I have used several CG balancers and found the "Rope and Plum Bob" method to be as accurate as the Teeter Totter devices. Here is a crude picture of waht I am talkning about. If one of AE's could explain how and why this method works I would really appreciate it.
Thanks,

It looks like you have to play with the sling till you get the plane hanging level. In order for the plane to hang level, the pick point has to be directly over the CG, with the string for the plumb bob hanging from the same hook or point as the sling, then it has to point at the current CG. Mr. Gravity would not let it point in any other position.

Seems to me, that a CG machine is alot easier than hanging a plane and playing with the string length to get it hanging level. then toss is another string for the plumb bob and wait for it to stop swinging.

You are right about how to operate the hanging type CG Machine. With the rope that the plane is hanging from looped around the dowel, you just rotate the dowel until the plane is level (checked with a bubble type level). You can get the plum bob to hang fairly steady in a short period of time. So the mechanics of operating the hanging CG Machine is easy. Establishing a hook to hang it from is normally not a problem in most work shops.

I don't sell these things so I am not trying to push them on anyone. I just think it is a safe, easy, accurate, and cheap method to find AND adjust the CG of a plane of any configuration. From what you wrote, Mr. Gravity will make the plum bob point to the correct CG no matter whether the plane is inverted, upright, single wing or bipe, 2 pounds or 40.

You answered my question about why it works with "Mr. Gravity". I guess there is really no need for any more details.
Thanks

What you are doing with this method is making the plane a plumb bob, as well as the independent weight. It is quite a simple concept that the CG of both objects will hang straight down - and their CG's will lie along the same line when hung from a common pivot.

AS far as acually locating the CG, it doesn't matter if the plane hangs level. The CG of the plane will hang under the plumb-bob no matter what the attitude. However, since most people are trying to find the CG longitudinally, it is easiest to find when the plane hangs level.

Thanks Johng,
That explains why the plane should be level when using this tool. Would you say that this method is more accurate than the Teeter Totter tools since there is some friction in the rotating parts of the those tools. I think for our purposes, somewhere within 1/8 of an inch is probably satisfactory unless you are balancing a model like the Stealth Fighter (I hear you got to be right on the money with that plane).

I am a believer in, but not a user of the plumb bob method. Vic's drawings show the model suspended by a four-point harness (the lines going up to the dowels) hanging from a two-point anchor (the dowels), which hangs by one point (from the pulley or skyhook).

My understanding of how the plumb bob method should be used is to suspend the model by two points, one forward and one aft of the likely cg. This could be accomplished by slipping a noose around the tail, and closing the noose not tight but close around the after end; then slipping another noose around the nose of the airframe and closing it. Now the model hangs by two-points, and is free to drop the heavy wingtip as well as settle with the front-to-back cg under the plumb bob.

My method, which works best when building a plane, but which can be retro-fitted to an ARF or a built plane acquired second-hand, is to hang from a one point suspension, that being a hard-point dowel glued into a vertical-grained balsa block, epoxied to a convenient point directly over (or under, in the case of a low-wing model) the recommended or intended CG. The dowel is pre-drilled with a pilot hole to receive a small screw-eye or screw-hook, which can be removed when the balancing is complete.

Using the one-point method allows me to put the framed up model together, hang from the ceiling, then place servos, batteries, other parts here and there until the plane is hanging perfectly level--front to back, and side to side. I can then install all these accessories in their approximate locations with full confidence that the finished and covered/painted model will be pretty close to the intended cg. I can then rehang from my screw-eye and place let's say the battery to balance almost to perfection, even correcting some side-to-side balance if necessary by putting the battery up against the light side of the fuselage--or screwing/epoxying a small piece of lead somewhere along the wing spar.

The point to all this is not that I have put the model together with a perfect CG, because what is "perfect" is a matter of pilot judgment and preference, which can only be determined by flying. The point is that I can easily set up my components, then eventually take off and trim-fly a new model knowing that the CG is in a safe and flyable range.

If later on I make changes or repairs which throw the cg off, I just turn the screw-eye in again, do another CG check, then remove the screw-eye.

I like the idea of the single point method and it will allow perfect balancing both side-to-side and the CG. There may be a problem mounting the dowel or screw eye on scale models without leaving some evidence (hole or dowel). You have also got to mount the screw eye or dowel such that it will support the entire weight of the finished model. Most of my models are in the 8 to 15 pound range. This must be "designed" into the plane (most likely the wing) during the building phase. I would be a little nervous about hanging my Top Flight Cessna 182 (13 lbs) from a single point on the top of the wing. Large, giant scale models (20 to 35 lbs) would be a challenge.

However, the rope a plum bob method does not require the installation of the single point to hang the model from AND you can use bigger or stronger rope as required. You can hang the model from a loop from the nose and tail OR from side to side (wing). The nose to tail method will allow side-to-side balancing as MajorTom described.

Again, I am not a sales person trying to sell the rope and plum bob CG Machine. I just think it is a great method to finding and adjusting the CG and lateral balance of a model. I found a pictorial description of this "CG Machine" several months ago and downloaded the pictures. I lost the web site address that I found these pictures. However, I did post this pictorial tutorial on the CG Machine on my web site. You can find the tutorial in the iFLYrc Magazine on my site at:
http://iflyrc.hypermart.net
Enjoy!

I understand your concerns about scale appearance. I cover over the dowel with whatever finish I'm using on the model. Since I know where the pilot hole is from the plan CG, I never have difficulty finding the hole under the covering, and the mark where the screw-eye goes in is pretty small when the hardware is removed. Nevertheless, I see how a nicely done scale model would suffer from any detraction from its appearance. As to the weight, I have gone as high as 19 lbs with no problem, and I expect I could do quite a bit more. As a precaution, a model hanging right side up needs to be just high enough to lift the landing gear off the floor, so if something does give way, the plane need not drop more than an inch or so. If it's hanging upside down, I lift it just clear of the cradle I build for every model, so again if it falls, it has a soft landing.

I'm not urging people to change what works for them, just describing an option that I have found useful.