Help with Weight and Balance envelope C172

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Help with Weight and Balance envelope C172

Help with Weight and Balance envelope C172

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Liam Sandie

posted 5/22/2013 10:33:15 PM

Group: Forum Members
Posts: 72, Visits: 261

Hi all, just subscribed to SD now Android is supported (hope the full range of features make it across).

Anyway I am having real difficulty setting up the w&b envelope for our aircraft. There is already an online envelope which uses a Microsoft spreadsheet. This can be found here:-

http://www.dukeriesaviation.f9.co.uk/downloads/wb/G-ROLY_WeightBalance.xls

Can someone please assist me with using this graph to set up my Skydemon as I cannot get it to appear correct at all.

Many thanks

Liam

https://www.instagram.com/liam548/

Replies

stevelup

posted 5/23/2013 3:00:40 PM

Group: Moderators
Posts: 593, Visits: 4.9K

The moment graph will lean to the right, the CG graph would usually have a straight right hand edge..

Personally, I'd ignore the graph in your POH and just use the textual information.

Liam Sandie

posted 5/23/2013 3:04:01 PM

Group: Forum Members
Posts: 72, Visits: 261

Sorry for mis understanding but ignore the graph? The issue I think is sd sets their graphs up different to most poh

https://www.instagram.com/liam548/

stevelup

posted 5/23/2013 3:32:27 PM

Group: Moderators
Posts: 593, Visits: 4.9K

The Moment Graph in your POH is there to help you do the calculation manually. That's why it is a Moment graph, and not a CG graph. You can easily tell one from the other - a moment graph will lean extensively to the right, a CG graph will almost always have a square right hand edge.

Have a look at this POH (Link)

On Page 6-11 is a Moment graph (this is what you are trying to use)

On Page 6-12 is a CG Limit graph (this is what SkyDemon needs)

It should be possible to convert a Moment graph into a CG graph by simply dividing the moment by the weight.

However, when I do that with your figures, it doesn't work (the right hand edge ends up sloping to the right by a considerable amount). That suggests that your starting figures are wrong.

Moment/1000	Moment	Weight	Arm
50.70	50700	1500	33.80
67.00	67000	1950	34.36
87.50	87500	2450	35.71
107.30	107300	2450	43.80
84.30	84300	1500	56.20

(The figures in bold should be the same)

If I do it to the figures in the moment graph in the POH listed above, it ends up with a square right edge as expected.

Moment/1000	Moment	Weight	Arm
52.50	52500	1500	35.00
68.00	68000	1950	34.87
88.50	88500	2300	38.48
109.00	109000	2300	47.39
71.00	71000	1500	47.33

Instead of trying to interpret the graphs, it is far simpler and more accurate to use the figures from page 2-7 of the book:-

'Center of Gravity Range'

Forward 35 inches aft of datum at 1950lbs with straight line variation to 38.5inches aft of datum at 2300, Aft 47.3

So from that, we can simply create...

35,1500, 35,1950, 38.5,2300, 47.3,2300, 47.3,1500

...which we know is right, and didn't rely on squinting at a graph.

Edited 5/23/2013 3:37:00 PM by stevelup

Liam Sandie

posted 5/23/2013 3:44:53 PM

Group: Forum Members
Posts: 72, Visits: 261

I see. Thank you very much for your help Smile

https://www.instagram.com/liam548/

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Help with Weight and Balance envelope C172

Liam548 - 5/22/2013 10:33:15 PM

The reason you are struggling is because the graph in your spreadsheet...

stevelup - 5/23/2013 8:18:53 AM

[url=http://linux.cre.co.uk/pub/G-ROLY.aircraft]Link[/url]

stevelup - 5/23/2013 8:22:21 AM

Thank you Steve. Is the link the link to the file for sd?

Liam548 - 5/23/2013 8:33:51 AM

Think I've worked out the figures to input into SD, but the graph (arm...

Lyndon - 5/23/2013 10:24:29 AM

Steve what does your point mean regarding the last figure?

Liam548 - 5/23/2013 10:44:36 AM

The moment graph will lean to the right, the CG graph would usually...

stevelup - 5/23/2013 3:00:40 PM

Sorry for mis understanding but ignore the graph? The issue I think...

Liam548 - 5/23/2013 3:04:01 PM

The Moment Graph in your POH is there to help you do the calculation...

stevelup - 5/23/2013 3:32:27 PM

I see. Thank you very much for your help :)

Liam548 - 5/23/2013 3:44:53 PM

Steve, pretty much there with the envelope now after reading your post...

Liam548 - 5/29/2013 10:22:45 PM

You should have a weight and balance record as part of your POH....

stevelup - 5/29/2013 11:39:43 PM

Hi again Steve I'm afraid that even after a year I am still unable...

fuzzflyer - 7/1/2013 12:30:57 PM

Easiest way is to upload the picture somewhere like photobucket and...

stevelup - 7/1/2013 6:04:02 PM

Hi Steve...

fuzzflyer - 7/2/2013 1:54:43 PM

thanks Steve, much appreciated. I will have to look in the POH...

Liam548 - 5/29/2013 11:43:32 PM

I'm pretty sure the W&B schedule forms part of your 'Approved Flight...

stevelup - 5/30/2013 8:34:52 AM

Empty weight is 1490lbs CofG empty weight 41.25 ins There arn't 2...

Liam548 - 5/30/2013 4:16:10 PM

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