversion graph for weight to moment index. Enter at 100lbs and go across to the rear baggage reference line then go down to the moment scale. That's a moment index of just over 15.

Or you can go to the example for Bravo and obtain the arm of the baggage compartment - 151 inches.

If you multiply the weight by the arm and divide by 1000 you get the moment. 100 X 151 รท 1000 = 15.1 moment index.
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• Mario
• Topic Author

Thanks Bob. I was using the Cargo reference instead of the Baggage line. Oops!

• Posts: 2469

Actually Mario, you have brought up a good point. I notice that if you check out the example, there is no mention of 'cargo' anywhere. There is also no mention of 'cargo' in the description of the aircraft - only 'baggage'. However on the weight to moment index graph, there are two lines, one for 'cargo' and one for 'baggage'. I have only just noticed that after all these years!! I don't know why the 'cargo' reference line is there. There is no extra compartment on the aircraft where you can put 'cargo'. That's a mystery to me.

I usually tell my classes not to use the weight to moment index graph anyway. It is much easier to simply multiply the weight in each location by the arm given in the example table. In any case, if a question asks for the MINIMUM weight that must be removed, you would have to use the baggage compartment because it is the most rearward compartment so any weight removed from there would have the greatest effect on the CofG movement.

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 876

Cargo/baggage ?

I would need to have a play with the loading information to form a definite view. However, a quick looksee at the moment line chart suggests to me that "cargo" applies for an aircraft where the rear seats are removed and all the floor area aft of the front seats is used for cargo .. the cargo arm would then be that floor area's fore and aft centroid for loading calculation purposes.

It may well be that, when whichever examiner in the past lifted the loading system details from whichever aircraft out there in the wild, the cargo line in the chart was not picked up .. so Bob, probably, is not alone in wondering why it is there ...

Engineering specialist in aircraft performance and weight control.

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 876

Just came across this thread while having a look here and there.

If I may now add to my previous post, having done some more digging about in the interim.

The OP initially has done his sums on the cargo arm (115) to get around 11 IU for the 100 lb change. Bob, then, has done his calculations on the baggage compartment (station 151) to get 15.1.

Unless you have figured out the history it can be a bit confusing.

(a) first, the Bravo is the Grumman AA-5A Cheetah. The examiner who set up the workbook in the Dark Ages pulled the data from an aircraft in the wild. Either he didn't think it out or the discrepancy came into play down stream with a loss of corporate knowledge. I don't have the original CASA workbooks, so I can't put my finger on just where the problem/confusion originated.

(b) the cargo compartment and the rear passenger seats are mutually exclusive. That is, you can either use the rear seats OR fold them down more or less flat to end up with a sort of cargo area which you can load. You CAN'T use both simultaneously as the example in the current CASA workbook suggests. (ref p11). A quick think about it shows that the cargo loading arm is around about where the rear seat occupants' feet would be.

The workbook situation is just a simple oversight on CASA's part. Either they should have clarified the position or removed references to the cargo loading option.

(c) I can't recall seeing any exam question reference which suggests that CASA has used the cargo option.

(d) from a training POV, it really doesn't matter all that much, but any critical examination will reveal the cargo/rear row conflict problem.

Engineering specialist in aircraft performance and weight control.

• Posts: 2469

Thanks for the history lesson John. Very interesting. That explains a lot. I suggested to Gavin that maybe the Bravo had either a cargo pod or a roof rack!!

Bob

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 876