## Sunday, February 12, 2017

### [HOW TO] Calculate the CoG from the weight

Given the number of questions I still receive :) , I see that the concept of measuring the CoG location by measuring the weight at the leading edge and X mm behind is not fully understood, so let's explain again with a real case, my Stribog CoG:

2 kitchen scales measure the weight of the plane, separated in my case by 293 mm. The first scale measure the weight at the vertical of the leading edge, the second one 293mm behind, so behind the trailing edge.
The sum of the 2 weight measured give the weight of the model. Now we apply the formula of a barycenter of a barrel that says: a x p = b x p'.

In other words, the if I measure p and p' , and also that I know the distance a+b (293mm in my case), so that b = 293 - a, I can easily calculate the CoG location.
So now, let's do it for real. My Stribog is placed on the device: I measure 1475gr at the leading edge, and 834gr at 293mm of the leadging edge. After entering the formula in Excel, it gives me a CoG located at 105.83 mm from the Leading Edge.

Now if you want to set the CoG of a Plane, the formula give you the ratio between the 2 weights. The only inconvenience is that as you add or remove some weight in/from the nose, you need to recalculate every time you do an action.

Also, I noticed something critical, at least with the model of kitchen scales I used. The link between the 2 scales must be the most free possible (vertically and in rotation) to not disturb the measure of one scale. Also, I had to remove the scales from the wood plate I prepared as I suspect the scale to need only 4 contact points to work properly. The weight reading was totaly false until I realized that.

1. Hi Pierre, It's a good explanation, but I think it's important to mention that the fuselage must be horizontal during measurement. Otherwise you can measure with a small mistake. In real aircraft weighing the aircraft pitch plays an important role. It is always taken into account during center of gravity calculation. (Boeing is using a correction table for this and Airbus has a formula with the alfa pitch value.)

1. Thank Roland! This is a very valid point that I forgot to mention.

2. Thanks for sharing wonderful information.
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4. Hi Pierre,
I made a similar CoG tool, with the automatic calculus of the additionnel weight to obtein the desirated position.
You can find it here : https://drive.google.com/file/d/0B3DuQRBtCWWJSG9zMlU0WjV3N2M/view
Franck

5. Hi Pierre,

it looks good. Can you provide me a source data for the 3D print, it's possible?
Thank you.

Petr H.

6. Bonsoir Mr RONDEL,

J'aimerai reproduire votre balance de centrage. Acceptez vous de partager les fichiers STL des pièces imprimées en 3D?

Merci,