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setting cg

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Old 02-18-2008, 08:13 PM
  #1  
chrisp32
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Default setting cg

Hey guys i was told by a friend that the " CG MACHINE " from great planes is and excellent tool for setting the cg, and it is amazingly accurate? is this true are there better ones. just wondering what people think about this product and also if there are any better ones what is thought about them also?

Thanks
Chris
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Old 02-23-2008, 09:47 PM
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Default RE: setting cg

I just made my own. If you search for CG you will find some other ideas. Some are complex and some amazingly simple. Though most use their fingertips for starters.
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Old 02-24-2008, 10:51 AM
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LesUyeda
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http://home.mindspring.com/~the-plum...%20Machine.htm

The most universal that I have ever used.

Les
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Old 02-24-2008, 12:34 PM
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Default RE: setting cg


ORIGINAL: chrisp32

Hey guys i was told by a friend that the " CG MACHINE " from great planes is and excellent tool for setting the cg, and it is amazingly accurate? is this true are there better ones. just wondering what people think about this product and also if there are any better ones what is thought about them also?

Thanks
Chris
Finger methods are quick, cheap & convenient. Put a piece of tape where you want the balance, and pick the plane up there with your fingertips. Add weight & repeat until balance looks right.

The CG Machine is also convenient. If the CG is 3" behind the leading edge, you set the little rulers on 3", and the vertical alignment wires let you quickly place the assembled model with the supports 3" behind the leading edge. Then you add weight until the little level bubble is in the middle, and you're done.

For swept wing models, some additional measurements may be needed. Models with more than about a 60" wingspan will be somewhat cumbersome to balance on the CG machine, and larger models will be too heavy for it. If you have one, and only do smaller models, you will be delighted with the simplicity and ease of storage.

For larger models, check out the previously mentioned Vanessa setup, for which you'll need an A-frame support, or an exposed rafter in the garge (for hanging the thing up.)

Good luck,
Dave Olson
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Old 02-24-2008, 01:44 PM
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Ed Smith
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Default RE: setting cg

The "CG machine" is a waste of money. It does not set the CG. All it does is provide two points on which to place the airplane. The picture shows something that is just as good and basically free.

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Old 02-24-2008, 03:28 PM
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Default RE: setting cg

after clicking on the link of post #3, i would say there is no doubt as to how to find the cg of a plane.

david
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Old 02-25-2008, 03:35 PM
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Default RE: setting cg

I have the GP CG machine. great, but over priced for what is it, really. Worried about it falling / tipping over..

I then made one by drillign two holes in my workbench. Got two 1/4" dowels, and added pencil erasers to the ends. Voila! A bench mounted balancer..

I then discovered the "Vanessa rig", and haven't looked back! (Les' post) Axial and some slight lateral balancing. Made one the for club! (But I gotta remember to bring it with me!!) There is [was?] a thread on RCU of the vanessa rig.
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Old 02-25-2008, 04:35 PM
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Default RE: setting cg

Try this thread. It shows some very basic CG machines. Its a little better than two vertically placed pencils in a block of wood, but not anymore complicated.

http://supercubclub.proboards78.com/...ead=1170341301
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Old 02-26-2008, 05:58 AM
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Ed Smith
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Default RE: setting cg

A note of caution.

All of the "CG" appliances mentioned will show the CG based on the mass of the airframe. This is not necessarily the same thing as the aerodynamic balance point of the airplane. That is a whole different subject.

Ed S
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Old 03-06-2008, 01:06 PM
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Default RE: setting cg

we all know that cg is in 30% of the wing chord from the leading edge. now my question is: how do we get a cg from a swept back double tapered or swept forward double tapered wing? does anyone knows the answer of this??? if so please teach us. I realy need the details. I have a strong feeling that someone from this thread can answer my question. Thanx in advance Cheers
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Old 03-06-2008, 01:52 PM
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Ed Smith
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Default RE: setting cg

we all know that cg is in 30% of the wing chord from the leading edge.
That is not correct. As you seem to have realised, 30% of the chord, where along the wing. A tapered wing will be different from a parrallel chord wing.

The balance pojnt of an airplane is measured along what is called the "Mean Aerodynamic Chord.

To determine the "MAC" use the attached diagram exactly as it is shown.

Ed S
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