A single-edge razor blade, a small hobby vise, and a gram scale.
Find the CG of each blade by balancing it on the edge of the razor blade (blade balanced diagonally on the razor's edge). Press lightly to make a mark on the blade's gel coat using the razor's edge. Rotate the blade 90 degrees, balance again. Make a second mark. Where the two marks intersect, you have that blade's CG.
Repeat on second blade.
In the pictures I posted, the yellow tape is there only to make the marks show up better for the pics, it's not something I add during the process.
Measure the distance from the center of the bolt hole to the CG, as well as from the leading edge to the CG.
The CG of both blades should be within a couple of millimeters of being in the same place. If so, fine. Then use a gram scale to weigh each blade. Use Trim Monokote placed at the CG of the light blade to bring it up to the same weight as the heavy blade.
If your CGs are way off, you can correct that too, but it's additional work.
With most of today's CF blades, I rarely ever find a set that needs additional balancing or work.
People will sometimes go to great lengths to balance their main and tail rotor blades, but will completely ignore the flybar balance. Walt Schoonard of Miniature Aircraft days-gone-by taught me about balancing the flybar.
Before you put the paddles on the flybar, measure to make sure the darn thing is centered in the head. THEN add a wheel collar to each side of the flybar. Install the paddles, make sure they are properly aligned and the same distance out from center. Disconnect all links that might restrain the flybar. The heavy side will drop. Slide the two wheel collars back and forth on the flybar until it balances. Use a small amount of blue loctite on the set screws when you tighten them down.
If you look at the flybar on this heli I used to own, you'll see the wheel collars on the flybar.
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