What follows is a copy of info related to a c.g. finding rig that may be of use to some. As requested I'm attempting to post the complete package. If this is deemed against the forum rules by all means the moderators may delete it with no hard feelings on my part for sure. So, for what it's worth here's part 1#. First the boring text stuff. This is what I wrote up and packaged with the units I produced for my local flying groups use.



Gents,
Some time last year the question was asked on the RCO forum in regards to the Great Planes C.G. machine. That question in essence was, “Should I buy the G.P. C.G. Machine? How well does it work? Who’s used it?” The replies to the posting were varied but most were complimentary, other then mine I think. I am an owner of said machine and I rather wished I’d saved my money. It sat in its box for the longest time, as I had no need of a c.g. finding device at that moment. With the completion of my SIG Magnum build I then had a need, so out of the box it came. Don’t get me wrong here, it’s a fine piece and comes with “sticky weights” and “spinner weights” all of which one might need. And the unit itself is of very good construction, no faults here. But what really didn’t rock my world was the manner in which the model would have to sit on top of this stand for it to work. Yes, I knew this when I bought it, but seeing and then doing, as in many things, are very different in deed. I believe that you would have to be close to balanced to get the model to set up there in order to have any chance of making full use of it. What if the model is out in left field related to C of G concerns? I had visions of “one hand on the model, one hand doing the adjustments”. Its manner of operation just did not set to well with me. (Personal preference to be sure here) To much work to be done on the underside of the wings for my liking (to get the model situated), but what other way is out there for finding the c.g.? Oh yes the two-finger trick. Yep I like that better then the C.G. Machine in truth. (Just a bit better but not much) But yet there must be a better way still. I think I’ve found at least one. Read on.

The C.G. Rig: How to’s and why for’s.


Firstly, this is not an invention of my own making, it’s comes from the Internet. The version I’ve built is based on the work of Jim Archer, and could be found on his web site. He calls it Vanessa for some reason or other. See www.jimarcher.cwc.net for the home page this came from, but you’ll not find the actual “Vanessa” file there, ..it’s no longer posted it seems.

Any way, changes that I’ve made included the following:

1) There are two sets of support loops, consisting of two short loops, and one longer loop. The two short loops are to be used as a pair while suspending the model via the wings only. The single longer loop is for use as a fuselage suspension system only, to support fore and aft ends of the fuselage. Profile models seem to work best with the two shorter wing support loops, and full body planes seem to work better with the fuselage loop (which is the longer one). Any thing can be made to work on any model with enough tweaking. See the diagram for the different set ups.
2) I’ve included a SliderLock bar on both the main “Inverted Tee” bar support line, and on the plumb bob support line. These SliderLock bars will enable easy adjustment of the system to suspend the model, and or adjust the height of the plumb bob above the model. Should the lines of either prove to be to long simply pull the required excess through the slider bar by the knot tied in it’s end, and then tie a new knot at the shorter line length as needed. The line lengths are sized to work in my basement via suspension from the ceiling/floor joist. These may be to long, or short for your area of operation. I tend to work with the model sitting on the floor hence the longer line at the Inverted Tee support.
3) The cross bar dowel of the Inverted Tee components is a friction fit, but I’ve added a tension screw that’s filed flat on it’s pointy end (tension screw is optional). A simple turn of the screw will bring more friction to the Tee, and thus stop the dowel from rotating freely. The dowel will need to be rotated during the C.G. finding process, so do not put this screw to tight.
4) Also included: An “S” hook, and a very short loop of rope. These are for hanging the pulley from any handy spot overhead, and/or securing the main Inverted Tee support rope to a stationary object so adjustments can then be made via the slider bar, for model height and such. Use them for either or both jobs.
5) Part of the process of finding and adjusting the model C.G. includes setting the models centerline, or thrust line to the neutral level position. Included is a small line level to help do this job. Simply place the level on the models fuselage at a point where its location is equal to the thrust line or zero/zero incidence line of the model then rotate the Tee dowel until the bubble is centered in the glass. Remove the line level after doing so, as its weight will effect C.G. location. Like wise: as the line level affects C.G. due to its weight, the actual model “level attitude” will be only close, but its better then the Mk 108 eye ball system I think.
6) Some small adjustment to the loops may be in order, check for wings level when equal turns are wrapped on the dowel and adjust as needed, they should be very close in any respects.

Tips:

1) The farther away the weight is from the C.G. location the more effect it has on the resulting change. If the model is tail heavy it will take less weight to balance it if you can get the required weight as far out front toward the nose as is possible. The moments of force are greater the further they get from the pivot point.
2) I’ve found that the technique of duct taping the rear fuselage loop to the underside of the full body fuselages works very well, the wing flaps are not fouled by the support loop this way. I’ve not tried this with a profile fuselage model yet, but it could be made to work. Flapped ships tend to be a bit harder due to flap-fouled loops, unless a means of locking the flap position can be found, and the flaps will support the load placed on them by the models weight.. Remember that if the flaps are locked, the locking medium could be part of the weight and balance of the model. Be aware of anything that you put on the model during the balancing, and where it is your putting it!
3) A neat way of adding weight without adding weight is to take a sandwich lunch bag, or Glad “Cheap Ware” container and pass a loop of string through the sides so as to form a handle of string. This handle can be hung over the engine crank shaft at the front, or tail wheel strut at the rear, and you just pile the weight into the container or baggy. With a known amount of weight arrived at, you can then attach this weight on the model to effect the desired balance. Helps to stop them lead bars sliding off your fancy paintwork and such.




Addendum: for SSW posting.

A Brief run down of the parts and cost follows.

1) 15/32” x 36” dowel stock: $1.45 (You only need about 14” as per my dimensions)

2) 1”x 2”x 8’ Pine: $4.72 (You only need 12”)

3) 1 Nail Setting punch: $2.98 (Optional, for plumb-bob, you can make your own from lumber)

4) 1 Line level: $3.98 (Optional, you can use the Mk-108 eyeball)

5) Cord for support lines: $4.29 (This gets you a 36’ pre-packaged roll, similar nylon braid cord is available for $00.12 per foot, or a slightly larger size at $00.18 per foot.)

6) Pulley: $2.86 (Optional, but I like the smooth operation myself)

7) Snap Latch: $2.00 (Optional, but works well with the pulley)

8) Carpet Thread $00.07 per foot (for plumb bob line)


(All amounts are in Canadian funds.)


That’s all, enjoy, ….I hope.