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Old Nov 04, 2013, 09:14 AM
Chuck 'Em and Chase 'Em
Fly2High's Avatar
United States, NY, Plainview
Joined Aug 2005
8,093 Posts
Discussion
Determine Cg distance - how do you measure so accurately and with what?

Guys,


Tools are everything. I have read plenty of guys reporting Cg locations down to the millimeter or less.

What are you using to be able to determine the Cg so accurately?

Photos and/or links for what you are using to determine location would be appreciated if available but not required.

Are you using a tool or determining it mathematically or what?

thanks

Frank
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Old Nov 04, 2013, 09:34 AM
Team Hong Kong F3K
ThomasLee's Avatar
Hong Kong
Joined Sep 2010
3,071 Posts
Honestly, I just use my finger(nails). I can get it accurate to about 0.5mm, but really, I can't feel any difference in flight with CG differences of less than 1mm (heck, maybe even 1.5mm)!!
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Old Nov 04, 2013, 01:47 PM
Thermal, where art thou?
BavarianCharles's Avatar
85045
Joined Oct 2005
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Quote:
Originally Posted by Fly2High View Post
What are you using to be able to determine the Cg so accurately?
Works really well ...

http://www.mountainmodels.com/produc...799b765ea89712

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Old Nov 04, 2013, 01:57 PM
Live for speed
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USA, CT, Bethany
Joined Mar 2004
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Multiplex CG Machine - if you can find one
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Old Nov 04, 2013, 02:10 PM
Launch high. Fly low.
United States, CA, Lake Elsinore
Joined Aug 2003
4,145 Posts
Quote:
Originally Posted by ThomasLee View Post
Honestly, I just use my finger(nails). I can get it accurate to about 0.5mm, but really, I can't feel any difference in flight with CG differences of less than 1mm (heck, maybe even 1.5mm)!!
Same here...

I just put a couple of peices of tape (Approx 5mm wide) on each wing where I want the CG range. And I just CG it with my fingers. For my Stobels, I usually range them at 70 to 75 mm. I like to fly a little nose heavy so I just CG the plane in front of the tapes...
Yeah, more power to yah if you can feel/see what 1mm makes.
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Old Nov 04, 2013, 02:51 PM
"...certainty is absurd."
kcaldwel's Avatar
Joined Jan 2007
3,598 Posts
Multiplex CG machine:

http://www.sussex-model-centre.co.uk...xd.asp?id=5319

http://www.wheelspinmodels.co.uk/i/194340/

http://www.rc-toy.de/lshop,showdetai...693054,5,,.htm

Or a less costly wooden version:

http://www.rcgroups.com/forums/showt...3#post26539240

Or the cheapest, possibly most accurate version requires string, some bits of wood, and a bubble line level:

http://iflyrc.hypermart.net/CG1.htm

Kevin
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Old Nov 04, 2013, 03:01 PM
Time for me to Fly...
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United States, MI, Fenton
Joined Jan 2000
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Quote:
Originally Posted by BavarianCharles View Post
^ I second this one. It's a great little tool.
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Old Nov 04, 2013, 03:09 PM
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A large pair of scissors. They come with handles and everything.
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Old Nov 04, 2013, 03:37 PM
G_T
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5,722 Posts
Upside down on the wing upper surface. Anything pointed.

Gerald
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Old Nov 04, 2013, 04:12 PM
Peter Madsen
pbm77's Avatar
Denmark
Joined Dec 2009
69 Posts
I also use my fingers. But only measures cg before the maiden.

Am I the only one that don't know the precise cg of my planes? I set it accordingly to the manual the first time and then adjust it from there, but must admit that I don't measure it after I have found the spot I like.
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Old Nov 04, 2013, 05:32 PM
let's FLY!
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United States, VT, Castleton
Joined Dec 2007
2,081 Posts
2 pencils stuck in a block of styrofoam.

Like pbm77, I don't know what my final CG is. I mark a line at the theoretical CG, balance it, then adjust from there to my liking, but never go back and measure where it ends up. It usually ends up slightly further aft than what SailplaneCalc predicts.
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Old Nov 04, 2013, 05:36 PM
G_T
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Joined Apr 2009
5,722 Posts
I just TLAR, take it out, and fly it. I adjust for conditions anyway. I don't care what it measures but how it flies.

When I get to fly that is...

Gerald
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Old Nov 04, 2013, 05:45 PM
let's FLY!
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United States, VT, Castleton
Joined Dec 2007
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Gerald: why upside down?
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Old Nov 04, 2013, 06:11 PM
G_T
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Unless it is a low wing, it will sort of balance on one point that way, rather than having to have one on each wing.

Gerald
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Old Nov 04, 2013, 06:36 PM
Registered User
Shoreline, WA
Joined May 2004
362 Posts
Great Planes CG Machine

The attached picture shows my Great Planes CG Machine. The URL with a better picture, write-up and a review is:

http://www.greatplanes.com/accys/gpmr2400.html

The plastic rulers are graduated in both inches and millimeters. I'm much better on setting the C of G with this gadget than I ever was without it. The accurate measurement lets you keep track of changes.

Richard S
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