SlimHeader

facebook_page_plugin
×
PPL Video Lectures (10 Jul 2020)

PPL Video Lectures covering Aerodynamics, General Knowledge, Performance, Meteorology And Navigation are now available through our website see front page for details.

× If you are studying for your BAK or PPL exams and need some help, please post your question here. Someone on the forum is bound to help you as soon as they can.

Aircraft Trim Sheet loading

  • John.Heddles
  • John.Heddles's Avatar
  • Offline
  • ATPL/consulting aero engineer
  • Posts: 477
  • Thank you received: 45

John.Heddles replied the topic: Aircraft Trim Sheet loading

.. the King Air is at the other end of the market and flown by experienced, professional pilots within a disciplined training and checking system overseen by regular CASA audits. Surely these pilots are completing the trim sheet? If it is no good, why aren't they finding difficulty getting it to work and raising questions within their operations management system?

Ray,

A valid concern and a good question, for which the answer, in principle, is easy .."it depends". Explaining things in some detail will take a bit longer.

What I'll do is spread the answer over several posts so that there is not too much to digest at the one sitting (plus it will take me a bit of work to prepare the background stuff). The initial plan will be

(a) compare the present sheet calculations with equivalent longhand calculations. If the sheet be correct, the final CG calculated in the sheet's machinations should be near identical to that coming out of the longhand calculations If not, then the error (delta) will be the clincher.

(b) review the present sheet to identify any errors

(c) redraw the present sheet as it should have been drawn originally by the designer. This probably won't be as I would choose to design the sheet from scratch, myself, but that is another consideration altogether.

(d) repeat exercise (a) and observe that the final calculated CG for the sheet is pretty close to the longhand calculation. If is worth noting that a well-designed and drawn trimsheet, completed with a modicum of care and diligence, functionally is as accurate as a longhand calculation. This, allied with the speed with which a trimsheet can be worked, is why they are so useful and have stood the test of time ... notwithstanding the recent preoccupation with all things being done using electronic whizz bang gadgets.

Engineering specialist in aircraft performance and weight control.
#51

Please Log in or Create an account to join the conversation.

  • John.Heddles
  • John.Heddles's Avatar
  • Offline
  • ATPL/consulting aero engineer
  • Posts: 477
  • Thank you received: 45

John.Heddles replied the topic: Aircraft Trim Sheet loading

Two random comparison examples -

(a) starting data weight = 3800 kg CG = 4700 mm

From a looksee through my archives, this is probably a reasonable starting guess for this aircraft. It doesn't really matter how close it is (or is not) as it is just a starting point for the comparison. As the sheet doesn't nominate an arm for the aft baggage, I have used the standard FS 325 for the longhand calculation.

The completed sheet is



The comparison longhand calculation (ie the normal tabular weight x arm exercise) gives the answers shown as blue dots. As the calculation is straightforward, I'll leave it for you to check if you wish. Data can be downloaded from the net without too much trouble.

(b) starting data weight = 4400 kg CG = 4785 mm

This might represent an aircraft with a bit of role equipment fitted and, say, just the rear rows for crew seats.

The completed sheet is



As you can see, quite easily, the delta between the sheet result and the longhand result is quite significant for both examples, particularly near the aft CG limit.

I would expect that operating crews using this document would have had the odd complaint or two regarding aft CG loading difficulties.

Next post we will have a look at specific errors in the sheet. Most errors the pilot has no easy way of detecting but it is useful for you to see some of the problems with which you might be faced later out on the line.

Engineering specialist in aircraft performance and weight control.
#52
Attachments:

Please Log in or Create an account to join the conversation.

  • John.Heddles
  • John.Heddles's Avatar
  • Offline
  • ATPL/consulting aero engineer
  • Posts: 477
  • Thank you received: 45

John.Heddles replied the topic: Aircraft Trim Sheet loading

I've had a look at the sheet in some more detail. The following overlay is based on an analysis to make the fuel grid correct so far as the IU equation is concerned.



The fuel line taken from the POH for this model is drawn in red and is based on each tabulated POH data point. The variation between the sheet and the POH data relates principally to an inappropriate linearisation process adopted by the sheet designer. The designer has taken several points from the POH data and then joined them with straight line segments. The expected outcome is seen quite clearly in the plot. By not looking at ALL the data, the designer has denied himself/herself the opportunity to assess the appropriateness of linearisation. It is worth your while noting this result .. if you wish to simplify a graph by linearising segments of lines, first look at the whole of the data before deciding which bits might benefit from linearisation. Had the designer done this, the two lines would have been quite similar. You might observe that the original sheet has a scaling error where the max fuel load has been plotted at 1750 kg rather than for 1629 kg.

Using the IU equation arising from an analysis of the fuel grid, the remaining trim lines show a variety of discrepancies with the declared seating locations. Despite playing with the data for a while, I cannot discern any pattern which might give us a clue as to what the designer has done. I have a sneaky suspicion, though, that the seating lines may have been lifted from a different sheet without due consideration to the differences in arms .. not conclusive, just a feeling.

The entry line and the envelope are just plain wrong so I have not wasted any further time with them.

In the next post, I will rework the sheet as the designer ought to have drawn it in the first place and repeat the earlier examples to show that a trimsheet is just as good as a longhand calculation (and a lot quicker) for practical purposes.

Engineering specialist in aircraft performance and weight control.
#53
Attachments:

Please Log in or Create an account to join the conversation.

  • John.Heddles
  • John.Heddles's Avatar
  • Offline
  • ATPL/consulting aero engineer
  • Posts: 477
  • Thank you received: 45

John.Heddles replied the topic: Aircraft Trim Sheet loading

One additional comment regarding the original sheet. This is not an "error", per se, but is unfortunate and indicates that the original designer didn't understand why we use sloping tick lines. As you can see in the graphic, the sloping tick lines don't overlap and this rather defeats the whole purpose of having them in the first place .. as we discussed in an earlier post.



The following graphics show how the original sheet might have been drawn using a consistent set of units throughout. The first repeats the earlier example one and the second example two. The longhand calculations, again, are plotted as blue dots. As you can see, the discrepancy between trimsheet and longhand calculations is negligible.





We probably have enough information, now, to have a think about Ray's question regarding how come the original sheet apparently was able to be used without undue operational problem. Let me come back with some observations in the next post .. if indeed, there is a sensible story to be told ... ?

Engineering specialist in aircraft performance and weight control.
#54
Attachments:

Please Log in or Create an account to join the conversation.

  • John.Heddles
  • John.Heddles's Avatar
  • Offline
  • ATPL/consulting aero engineer
  • Posts: 477
  • Thank you received: 45

John.Heddles replied the topic: Aircraft Trim Sheet loading

Let’s now have a look at the previous question posed by Ray: viz., how come the erroneous sheet didn’t attract strident operational complaint for whatever reason ?

First, may I state, explicitly, some points which have been implicit in prior posts ...

(a) a trim sheet is drawn on a vertical grid of IU (the incorrect sheet, in this case, is drawn on an inappropriate vertical grid of CG)

(b) the entry line, the background grid, and the envelope at the bottom of the chart are locked together .. if any element is required to be moved left or right, all elements must move in a single block The scale values are absolutes and specific.

(c) the intermediate trim lines, being the means of calculating and adding/subtracting IU shifts for the weight at the station, may be moved left or right singly or in any combination you might choose. This also applies to the fuel grid. The values are deltas only and the point from which the calculation might commence on the line is entirely arbitrary.

Now, moving on to the present question ..

(d) as IU, weight and CG are linked by the IU equation, the entry line IU scale may be presented as a weight by CG matrix, in lieu of an IU scale. You will see some designers do this although, in my view, it is a rather silly approach .. adds needless complexity and increases the risk of error in completing the sheet for no benefit that this engineer can detect.

If we put the relevant matrix on this particular sheet, lock it into the entry scale etc. combination .. and then shift that combined IU detail left or right to suit, we can position the two sheets (correct and incorrect) so that the entry lines match up. Once this is done, it is just a matter of looking at how the two envelopes overlap to make a call on operational problems.



You can see that the CG (incorrect) entry line aligns with the IU matrix for an empty weight of about 3833 kg. What this means is that, for an empty weight of 3833 kg, the incorrect CG entry line, coincidentally, provides a correct IU entry. It follows that, the further the empty weight is away from 3833 kg, the more the CG entry diverges from the correct IU entry.

Let’s put to one side the fact that the original chart has a number of errors in the trim lines. As we noted before, the intermediate trim lines were based on IU rather than CG. It would be necessary to discuss this with the original designer to find out just how it came to be: it doesn't make any sense, unless the lines were lifted from another sheet, as suggested previously ?

Now we can compare the incorrect CG envelope and the correct IU envelope to see what we might figure out.

It is clear that the IU envelope encompasses the CG envelope for weights above around 3600-3700 kg, That is to say, if the incorrect CG sheet is used, providing the load is calculated to be within the CG envelope, it will be conservative, when compared to the correct IU envelope for other than unrealistically low weights. The loaded CG numbers in the envelope will still be wrong, as shown in previous posts, but they will provide something in the way of a workable solution.

Note, however, that this will entail a considerable restriction on operational loading flexibility as the incorrect sheet prevents the use of the permitted areas shown in pink at the forward and aft limit regions. As I suggested before, I could see more than a few complaints and gripes from the pilots due to aft CG loading difficulties using the incorrect CG based sheet. However, it remains a moot point as to whether the operational folks would tie up the loading difficulties to the strange trim sheet envelope.

Unless there be further queries, I guess that we probably have exhausted the value in this sheet ?

Be there any more loading system questions ?

Engineering specialist in aircraft performance and weight control.
#55
Attachments:

Please Log in or Create an account to join the conversation.

  • rhlmcg101
  • rhlmcg101's Avatar Topic Author

rhlmcg101 replied the topic: Aircraft Trim Sheet loading

In contrast to the relatively complex King Air and its trimsheet, what is your assessment of the Manufacturer's system for the Jabiru, an aircraft at the other end of the size spectrum ? The Manufacturer's web site has manuals available for download,
#56

Please Log in or Create an account to join the conversation.

  • John.Heddles
  • John.Heddles's Avatar
  • Offline
  • ATPL/consulting aero engineer
  • Posts: 477
  • Thank you received: 45

John.Heddles replied the topic: Aircraft Trim Sheet loading

Have had a look at the OEM site. Several Jabiru models. Do you have a preferred model for me to review or are you happy if I flip a coin a couple of times to pick one ?

Engineering specialist in aircraft performance and weight control.
#57

Please Log in or Create an account to join the conversation.

  • rhlmcg101
  • rhlmcg101's Avatar Topic Author

rhlmcg101 replied the topic: Aircraft Trim Sheet loading

Perhaps you could look at the first model listed and then comment on the remainder should there be anything of note ?
#58

Please Log in or Create an account to join the conversation.

  • John.Heddles
  • John.Heddles's Avatar
  • Offline
  • ATPL/consulting aero engineer
  • Posts: 477
  • Thank you received: 45

John.Heddles replied the topic: Aircraft Trim Sheet loading

Good plan, that man. As with the previous sheet, we probably will run the answers over more than the one post to make the story a little more digestible.

Let's plan to look at the technical acceptability of the sheets, then whether the sheets are "good" and "well designed" loading systems ... and fit for purpose. Finally, might there be other, more appropriate ways to run the loading checks for this Type other than a trimsheet ?

Engineering specialist in aircraft performance and weight control.
#59

Please Log in or Create an account to join the conversation.

  • John.Heddles
  • John.Heddles's Avatar
  • Offline
  • ATPL/consulting aero engineer
  • Posts: 477
  • Thank you received: 45

John.Heddles replied the topic: Aircraft Trim Sheet loading

We'll start with the J160-C sheet.



Given that the sheet has been through both OEM and CASA assessments I am not anticipating any significant technical concerns.

I will, however, have more to say regarding its pilot and operational acceptability/usability after I have reviewed all the sheets in the series.

Engineering specialist in aircraft performance and weight control.
#60
Attachments:

Please Log in or Create an account to join the conversation.

Time to create page: 0.764 seconds