Designing the CG using moment arms

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By using the formulas for moments (from this page seeks to calculate the CG of a plane when given the weights and positions of various parts of the plane. By moving items like servos and batteries around, one can achieve the desired CG without adding weight.

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Example 1:
you have a bare airframe and want to know where to put the servos etc to achieve the desired CG without having to add weight. Weigh the airframe and find the balance point. (you can use this to do it) Fill in the table below with the weights and distances for each component. After pressing 'Update' the calculated CG point will be displayed. If this is not where the desired CG is, then move heavy items around till the calculated CG equals the desired CG. Say the calculated CG is behind the desired CG, move the battery forward and 'Update' to see the effect.
Example 2:
You have a mostly complete airframe on which you want to replace the tail surface servos with more powerful digitals. These are most likely heavier and will shift the CG. Where must they be put to keep the existing CG?
Weigh the complete airframe with the old servos in it and find the actual CG (balance point).
Remove the old tail servos and any tail weight and find the CG again.
Enter this data below. Now enter the data for the new servos at the old servos distances.
The calculated CG will now be behind (larger distance) the original CG.
Move the new servos forward (smaller distance from firewall) until the calculated CG is equal to the original CG. If you put the servos at these new psositions, the CG will be very close to the original CG, small differences will be caused by mounting hardware etc.


If you enter a 'Desired CG' position and then click the radio button next to any item (preferably a heavier one as it will make the most difference), after updating you will see a new position for that item in the Results box. If you then change that items position to the given value and update, you should see the calculated CG move to match the desired CG value.

It is entirely possible that the calculated value will move the item to a negative value position, ie off the front of the airframe if you are measuring from the firewall. If this happens you will have to move other items further away from the firewall in order to get the desired CG.

Note that while I have given examples using measurements from the firewall, you could also use measurements forward from the rearmost tip of the rudder.
Note that measurements are to the center of weight of the various parts of the plane. This would be the center of the battery pack, roughly the center of servos, and so on.


  1. - Weigh the fuselage complete with wings, and find the balance point. Measure from the firewall to the balance point. Enter this data below.
  2. - Weigh the motor, and measure or estimate the center of this weight from the firewall mounting. Enter this data with a negative distance.
  3. - For all other components of significant weight, enter the weight and a positive distance (assuming they're behind the firewall). Be careful with batteries, you may not be able to position a long battery pack as far forward as you think.
  4. - Press the 'update' button and the calculation results will be updated.

You need to enter the weights and distances from the firewall below.
Enter the weights in grams(oz) and distances in centimenters(inches)
Do not mix imperial and metric units

Error calculating CG
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Desired CG position =
DescriptionWeightDistance (center of weight of the item)Autoposition

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Last modified: June 25 2015 11:17:24.