Finding CG on custom build
10-02-2009, 12:43 PM
#1
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Finding CG on custom build
I was wondering if anyone remembers the link to a site that a gentleman had that would help figure the CG on any plane by entering wing info. Thanks.....
10-03-2009, 09:07 AM
#4
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RE: Finding CG on custom build
Dan...
The title you noted seems awful familiar. Used to be a German based web site that said you need their DOWNLOADER to get going full, for what you got was only a test or inovative version.
But it wanted a whole series of measurements, (in MM too) and asked that everything be weighed accurately to One gram, and a host of other requirement to say "IT" was here. Did not say where the CG was supposed to be at, only that based upon their analysis IT was here. The version back when wanted to know waht the design analysis was in some Wind tunnel (which I do not own either). I can figure the results easy enough via a thumb test. But not on a basic raw model still in the computer as requested.
Wm.
The title you noted seems awful familiar. Used to be a German based web site that said you need their DOWNLOADER to get going full, for what you got was only a test or inovative version.
But it wanted a whole series of measurements, (in MM too) and asked that everything be weighed accurately to One gram, and a host of other requirement to say "IT" was here. Did not say where the CG was supposed to be at, only that based upon their analysis IT was here. The version back when wanted to know waht the design analysis was in some Wind tunnel (which I do not own either). I can figure the results easy enough via a thumb test. But not on a basic raw model still in the computer as requested.
Wm.
10-03-2009, 11:15 AM
#5
RE: Finding CG on custom build
Try the website. It is very simple and a good help. I have placed the CGs of my planes at the 30% point of the MAC with great success. They have all survived the first flight which is all I wanted from the calculation. After that, I have trimmed the CG location with ballast just to improve the landing characteristics. Another way to locate the CG is to place 30% of the wing area in front of the CG. This point has so far agreed with the MAC location. I have to use the area technique for wings that have curved leading and/or trailing edges. It’s a tough way to do it but I have my designs in AutoCAD and the computer does it for me. As you know, there is much more involved to getting the CG right on an airplane but for model use this is adequate enough. Dan.
10-03-2009, 01:28 PM
#6
Senior Member
RE: Finding CG on custom build
That is a good app that considers all the things that control pitch stability. It's easy to use (only 9 measurements) and will quickly give you a reliable CG range. With the range, you've got an easier task adjusting your model prior to it's maiden. Anywhere in the range will be safe and reliable and it's easier to make weight shifting decisions.
To get the range, just plug in your 9 measurements and choose 5% for the static margin. Run the app and note the CG location. The change the 5% to 15% and run again, noting the new location. Those two are the safe range for the maiden CG location.
How is that easier? When you balance the plane and discover where it's CG is, you aren't stuck with just one pinpoint location to try and balance around. It's easier to see how much work (if any) will be needed shifting stuff, etc. It beats having no idea just how sensitve your CG location is too.
Over the last years, I've discovered that I can use the 5% location and the plane will be much more to my flying style than using either the recommended CG or one guessed at by just the MAC percentage. It's that accurate and reliable. Also cuts shortcuts the trimming that'll be needed with rule of thumb or mfg's advised locations.
