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Centre of Gravity Calculation Form

This is simple to use form for calculating the Centre of Gravity (CofG) for a model that is not suitable for the normal balancing method (e.g. a large model). The model is weighed under it's main undercarriage and tailwheel (or nose leg) in the required flight attitude. It is simplest when 2 sets of scales are available as the readings can be taken in one go. If only one set of scales is available the model must be weighed in the same attitude for both readings using packing as necessary. A suitable beam is mounted on the main gear scales allows a single scale to be used for these.

It is preferable to remove the wheels from the model during weighing for 2 reasons. Firstly it allows more accurate measuring of the lengths that need to be known, and secondly it stops the model rolling of the scales. The only drawback is that the weight of the 2 main wheels and tailwheel needs to be added to the measured weights. A sketch is provided at the bottom of the page to assist.

Centre of Gravity Calculation Form

Value

Units

Readings from Scales and Dimensions to be measured

Main Wheels

Wm

   
Tailwheel

Wt

   

Main Wheels to Tailwheel

L

   

Main Wheels to CofG Datum

d

   
Total Weight (as weighed) = Wm + Wt

W

   
Distance of measured CofG from Main Wheels = (Wt x L) / W

a

   
Distance of measured CofG from CofG Datum = d a

x

   

That is all that is required to check the CofG position of a completed model. Additionally, this method can be used to determine the correct position for the motor battery to be fitted on completion of all other construction. Using the data worked out above, the following dimensions and calculations are required. The position of the Motor Battery from the CofG datum (b) is to the CofG of the Motor Battery.

Battery Position Calculation Form

Value

Units

Reading from Scales

Motor Battery Weight

Wb

   
Required CofG position (e.g. from plan or notes) xr    
Calculated Total Weight (including Motor Battery) = W + Wb Wc    
Position of Motor Battery to achieve xr = [(W xr) - (Wc x)] / Wb b    

Any units can be used for the measurements provided they are consistent throughout. The following sketch shows the arrangement and measurements used.

CofG Drawing

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