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Old Apr 16, 2007, 06:34 PM
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United States, FL, Clearwater
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Question
CG Balancer for big planes??

Where do i get a balancer for big planes that won't damage the wings?

I saw a guy with a nice wooden one once at the field but i dont know what kind it was or where to get it. It had nice long arms with the center pivot where you place the cg mark. And those bubble liquid tube thingies that you see on a level.

I'd love to know where to buy that. Any help would be much appreciated.

,mike
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Old Apr 16, 2007, 06:40 PM
Deletedfor proving Nauga wrong
Joined Mar 2005
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broomsticks, 2X4's and make padded wing cradles for the broomstick's rounded ends.

Or, you can put each wheel on a scale, and using a bit of HS geometry, you can find the CG (and even how much weight is needed in which wingtip to correct lateral ballance)

Or.. you can hang the plane using cord tied to the prop and the tail of the plane. Tie those 2 lines together and hang from a single line to a hook. Hang a plumb bob from the same hook. The plumb bob will point at the CG.
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Old Apr 16, 2007, 07:05 PM
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USA, MI, Harrison Township
Joined Sep 2000
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I've always been a fan of 2 index fingers where the CG should be, usually 90% of the time on the wingspar. If the plane is low wing you do it holding the plane upside down.
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Old Apr 17, 2007, 02:13 AM
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Fort Collins, Colorado, USA
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Use the Great Planes cg machine. It costs $23 and can handle planes up to 40 pounds. It is immensely more accurate then the two finger method, although that does work for some people to get a general check of CG. broom stick are fine, but not very precise without adding some ruler type devices to the setup.
The Great Planes CG machine has foam pads that the wings rest on over the pivot points of the machine.

http://www2.towerhobbies.com/cgi-bin...&I=LXK259&P=ML
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Old Apr 17, 2007, 07:49 AM
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Rob Schaffer's Avatar
Lansdale, PA
Joined Jan 2002
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Easy solution, safe one too....

I used a CG Sling, made of some nylon rope, a plumb bob, and some spare wood. This allows me to hang the plane from the ceiling, move things as necessary, and see the CG accurately. Nice part, it's always supported by the rope so I don't have to worry about it slipping off the stand.

Here's a link to how to build and use one. I used Figure 'F' for balancing my Corsair.

CG Sling for Model Planes

Rob
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Old Apr 17, 2007, 08:05 AM
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Thanks. I'll probably go with the great planes balancer.

The plumb bob method is a wee bit too long for ADD, that and I don't know what plumb bob is. Or it could be the fact that i'm incredibly sleepy.

I'm hoping to win a Tower Hobbies $20 gift certificate from another site so i'll use it for that. (my odds of winning are very high i think)

,mike
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Old Apr 17, 2007, 09:41 AM
Fly allot, Crash allot, next?
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The great planes balancer is not good for large planes, the plastic rulers break, and if the plane if off balance it will topple over the balancer.

Build these and you will be fine. This is what I use.

3/4 inch bases
1 inch sprinkler pipe.
3/4 inch dowel.
Drill out the hole in the base for the sprinkler pipe.
Glue with Gorilla Glue
Drill out one end of the dowel, insert a large furnature roller.
Slip the dowel into the sprinkler pipe.
Drill holes in each dowel equally and adjust height by slipping a large nail or screwdriver through the hole.

Make another device to hole the plane stable just in case it is really nose or tail heavy. Adjust weights using masking tape on top of the plane until you get it balanced.

Al
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Old Apr 17, 2007, 10:28 AM
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now were talking.

i can build that.

Thanks Al
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Old Apr 17, 2007, 02:09 PM
Deletedfor proving Nauga wrong
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essentially the broom stick and 2X4 stand...

don't fprget to use a "protector" of some sort for each wing panel. If the end of the broomstick (or the above furniture roller) gets on unsupported covering with a larger model, its going to put a hole in the wing. The protector can be as simple as a 4 inch square of coroplast. (for up to 10 lbs... insert bamboo in a larger coroplast piece and you can support more weight. the idea is to spread the load and not ding the wing.)

CG has a recommended range because differnt taste in aircraft responsiveness will need different CG. The recommended range is typically a bit nose-heavy I usually end up 5% MAC behind the recommended range by the time I am happy with the way a plane flys.
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Old Apr 18, 2007, 11:33 AM
Awsome 2.5D Pilot.
bigbobed's Avatar
Foxfire Village, NC
Joined Nov 2003
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Quote:
Originally Posted by Rob Schaffer
I used a CG Sling, made of some nylon rope, a plumb bob, and some spare wood. This allows me to hang the plane from the ceiling, move things as necessary, and see the CG accurately. Nice part, it's always supported by the rope so I don't have to worry about it slipping off the stand.

Here's a link to how to build and use one. I used Figure 'F' for balancing my Corsair.

CG Sling for Model Planes

Rob
Here's a link to another build instructions for the sling. They are a little clearer than the link you posted.
http://iflyrc.hypermart.net/CG%20Machine.htm
I have found that the dowell that the sling strings go around must be exactly parallel to the wing center line or errors will creep in. I use two plumb bobs, one on each side of the fuse to make sure it's correct. My main like of this sling is that you don't have to worry about the plane falling no matter how far off you are to start. Keeps both hands free to move things around.
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Old Apr 18, 2007, 01:21 PM
Fly allot, Crash allot, next?
United States, CA, Corona
Joined Feb 2006
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Mike,

If you take a look at my stand, you will see the original bottoms. I was afraid that it might topple over so I screwed and glued the original bottom to a larger piece. Since then, I had to cut down one side of the larger bottom so that the stands could be pushed closer to the fuse.

Also, be sure you make some kind of tail or nose stand to support the bird until you get it balanced. It's hard to hold a big plane once you have it on the balancing stand, and move weights at the same time.

I forgot one other step to finishing the stands, I ground down flat spots on each furnature roller and glued on some packing foam to make the side softer where it supports the wing. No balsa breaks or cracks with these stands.

Al
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Old Apr 18, 2007, 02:45 PM
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yeah i noticed that. and i can always lengthen the surface the plane is gonna lay on if i need to as well. For now i'll just file it down and foam it.

,mike
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Old Apr 18, 2007, 03:18 PM
jrb
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Edina, MN, USA
Joined Oct 1999
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Great info guys!


Here’s another hint:

After you have the plane balanced; remove its wing and see where the fuselage alone balances!

Mark this spot, so next time you don’t need to put the wing on to check the balance! Note: its not likely to be at the CG.

This is particularly useful when trying to nail down the final position of the battery pack.


Its also useful when you 1st planning out your equipment positioning.

For example I assemble the plane and wing w/o any equipment installed; and then hang a weight up front to get it balanced.

Mark the fuselage and check it as you add things; might need to move the servos aft to get the pack to be centered very near the CG – this help when you change pack type, sizes etc.!


For my Cub I use a dowel rod in the window frames, and a line level on the wing saddle.

My Mustang works with a wire spanning the fuselage at the wing saddle doubler.

BOL

Jim
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Old Apr 18, 2007, 03:20 PM
jrb
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But this works too!
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Old May 21, 2007, 11:33 AM
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I saw a CG sling long ago that was made primarily of dowel rods and lines; gave it a try but wound up hanging it in the corner of the basement.

The sling ideas posted above got me to thinking about giving it another try.

I like the wooden post with the dowel rod shown; so I did a bit differently with a piece of PVC I had laying around.

Only needed to drill a few holes; even made the “line locks” as part of the pipe.
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