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## Flow chart or Graph?

• Topic Author

### wadner1 created the topic: Flow chart or Graph?

Please I would like your help regarding the Exercise 5.13 on page 5.54
Questions 3 & 5 flow chart was used despite addition of fuel and weight was required but in question 6 & 7 the graph was used to solve the questions.
1. why is it so?
2. How will I be able to differentiate when to use the graph and when not to?
Thanks
Sammy

• Posts: 1914

### bobtait replied the topic: Re: Flow chart or Graph?

What determines whether you use the flow chart or the graph to solve a balance problem is the limit which is causing the problem.

If the problem is the aft limit, you can always use the flow chart. Whenever any weight [including fuel] is being added or subtracted to put the CofG on the AFT limit, you can ALWAYS use the flow chart. That's because the aft limit remains in the same place whatever the weight.

If the problem is the forward limit, you can only use the flow chart if weight is not changing - i.e. you are SHIFTING weight to put the CofG on the FORWARD limit. In that case, you can always use the flowchart.

The only time you cannot use the flow chart is when you are ADDING OR SUBTRACTING weight to put the CofG on the FORWARD limit. In that case, you have to use the CofG envelope. That is because the forward limit of the CofG moves if weight is changed.

It is often a help in these questions to have the envelope beside you and roughly plot the weight and moment given in the question [even if it's just rough with you finger tip] to see which limit is likely to be the problem.

Remember it isn't the fact that you are adding or subtracting fuel that decides which method to use - it's the CofG limit that you are considering.

• Topic Author

### wadner1 replied the topic: Re: Flow chart or Graph?

Thanks for your response sir, i quite understand that part now but i am yet to know how to identify the required C of G.
On page 5.38 of the book the step D says Write the required C of G (in this case, the aft limit of 2680)
How can I easily know the required C of G?
Thanks
Sammy

• Posts: 1914

### bobtait replied the topic: Re: Flow chart or Graph?

The required CofG is always one of the limits. Either the aft limit, which is always 2680 mm, of the forward limit which has to be calculated for each weight using the forward limit formula - Present Wt - 2360 x .27 + 2400.

The question will always give you a present weight and present moment. If you roughly plot that point on the graph you will see whether you are going to hit the aft limit or the forward limit.

In this case, you are just outside the aft limit, so it is the one you require. The question will ask you to take weight out of the an aft compartment, add weight to a forward compartment or shift weight forward. In every case you can use the flow chart.

In this case, you are just inside the aft limit, so it is the one you require. The question will ask you to add weight to a rear compartment, subtract weight from the forward compartment or shift weight aft. In all cases you can use the flow chart.

In this case you are just outside the forward limit so it is the one you require. You may be asked to add weight to the rear or subtract weight from the nose. If you are adding or subtracting weight, you will have to use the graph to solve it. If you are told to shift weight, you can use the flow chart.

In this case you are just inside the forward limit. You may be asked to add more weight to the nose, or subtract weight from the rear. If you do you will have to use the envelope to solve it. If you are told to shift weight forward, you can use the flow chart.

• Topic Author

Thanks sir.

• Topic Author

### wadner1 replied the topic: Re: Flow chart or Graph?

Sir, i got this question today and i have not been able to sort it please help
An Echo aircraft has empty weight and moment of 1980 and 469 index units respectively. It is loaded as follows
Row 1 ....................... Pilot (77Kg)
Row 2......................... 4 crates of 40Kg each
Row 3..........................4 crates of 40Kg each
(seats in Rows 2 & 3 have been removed- an empty aircraft weight reflects that)
Wing Compartments ..............4 packages of 20Kg each
Rear Compartments................4 packages of 35 Kg each

Minimum fuel required for the flight stage is 100Kg
5Kg metal-bars are available as ballast.

If the passenger and Cargo load distribution is not changed, the minimum weight of fuel that is required in the Main tanks as fuel ballast in order to the COG with limits,is closest to

A. Nil required
B. 20 Kg
C. 50 Kg
D. 120 Kg

I keep getting 88Kg does it mean 50Kg is the answer?
Thanks

• Posts: 1914

### bobtait replied the topic: Re: Flow chart or Graph?

See full working attached.

• Topic Author

### wadner1 replied the topic: Re: Flow chart or Graph?

Thanks Sir,
But why is the "Minimum fuel required for the flight stage is 100Kg" included in the question?
Thanks

• Posts: 1914

### bobtait replied the topic: Re: Flow chart or Graph?

The 100 kg minimum fuel required for the flight does not actually affect the answer to the question that asks for the minimum ballast fuel required to correct the problem at ZFW. However it was included to remind you that you would require 144 kg of fuel in the tanks at take-off to legally do this flight. The 44 kg of ballast fuel is not counted as being available for the flight. Any fuel required, including all reserves and allowances, is over and above the 44 kg of ballast fuel.

Bob

• Topic Author

### wadner1 replied the topic: Re: Flow chart or Graph?

Sir,
I came across this question and I am yet to figure it out.
An Echo aircraft has an empty weight and moment of 1970kg and 480.00 respectively. It is loaded as follows
Forward Baggage compartment...........50kg
Row 1.....................................................Pilot + passenger
Row 2...................................................2 passengers
(assume each person on board weighs 77kg)
If the loading is not changed and no fuel is added to the auxiliary tanks, the maximum weight of fuel that can be added to the main tanks without exceeding the forward limit of the centre of gravity is closest to
A. 76kg
B. 93Kg
C.159 Kg
D. 215kg
E.190kg