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CG Calculation Differences

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Old 02-23-2013, 07:00 PM
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ron12901
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Default CG Calculation Differences

Why is the Ideal CG so different between the Adamone and the Geistware calculators in the link at the the top of this forum? I put i the same wing/tail numbers and the Adamone Ideal CG at 15% statisc margin is 5.08 in. while the Gistware number is 5.49 in. My wing root is 17.25 in (tip 12.25); this seems to be a significanr difference. Which one should I use?

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Old 02-23-2013, 07:33 PM
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Default RE: CG Calculation Differences

I've been using this one for years. It's right on with another method I use for a double check. Dan.
http://www.geistware.com/rcmodeling/cg_calc.htm#mac
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Old 02-23-2013, 07:39 PM
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Default RE: CG Calculation Differences

Also, you may have your static margin and percent MAC set wrong. Dan.
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Old 02-23-2013, 08:44 PM
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Default RE: CG Calculation Differences



Last year when I wanted to put together my own spreadsheet for Cg, I looked at a lot of different implementations.  What I noticed was that there are a couple of effectiveness/efficiency coefficients buried in the equations that each author has used as his best guess.  They tend to be slightly different between all of them and so you get different answers.  If you go look up sources, you will get opinions of what they ought to be based on aircraft configurations, experience, theory, etc.    So each author has chosen what he thinks fits model aircraft best.

I ended up using Adamone as my reference for crosschecks but they will all get you in the ballpark.  After that, you have to fly and test.

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Old 02-24-2013, 05:48 AM
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Default RE: CG Calculation Differences

Would be interesting to hear more views on this topic so hopefully more folks will chime in.
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Old 02-24-2013, 06:54 AM
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Default RE: CG Calculation Differences


Quote:
ORIGINAL: karolh

Would be interesting to hear more views on this topic so hopefully more folks will chime in.
I just verified both with different numbers and found out that the difference is in the location of the NP, which considers the moment introduced by the tail (it should be the same for both since it depends on tail area and fuse length).
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Old 02-24-2013, 02:53 PM
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Default RE: CG Calculation Differences

If you look at Adamone's version, there is an entry for Stabilizer Efficiency.Viewing the source for the page around line 178 you can see the values applied for stab efficiency, i.e., .85 for Std, .65 for Low. Opinions vary about what value to use. A quick look at Geistware seems to indicate thathe uses .80 in his neutral point calculation.He also notes published references for his method.

In IE, go to the View menu and click on Source to see the code.
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Old 02-24-2013, 05:24 PM
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Default RE: CG Calculation Differences

Here's mine
try it out

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Attached Files
File Type: xls
Ca80187.xls (91.5 KB, 8 views)
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Old 02-24-2013, 07:44 PM
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Default RE: CG Calculation Differences

Limey, your Excel file does not open. Does RCU support .xls files? Dan.
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Old 02-24-2013, 09:19 PM
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Default RE: CG Calculation Differences

try, right click, save as link.
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Old 02-26-2013, 06:38 AM
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Default RE: CG Calculation Differences

Limeybob, I tried your worksheet and got a cg right in the middle of the adamore and geistware values with a 15% safety factor. Thanks for attachment, I will use it. What I like is that I can save a spreadsheet for each plane and always have it available for future reference. The links can'f be saved with the data.
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Old 02-26-2013, 06:44 AM
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Default RE: CG Calculation Differences

OK, glad you like it.
The neutral point is a tricky one, had to use three different formulae and average them.
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Old 02-26-2013, 09:40 AM
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Default RE: CG Calculation Differences

Hi LB,

I tried using your formula but could not understand what the terms Tail Root Leading Edge and Tail Root Trailing Edge refers to.
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Old 02-26-2013, 09:44 AM
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Default RE: CG Calculation Differences

dimensions "e" and "f" on the instruction page.
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Old 02-26-2013, 03:25 PM
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Thanks
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