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Weight & bal drill exercises - p160 (May 2015 book)

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HPOUND created the topic: Weight & bal drill exercises - p160 (May 2015 book)

Hello!

Questions 1, 2 and 3 relate to load system ALPHA. Now I've filled out the load sheet 5 times, and I just am not getting anything close to the solution (p197). I'm wondering if someone can take a look at where I'm going wrong?

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Q2, the maximum amount of fuel that can be added, is a function of line drawing in Q1. I think maybe 5kg, but the correct answer is 35kg.

And for note only, Q3 asks for the additional fuel if baggage is relocated from the nose locker to the rear locker. This would enable to plane to fly at MTOW of 1,633kg. Given the current T/off weight is 1,562, the additional fuel is 71kg or just shy of 100 litres. The answer, however, specifies the total fuel (1,633 less 1,420kg = 213kg of fuel of 296 litres.

Thank you!
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John.Heddles replied the topic: Weight & bal drill exercises - p160 (May 2015 book)

Unfortunately, I don't have a copy of Bob's books/questions. Perhaps you might be able to scan the relevant pages to the thread ? regards, J

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HPOUND replied the topic: Weight & bal drill exercises - p160 (May 2015 book)

Thanks, John. Questions and answers attached.

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John.Heddles replied the topic: Weight & bal drill exercises - p160 (May 2015 book)

The problem is seen quite readily .. but is not of your doing so one should be a bit relaxed.

The two charts with which you are playing are different renditions of the same thing and are quite different in the precision at detail level. You can keep redoing your exercise as long as you like but similar discrepancies will remain evident.

Several considerations to discuss, I suggest.

First, what's what with the chart ?

Pre-PC days (pre-80s), we had no option other than to run up any drawings by hand and this included trimsheets. As best as I recall, the couple of other folk who played with trimsheets in the GA arena at that time all drew at full scale essentially by hand (ie pens, pencils, rulers, etc) on the office table. Indeed, my first couple of sheets (as far as I am aware, I was the first to introduce trimsheets to the GA side of things) were done that way.

It became apparent that the drawing quality was not really up to muster so I looked around for some way of getting a better quality (ie both precision and accuracy) of presentation. It so happened that a couple of ex-Navy mates had been in a photographic mustering and they had set up a business specialising in post processing work for the photographic industry. Of particular interest to me was their capability with photographic reduction, combinations and so forth. Henceforth, until the PC vector graphics world opened, my sheets were drawn on the board using a drafting machine at 3-4 times full size and then reduced photographically to final full size. In this manner, drafting errors essentially disappeared and the final result was quite precise, very accurate, and visually appealing.

Now the original Alpha sheet would have been lifted from a GA trimsheet by whichever examiner in the past. I suspect that the original was one of Norm Overmeyer's (a Sydney-based WCO now long gone to the great drawing office in the sky). Norm's time in the Industry predated the PC so his sheets were hand drawn. I am supposing that the sheet in the text information posted by you is of the original document. Certainly, the general level of drawing precision is not high and suggests a full scale hand drawn document.

(PS note - subsequently confirmed that the Alpha sheet originally was pinched from one of Norm's sheets)

It is evident that, at some stage, someone figured that a redrawn version, with better precision/accuracy, would be a good idea. The version of the sheet on which you have done your exercises is a (probably computer) redrawn version of the text version. A close look at it shows both considerably better drawing precision and accuracy. At the end of the day, the two sheets are not the same and you cannot expect to get closely compatible results using the two.

(PS note - subsequently confirmed that the redrawn sheet was done by Bruce Clissold as a favour to the then DCA theory examiner)

In the first instance, I suggest you locate a copy of the earlier chart and redo your plot to satisfy yourself that you can get an answer similar to that in the text. If you can't locate an original, you might try cleaning up the text version and using the cleaned up document for a rework of the exercise. Once you have done that if there remains any question regarding the subsequent exercises, we can look at them then. At this stage, those questions have little relevance to the revised sheet.

Second, a couple of comments on your sheet.

(a) a minor consideration only but you might find the row 2 calculation a tad easier using the bottom line where you would have 2 whole units (ie 2 x 77 = 154) to move. Not a biggie at all but something to keep in mind.

(b) for 197 litres of fuel (presumably usable rather than total), I note you have obtained 142 kg, ie at an SG of 0.72 whereas the text value is 140 kg (SG 0.71). I presume that the exam requires the use of one or the other so that consideration should be resolved before you run the final exams. No doubt Bob can sort that one out for everyone. I note that the trimsheet fuel line notes indicate that it was drawn for a presumed SG of 0.71.

It is worth noting a point for the back of the mind. In the olden days when we had all sorts of avgas fuels, there were two "standard" SG values in vogue; 0.72 for the lower octane ratings range and 0.71 for the higher (which included 100/130). It was considered acceptable to use these values for small quantities; typically up to around 700 litres (thinking the older large radial engined aircraft) rather than go through the exercise of fuel samples and hydrometer measurements. For smaller GA aircraft, a somewhat smaller quantity would be suitable.

Quite a while ago, the Industry introduced 100LL and that has an SG generally around 0.718. The SG value for 100/130 has been around 0.695-0.7 for a long time. It follows that, when one is out in the 172 or whatever, one should do the sums at whatever the actual SG might be. One can either carry one's own hydrometer (unlikely, although I could lend you mine) or check with the refueller .. the local fuel crowd certainly will have a very accurate SG for the current fuel stock as they pay on that basis.

Third, may I note some comments regarding the scanned images ?

(a) re Q2, the upper forward CG limit is a curve, rather than a straight line so, as an accuracy point of pedantry, one could put on a tad more fuel than the straight line graph would suggest. Something to be aware of only.

(b) re Q2 (and only a point of pedantry although it is a theoretically important point). The aircraft doesn't have an arm of zero for the fuel load, rather the trimsheet does. Were one to use a different datum in the loading system, then there would be an arm and a moment. So, in this case the comment in the text should be applied to the trimsheet rather than the aircraft, per se. It follows that the loading system designer can make any aircraft prismatic fuel tank have a zero moment provided he/she picks the appropriate datum position for the loading system to make that so.

(Note - tidy up edits - been on the list of to-dos for a while and finally got around to it …)

Engineering specialist in aircraft performance and weight control.
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HPOUND replied the topic: Weight & bal drill exercises - p160 (May 2015 book)

Thank you for your very comprehensive and prompt reply (and the backstory, too).

It occurred to me different charts could be the culprit, but I didn't want to attribute to this too quickly. I used the supplement contained in BT's book and guess it has been updated (it appears to be the same as the current CASA workbook) while the solutions in the book may be based on a slightly different/older version of the chart.

Hopefully in the exam the precision required doesn't come down to a millimetre here or there. Where the arm is given for W&B, it's very straight forward to do a precise mathematical calculation, but where only charts are provided, minor variation is inevitable if it comes to drawing lines on an A4 size page.

Thanks again.
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John.Heddles replied the topic: Weight & bal drill exercises - p160 (May 2015 book)

Indeed, but be aware that, with a well designed and well drawn trimsheet, executed with a bit of care, the end practical accuracy is as good as a longhand calculation.

Further, out in the field (as opposed to the exam nonsense), the accuracy of the starting data (ie empty weight and CG) is only as good as the folk who weighed the aircraft and will have a residual error .. so there is no sensible point achieved in figuring calcs to the nth decimal place.

For GA aircraft, a reasonably achievable accuracy probably is around 5kg and 5mm. In many cases, you will find rather worse data than that.

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bobtait replied the topic: Weight & bal drill exercises - p160 (May 2015 book)

Thanks again to John for his usual excellent reply. We do, of course agree that the position on the datum is entirely up to the designer of the trim sheet in the first place. However as far as we pilots are concerned, the trim sheet we have access to is usually in the POH or flight manual and therefore is the only one we are going to be dealing with for any particular aircraft. In that sense, a pilot may sometimes refer to the aircraft's datum rather than (more correctly) the trim sheet's datum.
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John.Heddles replied the topic: Weight & bal drill exercises - p160 (May 2015 book)

Bob, no argument from me on those comments. At the end of the day, it's a bit in the same paddock as the use of the term "weight" when the techo purists would opine that "mass" is the one to use. I can't ever recall an occasion where a pilot had a problem due to the popular use of the terminology.

The reason I raised the point harks back to two occasions in the deep, dark past, both relating to datum selection. On each occasion, I was called in to sort out the technical aspects of concerns raised by pilots who, it appeared, had done their theory training on a by-rote basis and held fairly strong, if somewhat misguided, perceptions of what was what.

In the first case, a professional chap out in the Western Districts (proud owner of a Baron, as I recall) was quite upset because the aircraft's loading system (which happened to be a trimsheet with a non-OEM datum) didn't follow the standard weight x (OEM) arm = moment mantra. It took quite some time to recalibrate his understanding of loading systems in general. However, he went off happy and, no doubt, the corrected understanding wouldn't have caused him any harm.

The second actually was quite close to the present thread discussion. A flying organisation had a number of aircraft, each with whatever loading system. Along comes another aircraft, of the same type and model as one of the extant fleet items ... different loading system with a different datum. One system used the fuel arm as the datum while the other used another location. A couple of the pilots had great trouble with the fact that one aircraft had a change in IU with fuel while the other didn't ... ie, why aren't they the same ? Again, I was whistled in to sort out the technicalities.

Point of the stories and, more importantly, the reason I raised the point regarding datum selection .. the typical pilot is better off having the information tucked away somewhere in the back of the mind so that, should a "problem" arise, the point can be recalled, even if not totally understood in the recollection after the passage of time, and the "problem" evaporates.

Engineering specialist in aircraft performance and weight control.
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