It’s actually pretty simple. The whole heli “hangs” from main blades via main rotor shaft. If the weight under the blades is not in balance the physics of the movement required (wanted by pilot) is not symmetrical. Well, the movement might be but the amount of force needed to accomplish it isn’t.
Basically, if the balance is spot-on it takes equal amount of force to flip heli in any direction. This makes the behavior symmetrical. And that makes the heli easier to fly since movements are always the same in every axis regarding flip’s and rolls. (In scale this is most likely not important but in 3D it actually is.)
For example. We have the heli in hover, we wan’t a two-stage flip. First flip with elevator so nose is straigth up, then continue to full inverted. Then let’s do the same flip but with aileron, meaning first turn the heli on it’s side, then to full inverted.
Compare the stick movements and heli’s reaction. If everything “feel’s the same” everything is ok. If not You might have an unbalanced heli.
(I’m not taking in to account the RX-settings, I’m assuming RX is configured similary on elevator/aileron axes)
The most seen method to balance is to hang heli from main blade-holders and see how vertical it is. This method is only so good since it actually allows pretty big balance errors before the error noticeable. To get the balance near perfect remove the head and put the heli on the table, resting on the main shaft. And start adjusting weight, most usually Your battery’s position.
Why this small post (and rant) about balance?
No particular reason. I just wanted to show this cool picture from yesterday when I balanced my 800 Xxtreme :)