Pre-made heli blades are sold in MATCHED PAIRS!!! This is why when you install a new blade in conjunction with an old blade the heli vibrates as the weights between the blades aren't the same. Like wood is not of the same weight/density as you know that balsa varies in both. You can match up blades from different sets---You not only have to make sure that they weigh the same, they must balance at exactly the same CG...You can do it yourself, however I suggest that the noobies stick to pre-balanced sets.

Kevin