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Difference between CG Limits & CG Moment Envelope

  • ray9ed
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ray9ed replied the topic: Difference between CG Limits & CG Moment Envelope

Bob, thank you for your explanation regarding datums. It cleared up my confusion.

John, some questions for you, please. What does skewing and stretching the envelope mean ? And how can you read things into the trim sheet when the diagram has no related information on it ?
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  • John.Heddles
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John.Heddles replied the topic: Difference between CG Limits & CG Moment Envelope

What does skewing and stretching the envelope mean ?

If you plot the normal weight by moment (or IU) chart based on the usual OEM datum (up near the nose of the aircraft), you end up with a narrow diagram sloping from the lower left to the upper right of the graph.

Problem with this is a compressed lateral (IU) scale which doesn’t help plotting or reading accuracy.

A way of improving this a bit without using a more graphically-suitable datum is to skew and stretch the chart. While this is straightforward, an explanation of the drafting might not be useful so let’s try with another way of looking at it …

(a) Start with the original chart – imagining that we have drawn it on a magical, very stretchy, material which can be stretched while staying flat.

(b) Skew ? - Grab the top of the chart with one hand and the bottom with the other. Move the top to the left and the bottom to the right. This has the effect of keeping the graph orientation constant while rotating the IU scale lines anticlockwise. The graph progressively stands up more vertically. Compared to the original chart, the typical angle needed for the IU lines is around the 60-70 degree mark to obtain the desired result.

(c) Stretch ? – grab each side of the chart and pull sideways to stretch it a suitable amount. This has the effect of making the diagram laterally fatter and easier to read

(d) Finally, trim both edges to get rid of the unwanted, pointy sections.

What you end up with is something like the attached Cessna chart at Fig 6.2. Point to note is that the original thin chart has now become nice and fat which makes it easier to plot and read .. the IU scale might be a bit weird but that is the price paid to achieve the fat envelope while using the original datum.

There are plenty of similar charts in various GAMA flight manuals. Some are drawn better than others but the intent remains the same.

…. how can you read things into the trim sheet when the diagram has no related information on it ?

All pretty simple if you know what to look for and where to look for it. The process generally is referred to as reverse engineering .. ie start with the result and figure out how it came to be ...

We have no knowledge about the specific aircraft so we need to do some Sherlocking to start off.

First up, is it a single or twin ? The trimsheet tells us the aircraft has a MTOW 3600lb. The typical 6 place twins are moderately heavier than this so we are looking at a single. My first guess would be the Cherokee 6 family ? So let’s start there and see where we end up …

A run through US TCDS A3SO for MTOW and fuel capacity indicates that we are looking for one of the Lance models. A quick back of a fag packet calculation indicates that the sheet is metric in design and has the form

IU = W x (FS – 2377) / 1000

where W is in kg and FS is the station (arm) of interest and is in mm. As the sheet hasn't been tidied up to get rid of the negative IU values, there is no constant in the equation to shift the IU entry scale.

Looking at the TCDS and the trimsheet, the various loading arms match up nicely although there are some discrepancies in the CG envelopes. The most likely candidate is the Turbo Lance II which has a similar envelope with the aft limit agreeing and the forward limit's being conservative below MTOW.

This I can see being an expected response from the DCA Certification folk were the US data package supplied for the Australian TC a tad light on for stall data.

Should someone have an old copy of the original Australian flight manual for the model, we could check out the envelope discrepancy quite easily.

Engineering specialist in aircraft performance and weight control.
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