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/

stevelup

posted 5/23/2013 8:18:53 AM

Group: Moderators
Posts: 593, Visits: 4.9K

The reason you are struggling is because the graph in your spreadsheet shows the moment, not the CG.

I've tried converting the moment back (by dividing the moment by the weight), but it still doesn't make sense - I have no idea where the last figure came from...

I'd suggest carefully checking this against the POH instead.

Disregarding the last figure, the graph comes out OK and looks sane when compared with the numbers in a random POH I found on the internet, but you must do it properly using your figures.

stevelup

posted 5/23/2013 8:22:21 AM

Group: Moderators
Posts: 593, Visits: 4.9K

Link

Liam Sandie

posted 5/23/2013 8:33:51 AM

Group: Forum Members
Posts: 72, Visits: 261

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

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

Lyndon

posted 5/23/2013 10:24:29 AM

Group: Forum Members
Posts: 1, Visits: 3

Think I've worked out the figures to input into SD, but the graph (arm length vs weight) comes out a little differently shaped to the one on the spreadsheet (moment vs weight).

instead of moment x 1000 weight of the POH

50.7 1500

67 1950

87.5 2450

107.3 2450

84.3 1500

I get this arm weight in sky demon

33.8 1500

34.36 1950

35.71 2450

43.8 2450

56.2 1500

I've tried a few plane configurations of various weights in different locations on both the spreadsheet and SD and they do appear to be working the same so I guess its right, its just a little disconcerting they look so different.

Can anyone check this for me please?

-Lyndon

Liam Sandie

posted 5/23/2013 10:44:36 AM

Group: Forum Members
Posts: 72, Visits: 261

Steve what does your point mean regarding the last figure?

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

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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