Hello everyone, hopefully you are all doing well. I'm really confused about performance charts especially with baggage and passenger weight. I've read the book and did it with that method (such as adding and subtracting a 100 kg/Ibs) plotting my new ZFW and with the original ZFW and thats how much baggage/passenger i can carry. But some questions add 50 kgs/Ibs and that doesnt make sense to me at all. I've asked my instructors but they just said use common sense which led to more confusion . Performance is the only subject i really struggle with and i have no confidence at all after multiple attempts. Hopefully I've explained properly

Kind regars,
Barkin

I've read the book and did it with that method (such as adding and subtracting a 100 kg/Ibs) plotting my new ZFW and with the original ZFW and thats how much baggage/passenger i can carry. But some questions add 50 kgs/Ibs and that doesnt make sense to me at all.

You're missing the point with this stuff. The technique is only a geometric means to an end and has no other significance.

If you run the initial sums and find yourself not where you want to be (CG-wise) you have to rework things to fix that problem. Techniques which spring to mind include

(a) algebraically - too much like hard work so let's give that a miss.

(b) trial and error calculation - fine, if you have set it up in a spreadsheet and then just play with the numbers. A real pain if you do this longhand. Except, perhaps, for some final finessing, let's give that a miss.

(c) set it up with bits of transparent graph paper overlays and run the sums graphically. Works fine but a bit of a pain to set up unless you are going to do a heap of exercises.

.. and you can come up with other ways to the solution but they are too involved for routine use and, for the exams, impracticable or not permitted. So we need a simpler way which is reasonably accurate.

You can plot the CG/load on a CG envelope providing that the envelope is weight x moment (or IU) as when we do the calculations longhand we add weights and add moments (or IUs). It follows that we can do the same, graphically, on a chart presented as weight x moment (or IU). Mathematically, this technique is incorrect if the chart is presented as weight x CG but the error usually is small and we can get away with it if we can accept a small error at the end or run a longhand check calculation.

The technique is based on the fact that, for a constant arm (eg a passenger seat) the plotted line on the weight x moment (IU) chart is a straight line. So, we can add any CONVENIENT weight to see where the straight line goes and we can observe where it crosses whatever CG position we might desire, typically the forward or aft CG envelope limit. It is a simple exercise, then, in the typical problem, to work out what extra weight we might need to add (or what weight we might need to remove if the original load put us outside the limit) to figure out the correct loading.

To minimise plotting and reading value errors, it is best to use large weight increments but, at the end of the day, it really doesn't matter what weight you use so long as you get a useful intersection which will allow you to read off the required information.

So, you pick a suitable weight increment. Whether that is 100 kg, or 50 kg, or 60,3749 kg (that would be a bit silly but you should get the idea) so long as it does the job, it will be fine. If you have done the job on a CG chart presented as weight x CG, it is better then to confirm the result by a quick longhand check calculation.

Hi John, hopefully I understood and won’t be a bother as well. So essentially I should use any weight for the loading charts ? For example if I had a question about baggage weight and I added a 100 kg/Ibs but the actual answer just added 50 kg/Ibs would the answer still be the same? I know this might sound dumb but this is how I understood from your text.

Kind regards,
Barkin

I think I just don’t know when or what kind of questions to add 100 and 50 kg/Ibs. That’s the problem I’m struggling with

Adding or subtracting weight in any loading station causes the centre of gravity to move along a line with a particular slope. That slope is unique to each particular station.

When you add or subtract weight to a particular compartment you are simply locating a point on the line associated with that compartment. It doesn't mater what weight you add or subtract, all you need is another point on that line so that you can draw that line on the chart.

Once you have established the line, if you locate the point where it crosses the forward or aft limit on the envelope, you have found weight that needs to be added or subtracted.

hopefully I understood and won’t be a bother as well.

Barkin,

Everyone has some level of difficulty getting the head around some of the theory stuff in flying. Just don't give up .. keep getting back up on the horse until you have it all under control.

I should use any weight for the loading charts ?

If you are re-jigging the numbers, in essence, yes. The only real caveat is that you should rigorously avoid using a small increment which then has you extrapolating outside your two points. That is a recipe for exaggerating errors.

For example if I had a question about baggage weight and I added a 100 kg/Ibs but the actual answer just added 50 kg/Ibs would the answer still be the same? I know this might sound dumb but this is how I understood from your text.


That's the thing. You aim to add more than you need to achieve whatever it is that you are setting out to achieve and then read off, as best you can, the figure you are after. Precisely what you add, subject to the previous caveat, is pretty much immaterial. Nothing dumb about it, we all had to wade through this stuff until it started to make some sense to us.

I just don’t know when or what kind of questions to add 100 and 50 kg/Ibs

It doesn't matter. Providing that the loading arm doesn't change with varying weight and you avoid the extrapolation trap, just pick a weight which you find convenient. The next student might well pick another weight .. no problem there. You might choose a trial weight of 100 kg, I might choose 200 kg, Bob might run with 150 kg. All should give a reasonably useful answer providing we all exercise due care with plotting and reading stuff on the chart.

Bob's post provides a nice graphic to assist understanding. If I were to be a bit picky (we engineers tend to be that way, sometimes .. upsets people no end, on occasion) I might observe that his first point really should have the caveat qualifier that it requires that the loading arm doesn't vary with load. This problem often arises with fuel loads and cargo/baggage loads where the shape of the tank or baggage area isn't a nice regular boxy shape. Buzz word we use is "prismatic". You can still use the technique but only if your line reflect the arm changes. Generally, if we are running the problem longhand, this means we need to have a transparent graph which we can overlay on the basic chart to get the answer. Not at all difficult and, before we had all the nice graphics packages, was the usual way we approached such problems on the drawing board.