To get the CG I do it the same way they do it on a full sized aircraft. I measure the reference distances, then I place all 3 wheels on scales and measure the load be taken by each wheel. To calculate this takes a bit of Physics…You have to know about moments and force arrangements. I’ll try to make it simple. Assemble the model with all its parts and bits. Ready to fly except the battery pack is not mounted yet. Its loose as in the photos above.

I run the models nose up tight against the wall, and measure my references from that wall. These numbers will vary on each model. The key is you measure from the same reference. We are going to calculate the moments around the tip of the spinner. Then divide the total moment by weight of the model. This will give us the distance from the tip of the Spinner that the entire mass is centered on. This is the Center of Gravity. The problem is we can’t easily measure from the tip of the spinner to a reference on the side of the plane of the wing. But if we measure the distance say from the LE of the wing to the tip of the spinner then its easy to calculate how far back our CG location is from the Wing LE. The designer referenced the LE of the wing at the root and says the CG location should be 6.5” aft of the LE of the wing at the root.

Another thing to realize is the model needs to be “flying level” while sitting on the table or the mass would not be distributed absolutely correctly. Will it get it super close….yes much better than the guy sticking his fingers in the wing tube socket of the fuse and trying to determine how far the tail hangs down from level.

#1 Push the model up against the wall. And block the tail up so the model is level. I used some empty JR servo boxes as my blocks. These will sit on to of the scale when getting the weight from the tail wheel. Since we will have to “TARE” or zero the scale with the blocks on it…we don’t want heavy blocking material. Foam works the empty servo boxes are also light. I think I used 4 servo boxes. Get the model close to square with the wall surface.

#2 Measure from the wall to the Main gear axles, and to the tail wheel axle. These are the distances that will be used with the weight carried by each wheel. In my case this was 14” from the wall-tip of the spinner to the main gear axles. Tail wheel axle is 64-5/8” or 64.625”

#3 Measure the distance from the wall to the LE of the wing. This became a slight problem as the landing gear was in the way of my square. So I measured to the LE of the Landing at the fuse then used a ruler to get the extra distance back to the LE of the wing. This is shown in the photos. The distance to the Landing gear is just under 13.75” and the distance from the landing gear to the LE of the wing is just over ¼” The result is the LE of the wing is 14” from the tip of the spinner. This is the same distance as the main gear axles. This doesn’t have to be the case but on this model it worked out to be that way. So the LE of the wing is 14” from the tip of the spinner.


#4 You have to have a scale. I use actually 3 digital scales. One scale placed under each wheel. You can use just one scale. I would recommend getting a small foam or balsa block that is the same thickness as the scale to go under each wheel that is not on the scale at the time. This will make sure the model is not sitting at an angle and distributing the weight unevenly. I place the empty servo boxes to block the tail up. Then each scale is the same so the model is sitting level on the table. NOTE you can’t do this on carpet…the carpet will give a low reading on total weight. I did this a few years ago and the model weight was about 6oz off. I thought the plane was 6oz under the limit and it was really about 2g under the limit. So make it a hard surface….My bench is what I use.

#5 Turn on the scales one at a time lifting the models wheel off the scale while it calibrates and zeros its self.. With the tail wheel one zero it with the servo boxes on it and not the tail wheel. Write down these numbers. I use grams….the scales are more accurate in grams and are good to 1g increments. The mass will not matter in the CG calculation as it will be divided out. You could use oz’s lbs, tons if you wanted. The unit you choose should be as accurate as possible. We don’t want to record a number that is 0.0050tons.

The scales you choose will either make this easy or hard. A scale with 1g resolution is excellent for this application. Fishing scales that are accurate to 1/4lb is probably not the best tool . Same as the bathroom scale. The bathroom scale is designed to read 150-250lbs….your model is lighter than that…so get a good scale that is for reading small numbers. The ones I have are EBay Specials about $35 each and are 1g resolution to 13kg. They also ready lbs and ozs. Being an Engineer type I prefer the grams…its easier…1000g is a Kg and the weight limit is 5 of those Kg. The weight limit in F3A doesn’t say 11lbs it says 5kg so this is the mass unit we will use. The reason for this is resolution. We want the Cg as accurately as possible. If you are like me and have 3 scales…great if not you need to move the scale and your balsa or foam block around to keep the model level while measuring the weight on each wheel. Write down the numbers for each wheel.

The next post will show my numbers and the math to get the CG location. You now have all the numbers you need.