The quickest way to turn a finished glider into a pile of balsa is to launch with the CG in the wrong position. In the previous post I went over how the CG location affects stability, if you haven’t seen it start here. Now that we can all appreciate the importance of the CG location, we need a reliable way to measure it. These balance scales that do just that. They were designed for the Glider Kit but they will work for any small model. Here is a video of the scales in action.
Step 1 – 3D Print the CG balance scale parts
Download the Thingiverse files, the scales come in 4 pieces.
Base
Slide
T-Nut
Knob
The Slide and T-Nut have to be printed with the threads in the vertical direction. On my small 3D printer all parts fit just fine. I only added support where touching the build plate, I didn't have any problems with the base not being supported. Print two of everything for a pair of scales.
Clean off the excess material from the T-Nut where it was resting on the build plate. A nail file works great for this. Make sure the T-Nut can slide smoothly in the channel. Carefully thread the T-Nut onto the slide screw a single thread. Put the arms of the T-Nut thought the base and continue to move the slide down. The knob slides over the end of the bolt, if needed add some glue to hold in on. Once you are done you should have a scale that looks like this.
Step 2 – Print the Ruler
Print and carefully cut out the rulers. Glue the rulers to the outside of the slide, there is a left and a right.
Make sure the PDF printer settings are “Actual size” not “Fit”!
Very Carefully align the “0” on the ruler with the back edge of the forward stops.
You will notice the scales have a large circular opening on the base. That is for the large neodymium magnets, they attract through the workbench and hold the scales in place.
Step 3 – Place the Model on the Scales
With the two scales attached to the workbench, level the model. Set the model on and adjust the balance point on both scales at the same time. Turn clockwise to bring the nose up and counter-clockwise to lower the nose. For extra precision, add counter weights to the backside of the scales. This will cancel out any influence the balance arms are having on the CG location. When the model is flying level, read the ruler just over the center of the pivot point, that is how far back the CG is from the leading edge. Simple Enough.
As we add new equipment to the model we will have to make sure the CG remains in the safe range. Stay tuned for many mods in the future.
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