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Alpha Loading

  • jukzizy
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jukzizy created the topic: Alpha Loading

Sir, could you please explain how to go about this type of question. I'm confuse with it.

My answer is 22 Kg but I'm not sure if it's correct.
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  • John.Heddles
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John.Heddles replied the topic: Alpha Loading

I've only just now spotted this question. It's a fairly standard use of a trimsheet and the answer falls out relatively easily.

How about you post your solution and I can then both comment on it, specifically, and post my own for comparison which I can do when I post comments on your approach

Engineering specialist in aircraft performance and weight control.
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jukzizy replied the topic: Alpha Loading

my zero fuel weight is 1355 KG

330 Litres× 0.72= 230 KG

My Take off weight= 1585 KG

so I add 40 KG in the nose compartment

which will also add to my current take off weight 1585 KG(1585+40)=1625KG
so i plot two points and got 10 KG

is there anything i'm missing here?
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John.Heddles replied the topic: Alpha Loading

So we can see exactly how your solution is travelling, can you post a picture of your sheet ?

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jukzizy replied the topic: Alpha Loading

Sir..check my loading sheet here..
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John.Heddles replied the topic: Alpha Loading

You have the basics under control there, good sir.

However, several comments if I may -

(a) your execution of the sheet needs to be a little more precise - did you use, for example, a Douglas protractor (or similar device), to run the lines ? It is essential that you run vertical lines vertical (the sheet is drawn to a background vertical scale of IU).

(b) be careful counting divisions left and right - for instance, your 40kg added to the nose locker is more like 70 kg. If the sheet's design is not well done, interpolating can be a bit of a pain. The Alpha loading system is one of Norm Overmeyer's sheets (lifted from a GA aircraft by the then examiner and subsequently modified by Bruce Clissold at the examiner's request) - Bruce is, and Norm was - jointly competent trimsheet designers.

(c) while I appreciate that you are trying to save a few photocopies when practicing, it would be better, I suggest, if you squandered the inconvenience and dollars while completing the sheet with greater clarity. If nothing else, it makes it much easier for you to see what you are doing and minimises inadvertent errors. Your sheet, on this occasion, is a little difficult to follow in detail.

(d) when you end up with whatever front locker load from the trim line, it is worth keeping in mind that the envelope upper forward limit is not drawn "correctly". Were it correct, it would be a (quadratic) curve bulging out a bit from the envelope. Drawing the limit as a straight line is a very common conservative convenience in design and, in general, doesn't lose all that much in the way of loading flexibility.

When the designer uses a trim datum in the rear half of the envelope, as Norm did here, the discrepancy becomes much more noticeable. If (very poorly) drawn using the OEM's datum, the discrepancy is barely discernable. However, had the sheet's limit line been drawn correctly one would be able to crib a little bit more weight in the exercise. For the exam, and in real life, however, you use the sheet as it is presented.

I will complete the exercise later today, if I get some time, and post it to the next post for comparison.

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jukzizy replied the topic: Alpha Loading

Thank you sir..I don't have yet a douglas protractor..now it's clear.
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John.Heddles replied the topic: Alpha Loading

Now, if I've not made too many mistakes, this should be somewhere near the mark ..



The red line indicates the initial load calculation with weights shown as the blue lines. (I've used an SG of 0.71 as the sheet is drawn for that SG. If you use 0.72, the difference is immaterial)

Clearly, the TOW loading is going to be critical so we need to put some weight into the nose locker to force the CG to the forward limit.

Pick any convenient weight - doesn't matter what it is as we only want to get a line on the trimsheet to see where it crosses the forward limit - however, you need to straddle the limit line so that we are interpolating rather than extrapolating as that gives a better potential accuracy. We run back up the final IU line and then add the nose locker load, shown in green, run it back down to the envelope with the weight shown in mauvre. Join the end points with a straight line as the arm is constant for the load - shown as an orange-ish line.

We see that the forward baggage line intersects the forward limit pretty well right on an added weight of 20 kg which is my answer for the question.

General note - this question highlights one of the principal strengths of trimsheets. The format adds IUs graphically and it doesn't matter an iota as to the order you add or subtract IU values (so long as we keep the ZFW and fuel bits separate). If the trimsheet designer knows what he/she is doing, then the trimsheet, functionally, is as accurate as (and, if both are done manually, then generally quicker than) a longhand calculation. While I never did like his stylistic approach overly, there is nothing wrong with Norm's trimsheet design techniques, of which this is a stock standard example.

(Note for me if I forget again - graphic formats display integrally, pdf/doc etc as a link)

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jukzizy replied the topic: Alpha Loading

Thank you very much sir for your time and help..now i got the idea and where I went wrong..Thank you very much
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John.Heddles replied the topic: Alpha Loading

I don't think that you were "wrong" at all. However, a bit more attention to detail and your answer would be just fine.

The actual answer in kg will vary a little between solutions due to the fact that we are not going to be able to plot things absolutely perfectly.

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