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 Full Scale Ratsrepus
 Bob Pauley 3-View Sketches
45% Ratsrepus - July 21, 2001
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Calculating The Center of Gravity on Your Aircraft

Level the fuselage at the top; forward and aft, left and right. Pick a datum point, I used the front of the engine box in the pictures above. Any point will work. I like to use the crank flange on full size aircraft. Anything added forward of the datum has a negative moment, anything rearward has a positive moment arm.

Use an 8 foot scale to measure the points needed. Use a square to measure straight down to the 8 foot scale. Set the 8 foot scale so it reads 0 inches at the datum. Then measure the distance to the center of the main gear axles from the datum. This will be the arm for the main gear. Now measure from the datum to the center of the tail wheel axle. This will be the arm for the tail wheel. Record these measurements, they will not change unless you change your datum point or gear, tail wheel location.

I used a single scale. Make a block to shim up the opposite side to keep it level while weighing. Once you weigh the mains, level the aircraft with the scale under the tail wheel. This will give you the weights you need to do your CG calculation. You can weigh both mains one at a time or just multiply the first reading by two to get the weight at the main gear location.

To get the moments at the main gear and tail wheel locations, multiply the weight by the arm: Weight x Arm = Moment at each location

Calculate the total moment for the aircraft by adding up the individual moments. Be careful to keep track of the positve and negative moments.

Calculate the total weight of the aircraft by adding up the individual weights.

Center of Gravity location from the datum equals:
CG = Total Moment / Total Weight

For a completed aircraft the steps are the same. Weigh the aircraft with the wings at 0 angle of attack, aircraft in level flight. Use the spinner tip as the datum. The aircraft needs to be level at all times during the weighing process. Left and right main wheels should weigh about the same. Take all arm measurements from the datum while plane is level. Measure the arm locations of the mains and tail wheel. Take the weight at each wheel location. Use this info to calculate the CG, as explained above.

Doing the CG calculations during the building of your project allows you to check that your CG is coming out where you had designed it to be. If it is not coming out correctly you can re-configure things before it is set in stone. i.e., Move the engine forward or aft, drill holes in the fuselage where required. You can use the formulas to calculate where and how much weight needs to be added or removed to get the CG in the range that your design requires.

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