Yes John, I was the one who posted about vertical cg.

I haven't tried that  but I am sure it is possible.  For instance, if you have a piped plane in the drawing stages, you know there will be more weight on the bottom than on the top.  For me before I start to paint an airplane, I check the vertical cg to see if I should add more paint to either the top or the bottom to help align it.  My grandfather has a black tiger and he couldn't figure out why the plane wouldn't fly right.  Actually it is a couple of reasons but he hung the plane from the leadouts and found out he had to lower them to get the cg right.  When he did this, the plane flew so much better.
Now it isn't flown anymore and is in the repair process because when I flew, I couldn't figure out why it wouldn't turn.  Turns out the controls had sooooooooo much slop in them (almost a 1/4 inch both ways) it wouldn't turn.  He opened it up and saw that the horns weren't bushed and were getting worn out.  If it had kept flying, I'm almost positive that horn might have broke and the plane would have gone in.  Little off topic, but had to get off my chest, 

Now days, I use the same method with my CAD drawn designs, but it's a lot easier now. I create a scale outline of the plane, complete with all items attached. There's a pull down menu for inquires, that will exactly figure things like Area, and here's the one wer're interested in for this post, Center of Mass. The computer will find the exact center of mass for you. Still, it's not exactly perfect, but close enough for our needs.

John, unless it is a solid modeling program like Solidworks Pro-E etc., how does the cad program know the center of mass?

probably the most accurate method I have used, is one taught to me by Pat Johnston,, its called compariing, I simply compare the layout to an existing design, and compare the distance from your reference lines. It will give you a really good idea of where to put your wintip.

Finding the center of mass using a cutout, is not my idea, rather one passed on to me by someone I respect.

Of course there're differrent weights and masses involved, but the cutout represents an average. Also, it was never claimed that the method will give the precise solution. Instead, it can get you close enough that it's much easier to do corrections, without 4 blackboards filled with equations.

Computer modeling id 3D space, in a program such as Solid works will allow you to closely locate the center of mass in one step. You can also get close usinng 2D as well. That's why a head on view, and a side view. You may have to do a little extrapolating with your grey matter, but it's far better than a WAG.

Comparing works, until you get to something way different.

Actually, it is not that difficult to mathematically determine a reasonable close approximation of the vertical CG of our models.  And the more accurate you can estimate and/or measure the weight and determine the center of mass of the various components, the more accurate you can determine the vertical CG.

Establish some longitudinal reference line.  Any will do, but the thrust line is usually already on the plans, so that will do.  Then weigh each of the major components of the model.  If the model has not been constructed, a fairly close estimate can normally be made of the painted wing, horizontal tail and fuselage.  You should know the weight of the engine, muffler, wheels and LG legs/gear covers wheel pants or whatever other major components you have in the design.  You should also be able to determine the approximate location of the CG of those components on their vertical axis.  Measure the distance of that vertical CG position of each component relative to your reference line (the thrust line in our example) and multiply that distance by the component weight.  You will then have a moment measured in inch-ounces for each component.  (for English units, the terms can be in inch-ounces.)  For those items that have their CG above the reference line, give those a positive moment.  For those items below the CG, give them a negative moment.  Add these moments and divide that value by the total weight of the components.  You will get a linear measurement that represents fairly closely where you vertical CG will be relative to your reference line.  It will be as accurate as your estimate of component weights that you cannot weigh and your estimate of the center of mass of those components.  For the fuselage, you can come up with some guess as to its completed weight (painted) and an "eyeball" location of its vertical cg relative to the thrust line.  With the engine/muffler, weigh the whole thing with the spinner and prop, then sort of do a balancing act with your fingers to see where its vertical CG is relative to the thrust line.

This approach works and can give you a very usefull initial point for your design vertical CG.  You can then adjust various components, like primarily the wing, so that the leadouts will be very close to that vertical CG location.  With the wing, you can simply move it up or down relative to the reference (thrust) line, or you can use combinations of  different amounts of dihedral and vertical position of the chord line to get those leadouts where you need to have them.  When you start adding dihedral, you need to compensate where the average chord is (or to be more precise where the mean chord is) relative to the reference longitudinal line.  In other words, for a wing with dihedral, the wing center of mass will be above the root chord line.

Or, you can look at proven designs - with and without dihedral and those with different locations of of the wing relative to the thrust line to get you in the ball park of correct vertical CG postions, at least close enough to allow appropriate trim adjustments.

When you consider these calculations, it becomes easier to see why light or heavy wheels can make a big difference on the vertical cg location on the model.  For example of a "typical" size and configured model (flat or no dihedral wing), let's say that your complete wing weighs 12 ounces, you can estimage that its center of mass is on the chord line and that chord line is, for example, 1 inch from the thrust line.  (Other than the engine/muffler, LG components, horizontal tail and fuselage, the wing is one of the most significant weight components to determine the vertical CG.)  So, we have a -12 inch-ounce moment.  Now, let's say that the two wheels weigh 2 ounces total (these are heavy ones!  The LG legs should be calculated separately) and the axles are 6 inches from the thrust line.  So we have another -12 inch-ounce moment, the same as that huge wing that is 6 times the weight of the wheels.  The combined moment of just those two components then becomes -24 inch-ounces, divide that by the total weight of those items (14 ounces) and you get their combined CG position (for just those two items) to be 1.7 inches from the thrust line.

In my opinion, it is easier to run these excursions on paper than it would be to make "cutouts" of the various components and, given the accuracy of your estimating capability, a more accurate determination of vertical CG than what "cutouts" will allow.

Something to ponder

Center of Mass: the single point the entire object (model, for us) will "balance" on in any direction.

Our lines, and the leadouts from the tip guides, deliver the line-pull towards the CG (a short term for Center of Mass). If the CG, vertically, is not in line with the line-pull, the model will roll - or try to - until the CG and pull line up. Same for yaw: if the pull aims ahead of the CG, the model will yaw nose-in until these forces line up. ...And of course, vice versa...

(It also works with roll from maneuvering lift/drag, but that gets complicated.)

If the vertical CG is much off, the model will try to fly rolled a bit away from level to the imaginary line from the flier to the CG. If the roll tendency is to the right, the lift force aims a bit away from the flier at the center, and vice versa. The model will increase, or decrease, pull according to which way it is rolled when maneuvering lift comes on... Not the best solution...

And, such tabs as are often called 'warts' act best only at ONE speed and load condition. Anything else, and they are less than perfectly effective... Best solution is to get all the forces to pass through the CG, in all directions that we can affect.

Compared to nose- or tail-weight ballast, we don't have a very long radius to add lead to for the vertical CG... Keep that in mind. Heavier (or perhaps lighter) wheels can help some. If there is a way to shift the leadouts vertically while building the wing, that can also help.

However, pull force is one of the strongest acting on the model. (Lift, of course, in maneuvers, and possibly the associated drags, are quite large, too.) It doesn't take much "aim error" for the pull force to shift the model's attitude. Some. CL models fly well, regardless of minor errors in things like this. It only becomes critical, when YOU develop the skills and critical understandings that make it critical.

We can make them fly, and have a ball! But, if we want to out-trim, out-adjust, out-fly a guy like Brett Buck, we got some work to do.

For usable practical results, this doesn't need to involve cutouts or calculations.  All of my airplanes with inverted engines have the leadouts between 1/2" and 7/8" below the thrust line.  Again, practically, if the leadout exit is properly located, the shape of the wing and bellcrank location doesn't matter.  This alone will be sufficient for most models.  If you want to get a more accurate location of the vertical CG, go ahead and build with a design location in this range and refine it in either of two ways.  One way would be, before gluing the inboard tip on, hang the model by the leadouts next to a wall and adjust the leadout exit in the tip.  The other way would be, to hang the model and substitute wheels of various weights.

If the leadout location is in the suggested range, adjustments are not usually necessary.  Anywhere in the suggested range will give a bank angle error less than a tiny warp and can be adjusted with a small flap tweak.

With an upright engine, I'd think that simply making the leadout exit right on the thrust line would be near enough correct with the wheels providing the compensation for the mass of the engine above the thrust line.

Al