Easy way to find C.G. - RC Groups
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May 02, 2002, 08:13 AM
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thor4944's Avatar

Easy way to find C.G.


I am not the sharpest tool in the shed so I wanted to find the c.g. for my plane...I am a Biology teacher and I asked our Physics teacher how he would find the C. G. for my airplane. He said the easest way to do it is...
1. Tie a heavy object to a string
2. Put a pin in one wing (did not want to put a hole in my plane so you can hold it up with two fingers so it can swing freely, does not matter where, I like the top of the wing)
3. Where your finger is hold the string with the heavy object hanging like a swing. and draw a line where the string crosses the plane.
4. Do the same thing to the other wing and where these lines cross there is your C.G.
* I dont know how to put more than one pic on a thread so I have them numbered as the steps are...what do people think of this way.....?
This pic is pic 1
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May 02, 2002, 08:14 AM
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thor4944's Avatar

Picture 2


Pic 2
May 02, 2002, 08:16 AM
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thor4944's Avatar

Picture 3


Pic 3
May 02, 2002, 08:17 AM
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Pic 4


Picture 4
May 02, 2002, 08:18 AM
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Pic 5


Picture 5
May 02, 2002, 09:08 AM
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mtayl's Avatar
Sure enough. That worked great on an old 400X with the battery in it. It crossed exactly at 8". Very cool. I need to try this with all of my other wings. I had to keep the battery in it, otherwise it wound up at 10 inches or so.
Last edited by mtayl; May 02, 2002 at 09:12 AM.
May 02, 2002, 09:19 AM
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mtayl's Avatar
Where the superimposed red line is and the ribbon is exactly 8". (My bottom spar is a bit forward)
Last edited by mtayl; May 02, 2002 at 09:24 AM.
May 02, 2002, 09:22 AM
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mtayl's Avatar
I just had a thought though. Let us assume that you build a new wing with no instructions. There are calculations to figure out where the CG should be. This will only show you where it is, not where it needs to be. Right?

Even so, this is a lot more accurate for me than balancing it on my fingers.
May 02, 2002, 09:30 AM
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thor4944's Avatar

Yep


Yep......it shows you where the C.G. is in your plane..what is pretty cool is you can hold the wing any place as long as it freely swings and you place the string with the weight were your hand is and the C. G. should be the same place.....I just know I am not a very smart person (at least with planes) and this was a very cheap and easy way to find it...I had problems with the pencil on the bottom of the plane.........certain problems keeping the marking it once I found it....Hope this helps...........
May 02, 2002, 10:53 AM
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Sparky Paul's Avatar
That is a very sensitive method.. works with wing-tail-fuselage planes also.
May 02, 2002, 11:00 AM
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Quote:
Originally posted by mtayl
I had to keep the battery in it, otherwise it wound up at 10 inches or so.
Well, of course. The battery has mass, which will affect the C.G.. Always find the C.G. of an airplane in ready-to-fly condition. The only exception is in the case of a slimer with the fuel tank in front of the C.G.. In that case, you want to find the C.G. with the tank empty because the plane gets tail heavy when as tank empties.

Nose heavy flies poorly, but tail heavy flies once.
May 02, 2002, 12:54 PM
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Larry Dudeck's Avatar
Mytal has it correct.

Your method will, in fact, show where the CofG is.

Bear in mind, though, that CofG location may be absolutely incorrect for flight.

Take your ZAGI, move the battery pack either way, do the CofG thing again. You will find the CofG is now in a different location, one that may be unsafe for flight.

I know of two ways to determine the correct CofG.

One is to build a 'chuck' glider of your model. Balance this until it glides in a nice flat trajectory. Hang it by a string, adjust the hang point until the plane hangs level, note the location of the hang point. Transfer that location to you full size model. Bingo, CofG location is solved.

The other method involves calculations of the wing area, horizontal stab area, tail moment (which requires the mean aerodynamic chords of both the wing and horiz.stab). If the wing has either forward or aftward sweep, that has to be accounted for in the M.A.C.

Then plug all the values into the formula (see May 2000, EFI). The answer is expressed in inches aft of the leading edge, measured on the MAC.

I have used both methods while building a 48" WS twin EDF jet with a swept back wing. The numerical value placed the CofG about 1" farther forward than the chuck glider.

Either location would be suitable for the initial flights, the mathematical location being the safer of the two.

After the initial flights, the CofG is then tweaked to suit your flying needs.
May 02, 2002, 02:31 PM
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JasonJ's Avatar

Neat trick


Thats a cool trick.
I know of two other way to find where the CG should be.
one is a graphical drawling of the wing show on page 2 of this post 6th post down
https://www.rcgroups.com/forums/show...t&pagenumber=2
The other is to use a program that can find the C of G of a drawn object,, $$$$$ or a cool little freeware program I found linked in the above thread and was so kind to attach to this post
BTW,, the freeware prog lets you do forward and rear swept wings as long as you know the angle,,,,,, spits out the Mac and you just X that by you .25 for distance from LE.
May 02, 2002, 02:55 PM
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Jim McPherson's Avatar
Uhh... what ever happened to the balance it on your finger method?
May 02, 2002, 03:04 PM
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thor4944's Avatar

Wait


Now wait....this is why I just do it my way...I get a plane and put the battery and everything in where I want it when I fly...balance it out and find the C.G. if I dont like the C. G. I add weight to the nose..and do it again....calculators and measuring stuff...naaaa I would rather fly!!!!!


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