CG with a cradle or pendulum
 

I keep seeing the question on how to do this and this is how I learned so many years ago from RCU when I started flying so I figured I'd post a pic so you could see what all the people are talking about, Works really well on high wing, Low wing and even biplanes like mine. The only thing that might be tough is finding a good plumbob, I made mine at work but many don't have that opertunity as I do in a machine shop but if you find a good alternative please post so others can use the ideas.

Enjoy
Dauntae

RE: CG with a cradle or pendulum
 

A long wood drill from the toolbox works well.

Bill

RE: CG with a cradle or pendulum
 

Bill,

I have used your method and haven't been very satisfied with the results. Dauntae's is very similiar in principle. I don't seem to get consistent results, particularly when checking it against the old finger on the wing tip technique. Using the hanging method, I level the stabilizer and add weight to bring the CG close, then relevel the stabilizer and adjust the weight again if needed. Is this what you do or am I missing some step?

Chukc

RE: CG with a cradle or pendulum
 

So what y'all are saying is hang the model with a string bridle, hang the plumbob from the same hanging point of the bridle, adjust the model till it's level and the point where the plumbob intersects the model is the model's cg?

RE: CG with a cradle or pendulum
 

Not quite - the plumb-bob will always point through the center of gravity, regardless of the plane's orientation. That is why it doesn't really matter how you suspend the model. But by suspending it from the nose/tail, you can also check lateral balance as well as fore/aft CG.

As for a plumb-bob, I just bent a small loop in the end of a piece of music wire about 8" long. When suspended with thread from the suspension point, I find it very accurate. Certainly faster, cheaper, and easier than any other means IMO.

RE: CG with a cradle or pendulum
 

Tashley, Where the plumb bob points is where the plane is currently balanced. IF the PB points too far back, the model is nose-heavy, too far forward = Tail-heavy.

So you can adjust weights accordingly

RE: CG with a cradle or pendulum
 

Chuckc,

Those are the steps that I follow. Only problem is that it takes some time for everything to stop swinging. I like the method because I can continuously experiment with placement of parts during construction.

Bill

RE: CG with a cradle or pendulum
 

Got it! I like it and want to try it. Just seems a bit more precise than using a cg stand.

RE: CG with a cradle or pendulum
 

Tom,

I especially like the fact that it will not fall off anything and can be left for hours while experimenting

Bill

RE: CG with a cradle or pendulum
 

do a google search for The House Of Balsa Dust, it gives directions and illistrations of a c.g. machine

RE: CG with a cradle or pendulum
 

Thanks for the plug . . .

The original device was called "Vanessa" by it's designer, the late Jim Archer. I don't remember why Jim called the rig "Vanessa", so your guess is as good as mine.

I managed to preserve a verbatim copy of Jim's original design, currently available at [link=http://home.mindspring.com/~the-plumber/]Vanessa[/link].

RE: CG with a cradle or pendulum
 

hey your welcome guy, built the vanessa a while back, works great, thanks

RE: CG with a cradle or pendulum
 

Home Depot and Lowes carry plumb bobs.

RE: CG with a cradle or pendulum
 

Fred - Great link and machine! I have always used more conventional methods of getting the CG right - such as fingers, hand made setups with pencil erasers at the tips, and the Great Planes CG machine, and found them OK for smaller planes, but with the obvious limitation that the model will - not may, but will - fall off the machine when you have turned around!

I'm building a 104" PBN Nomad and hadn't really thought about how I would set up the CG on this ~20# model, but now I know what I'll be using.

A couple of comments about the write up on your site...

I think that there's an error where the instructions stated "Drill a hole parallel to the long dimension near the end of the 1 x 2." I believe that it should read "Drill a hole perpendicular to the long dimension near the end of the 1 x 2." Drilling the hole parallel to the long dimension would mean to drill the hole length-wise, not across the 1 x 2 piece of wood as is shown in the drawing.

It is suggested that the line holding the model be coiled around the dowel a couple of times to hold the model in position and that this also allows the dowel to be turned to allow easy movement of the model to achieve a level setting. I like that idea and think that another alternative that would accomplish the same thing would be to glue rough (say 80 grit) sandpaper around the circumference of the dowel. What do you think?

Finally, I think that another possibility to prevent damage to ailerons and/or flaps is to have the rope that is acting as a cradle for the plane to be inside foam that is used for pipe insulation. This is how I hang my larger models for storage as shown in the photo.

For fine tuning the lateral balance I would suggest having one of the ropes around the prop bolt of a single engine model.

I’d be interested in your comments about the above. Thanks again for the article.

RE: CG with a cradle or pendulum
 

Here's my version of a similar setup, using nylon webbing "slings".... the really important part of using strings or slings is that one must adjust the airplane horizontally so that whatever the longitudinal "datum" reference line is - is parallel to the ground. That is the ONLY time the plumb bob will point to the real center of balance. If you set it up with nose slightly higher than actual reference, the bob will show "tail heavy".... get the tail a little high, and one could be in REAL trouble, 'cause the bob will show "nose heavy", and that could be ugly.

Pics show how I did my Ultimate bipe and a Goldberg Yak...

RE: CG with a cradle or pendulum
 

That was exactly my question earlier. The pointer will show the models cg but it will not show TRUE cg unless the model's datum line is level. Nose up, forward cg. Nose down, rearward cg.

RE: CG with a cradle or pendulum
 

What I actually said was:
The true CG will not be located on surface of the model, but rather somewhere between the top and bottom surfaces. If you want to locate the projection of the CG on the top surface, then the datum line must be horizontal (perpendicular to the plumb line).

RE: CG with a cradle or pendulum
 

Right.

RE: CG with a cradle or pendulum
 

Indeed, that portion could use a bit of correcting, but I chose to leave the article as written by Jim Archer if for no other reason than to preserve his mannerisms of presentation - translating English to 'murriken is sometimes part of the fun !

Wouldn't hurt a thing.

I use a couple of ply 'planks' cut to fit the chord + an inch or so, to serve as cradles on the larger models, like my 140" L-4. But yes, some flavor of padding is probably in order.

Figure F will work for lateral balance, if the model is slung with the fuse parallel to the dowel.

RE: CG with a cradle or pendulum
 

I believe that the difference between parallel and perpendicluar is the same in either Brit's English or US English.