Determine Cg distance - how do you measure so accurately and with what? - RC Groups
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Nov 04, 2013, 10:14 AM
Chuck 'Em and Chase 'Em
Fly2High's Avatar
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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Nov 04, 2013, 10:34 AM
ThomasLee's Avatar
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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Nov 04, 2013, 02:47 PM
Charles Martin
BavarianCharles's Avatar
Quote:
Originally Posted by Fly2High
What are you using to be able to determine the Cg so accurately?
Works really well ...

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

Nov 04, 2013, 02:57 PM
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GoFaster's Avatar
Multiplex CG Machine - if you can find one
Nov 04, 2013, 03:10 PM
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Quote:
Originally Posted by ThomasLee
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.
Nov 04, 2013, 03:51 PM
Cognitive dissonance
kcaldwel's Avatar
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:

https://www.rcgroups.com/forums/show...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
Nov 04, 2013, 04:01 PM
Time for me to Fly...
Mr. Wiz's Avatar
Quote:
Originally Posted by BavarianCharles
^ I second this one. It's a great little tool.
Nov 04, 2013, 04:09 PM
Registered User
A large pair of scissors. They come with handles and everything.
Nov 04, 2013, 04:37 PM
G_T
G_T
Registered User
Upside down on the wing upper surface. Anything pointed.

Gerald
Nov 04, 2013, 05:12 PM
Peter Madsen
pbm77's Avatar
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.
Nov 04, 2013, 06:32 PM
It's time for me to fly
JimZinVT's Avatar
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.
Nov 04, 2013, 06:36 PM
G_T
G_T
Registered User
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
Nov 04, 2013, 06:45 PM
It's time for me to fly
JimZinVT's Avatar
Gerald: why upside down?
Nov 04, 2013, 07:11 PM
G_T
G_T
Registered User
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
Nov 04, 2013, 07:36 PM
Registered User

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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