Below are the numbers.

Left wheel 1686g
Right Wheel 1664g
Tail wheel 489g

This along with our distances we can determine the Center of Mass of the model. This is out center of gravity. To get engineering terms…the Center of gravity is where the entire mass of the plane will be acting. This is one point on the model. Right now we have 3 points that the mass is acting and its not acting evenly at these points either. The majority of mass is being carried by the main wheels. Then a small portion is acting on the tail wheel but this lever arm is much longer than the one acting with the main gear. We are going to calculate the moments around one point. That tip of the spinner reference. Engineering education tells us that is we add up all moments about a point it’s the same as one big moment around that same point. This makes it easy for us.

I use a spread sheet and just plug the numbers in for each model and the sheet calculates it for me. But the math is simple and can be done by hand.

Mass left wheel + mass right wheel = mass of the mains

Mass mains * distance to mains = moment of the mains

1686g + 1664g = 3350g


3350g * 14 in = 46900 in-g -----�* don’t worry about the units it will end up with inches.


Tail wheel moment

Mass of the TW * distance to the tail wheel = moment of the TW

489g * 64.625 in = 31601 in-g


Total moment

Moment of Mains + moment of TW = Total moment around the spinner tip.

46900in-g + 31601in-g = 78501 in-g

This is the number we need. We now know the total moment about the spinner tip and the total weight of the model 1664g + 1684g + 489g = 3839g

So now we need to point at which this moment would be centered knowing the total weight of the model. So we divide the 78501 in-g / 3839g this gets the mass units to go away and the resulting number is just inches.

78501 in-g / 3839g = 20.44inches

So the CG is 20.44in from the tip of the spinner. Dave Guerin designing the model tells us we want the CG to be 6.5” behind the LE of the wing. We know the LE of the wing is 14” behind the reference we used as the tip of the spinner. So 20.44 in is 6.44in behind the LE of the wing at the root. WOW

It came out just about perfect. The CG is about as close as we could get. Its within about 1 to 1.5mm from the designed CG. This is with the Heavy battery 5 cell 2300mah right on the center of the wing tube. I could not ask for a better balance than that. Usually you have to move the battery and repeat the math to get your new CG…That is why I plug the numbers in Excel spread sheet and let it do the work for me. You may have to move things around. The cool thing about this method is you can add weight and fly it….changing the CG then come home remove the weight and shift battery packs to get the CG back in the exact spot you had when you flew it. This allows for a reference.

Notice you could use the wing tube as the reference. The trailing edge works too. You just have to keep track of the numbers in relation to that tip of the psinner location. Once you find these distances you never have to get them again. The relationship between the mains and the tail wheel will not likely change so you just keep the references and use a spread sheet to calculate it fast and easy. However the math is not hard to do or understand.

I hope I have not confused you guys too much.

Of by the way we also have the weight of our model. It will be pretty close within a few grams. Each scale has a error associated with it. To reference it comes out at 3839g which is 8.46lbs or 8lbs 7.4oz. Reminder this is with the big 5cell pack.

So the CG of the model came out excellent. The weight of the model came out excellent. Now why is this. First using smaller servos in the tail. Being light with the glue in the tail, Using the Hyde Mount with the YS engine gave enough mass in the front to balance it considering the lightweight servos in the tail.


Digest all this and tomorrow I'll have the radio programming stuff on the JR X9303 2.4ghz system. I also have flight reports as we are planning to get it in the air tomorrow. Since I'm lucky enough to be in Phoenix with temps suppose to be in the mid 60's I think we can get it in the air. Things are looking good on it though.

I have not put any graphics just yet. I'll likley have some graphics and stuff on it in the morning.

Radio programming is very simple and straight up.

Troy Newman