Visual CG Calculator
By: Ralph Shultz

Every pilot knows that aircraft CG is important but how do we ensure that we build our planes with a correct CG? Have you ever wondered whether the battery should be placed up front or to the rear in your particular aircraft? How will the weight of a fiberglass nose cone, ballistic chute, better seats, cabin heater, propeller or floats affect the finished aircraft's weight and CG position? There are many decisions of this sort that must be made during the planning and building of your Challenger II. Wouldn't it be nice to have a graphical tool to help predict the effect of these building considerations?

This article describes such a tool: an Excel spreadsheet which can be used to predict and control your Challenger's center of gravity through its planning, building and then after completion its real world weighing and CG calculation.

Spreadsheet programs like Microsoft Excel are excellent for manipulating weights and moments and determining the aircraft's CG. However, the attached spreadsheet goes one step beyond simple tabulations and algebraic functions. It will also allow you to do all of that as well as provide a visual representation of your aircraft and it's CG position, automatically updating as you add, remove, or move components and accessories.

Figure 1 below shows a typical screen snapshot from the spreadsheet calculator:

Figure 1

This snapshot shows a hypothetical Challenger II, equipped with a forward gap cover, instrument panel and instruments, 60-inch prop, 10 gallon fuel tank with 2 gallons of fuel in the tank, battery under the engine, ELT, 16.75 inch diameter main wheels, 10-inch diameter nose wheel, and a 135 lb. pilot in the front seat. Looking at the drawing, only the components that have a weight entered into the proper space appear. The rest are hidden. For example, wheel pants are not shown because their weight is listed as 0 in figure 2. Change the weight to 5.2 (or whatever your wheel pants actually weigh), and the pants appear on the drawing, and the plane's CG is recalculated automatically.

Figure 2 is screen snapshot of the Weight-Arm-Moment table.

Figure 2

This table essentially keeps a summary of the aircraft's components and their weights as well as their horizontal and vertical arms and moments. The spreadsheet then uses this information to calculate the airplane's CG, both horizontally and vertically, which appears on the drawing as a small black triangle.

Now that we have seen the main display and working elements of the spreadsheet let's investigate how to make things happen.

If you want to add or remove a component in the airplane, simply add or remove its weight. If you want to move a component from one place to another in the plane, just change the horizontal and/or vertical arm dimensions for that component in the Weight-Arm-Moment table to wherever you want the component to reside. Every component has its own center of gravity, and each of those CG points have been estimated and are shown on the drawing as a small dot inside the component. Additional components or accessories can be added at any time into the table. What effect will adding that Norton Bomb Sight to the nosecone have? Add it to the table, then enter the weight and arm dimensions to find out.

The spreadsheet can also be of help when one is trying to decide whether to build using a fuel tank with more than a 10-gallon capacity such as a 17 gal. tank. Just add the fuel weight along with some representative pilot and passenger weights. You may surprised to note that an iffy CG condition can exist with a light pilot, no passenger and a full tank of fuel in a large capacity tank!

Some components and airplane parts are not movable. This has been done to protect the integrity of the original design and protect against doing something that may adversely affect the structural and operating safety of the finished airplane.

After the airplane is completed and has been weighed, which must be done in the flight attitude, the wheel weights can then be entered in the proper places in the blocks to the right of the weight, arm and moment chart. When weights are entered in these blocks the spreadsheet will use these weights in place of weights in the chart. Weights added to the payload section of the chart will automatically be applied to the completed airplane weight and its CG point recalculated. Figure 3 shows these blocks.

Figure 3

Do you want to install a ballistic chute, but don't like the generic shape the spreadsheet has drawn? Customizing the view of your chute is fairly easy. Figure 4 depicts a sketch of the chute. Major dimensions are shown in BLUE, and an updating view of the chute is shown to the right of the sketch to help you visualize the effect of each change. A few minutes changing dimensions will adjust the shape to your needs. Note the chute view will only be displayed on the drawing when a chute weight is entered into the Weight-Arm-Moment table.

Figure 4

That about covers the main features of the spreadsheet calculator. Let's now look at some of the less important contributions it has to offer.

Having a means to both record and control component weights and locations can help in making better weight and balance placement decisions during construction. This calculator can help you to avoid building an airplane with an undesirable CG location.

A convenient weight look up table is provided to aid in finding the weights of components. A reminder is offered on how to calculate weight estimates. Basically this involves estimating the component's volume in inches and multiplying this volume by its material density. The following general class densities are close enough to use:

The green line, which shows up in Figure 1, is a ground reference line. It is drawn only when the CG point (the small black triangle on the view) is aft of the main wheels, indicating that the plane would be sitting on the mains and the tail wheel.

Note that if a line is drawn through this CG point and perpendicular to the green ground line (making it a vertical line relative to this reference ground line), it will pass farther aft relative to the main wheels than when the plane is sitting on the mains and the nose wheel. This may help explain why the tail will often stay down until the engine is started and rev'ed somewhat even if the plane's CG is slightly forward of the acceptable aft CG limit. The two vertical red lines in the side view drawing represent the C-II's acceptable forward and aft CG limits.

Figures 5a and 5b show some interesting wheel loading information on hypothetical Challenger loadings. This information is displayed below the weight, arm and moments chart. They show the weight resting on the mains and the nose wheel or tail wheel which ever is appropriate considering the CG location. In the case where the tail wheel is resting on the ground, the weight that must be lifted to raise the tail off the ground when lifting just ahead of the horizontal stabilizer's leading edge is also displayed. No wheel weight loads are displayed when the plane is on floats.

Figure 5a

Figure 5b

Well folks, that's about it. Try the spreadsheet, experiment with it, but I warn you, it can be habit forming and lots of fun.

Questions about this article or the spreadsheet, which can be found in the 101 download section, Visual CG Calculator, should be directed to the author and developer, Ralph Shultz at: