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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fuzzflyer

posted 7/2/2013 1:54:43 PM

Group: Forum Members
Posts: 37, Visits: 157

Hi Steve

https://plus.google.com/photos/113724982439309122239/albums/5408522887582475457/5896000114774703330?banner=pwa&pid=5896000114774703330&oid=113724982439309122239

hope that works

I loaded the plane from the cloud where I use it on Android.. it seems the W&B works ok there though I'm not certain as no matter how I load the aircraft the cross in right on the bottom of the polygon

help mogas

stevelup

posted 7/1/2013 6:04:02 PM

Group: Moderators
Posts: 593, Visits: 4.9K

Easiest way is to upload the picture somewhere like photobucket and then paste the link

fuzzflyer

posted 7/1/2013 12:30:57 PM

Group: Forum Members
Posts: 37, Visits: 157

Hi again Steve

I'm afraid that even after a year I am still unable to get to grips with W&B in SD. I have a screen shot of what happens when I enter my details in the report but can't seem to attach it here. is there another way?

buy the way when I enter all the weights etc nothing shows in the box.

help mogas

Liam Sandie

posted 5/30/2013 4:16:10 PM

Group: Forum Members
Posts: 72, Visits: 261

Empty weight is 1490lbs

CofG empty weight 41.25 ins

There arn't 2 lots of figures listed like SD asks for so I presume I put the 41.25 in the lon box Steve?

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

Edited 5/30/2013 6:24:19 PM by Liam548

stevelup

posted 5/30/2013 8:34:52 AM

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Posts: 593, Visits: 4.9K

I'm pretty sure the W&B schedule forms part of your 'Approved Flight Manual' so should be with it.

Liam Sandie

posted 5/29/2013 11:43:32 PM

Group: Forum Members
Posts: 72, Visits: 261

thanks Steve, much appreciated. I will have to look in the POH although as I understand aircraft have to weighed periodically anyway so maybe it is recorded elsewhere.

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

stevelup

posted 5/29/2013 11:39:43 PM

Group: Moderators
Posts: 593, Visits: 4.9K

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

This is aircraft specific as the details in there are entered as a result of weighing it.

That would be the definitive source for the empty weight, CoG and arm positions.

Liam Sandie

posted 5/29/2013 10:22:45 PM

Group: Forum Members
Posts: 72, Visits: 261

Steve, pretty much there with the envelope now after reading your post and comparing to the POH you provided.

I will check these POH figures against our 172N but presume they will be the same.

Can you clarify, in Skydemon there is an option above the envelope figures for empty weight, Centre of Gravity (lon), and Centre of Gravity (lat).

Where in the POH link will I find the correct figures and how do these affect the graph itself?

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

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/

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

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