by Rick Bell

CG Machine Photo


In the excitement of flying the latest masterpiece that you've labored perhaps months over, do you take proper steps to determine its center of gravity (CG)? Most of us usually do a quick check of the CG using our fingertips. If the plane is too' large to do this by yourself, you have your wife, girlfriend, flying buddy, or one of your kids help with the task. The results are often less than perfect. The CG has a profound effect on the way the model flies, handles and responds to control inputs. As most of us know, a nose-heavy model flies poorly; a tail-heavy model flies once. Those who do not know this find out the hard way!

Great Planes has recognized this problem, and with their C.G. Machine, you can now accurately and easily balance your model according to the manufacturer's recommendations. You can also make precise changes to the CG location to alter your model's flight characteristics to match your skills and preferences.

Product:C.G. Machine
Manufacturer:Great Planes Model Distributors Co.
Comments: The Great Planes C.G. Machine is an easy to-use tool to accurately balance high-wing, mid-wing and low-wing models. It can accommodate models weighing up to 40 pounds. Good instruction booklet with lots of photos.
Hits:Easy to assemble/use; Accurate; Good instructions; Accommodates many model types.
Misses:None noted.


The machine goes together easily; however, there are a few things to look out for. The instructions direct you to thread the pivot balls into the 1/4-inch rods with locking compound. Before doing so, you should just screw them in, snap on the socket caps and check them for freedom of movement. If the sockets are too tight, when the model is placed on the machine, the friction will not allow the model to rock easily, and this will give a false reading. Mine were a little too tight, so I chucked them (loosely by the threads) in a hand drill and used a piece of 600-grit wet or dry sandpaper with light oil to polish them.

Next, the two plastic rulers come connected by three tabs. You must carefully cut them apart and trim away the tabs flush with the ruler edge. This is important so the rulers slide easily in the ruler holders. You can then seat the rulers into the endcaps and glue them with thick CA; make sure that the rulers are 90 degrees to the pointers before you do this.


There are two ways to use the machine: balance your model at a predetermined location or measure where your model currently balances. For new, unflown models, the predetermined location method works best. You can balance your model slightly nose-heavy for those first test flights and then go back and move the CG forward or aft to fine-tune the model's feel to your tastes.

Predetermined method. Measure on the plans the distance from the wing's leading edge to the balance point. Spread the width of the uprights to just clear the sides of the fuselage, keeping them as close as possible next to the fuselage. Remember, the CG on the plans is where the wing meets the fuselage. Slide the rulers to the distance measured and place the model on the machine in a level attitude. Pivot the rulers until they are horizontal, then move the model until the wing's leading edge touches the upright pointers. This is the spot where your model should balance. If it is nose- or tail-heavy, move internal components (if possible) to achieve balance.

Check where your model balances. Slide the rulers out to the 7-inch lines. As before, place the model on the machine in a level attitude, pivot the rulers until they are horizontal, and slide the pointers back to touch the leading edge. Read the distance in both ruler holder windows to find the current CG of the model. If the reading is slightly different between the rulers, use the average of the two values.

Included with the C.G. Machine is a bubble level that can help you accurately determine when the model is level on the machine. You can tape the level to the side of the fuselage relative to a reference line (line must be 0 degrees) or place the level on the stabilizer. Again, the stabilizer must be at 0 degrees. Then make your adjustments to the CG until the bubble is in the center of the level.


To really test the machine, I used an unflown GP Extra 300S that I had recently finished. I balanced the model using my fingertips (the way I usually do), making it slightly nose-heavy. Then I test-flew it a couple of times. The model flew well, but it was sluggish for the type of flying it was designed for. It was also difficult to slow down for landings. Back at the workshop I checked the CG using the C.G. Machine and found that the CG was in front of the recommended CG range, so I moved the battery pack aft a few inches to achieve the desired CG.

The results were amazing! The Extra now performs as I hoped it would—crisp and precise. Landings were also much improved and easier to slow down.


The GP C.G. Machine is a tool that improves the way we balance our models. You can now have better control over the placement of the CG and remove some of the "pucker factor" of those first flights. It's easy to assemble and easy to use. It is a must-have for any workshop or club. Try one and see!

Reprinted with permission.
July, 1997 Model Airplane News
Editor: Gerry Yarrish