The lengthwise CG of a helicopter should be at its main rotor shaft or slightly forward. Otherwise, the heli will have a tendency to drift forwards or backwards.

To check this, first install the heli's batteries where you think they should go. rotate the main rotor so the blades are perpendicular to the body. Grab the main rotor blade grip closet to you and raise the heli up to eye level. Now find a wall corner or door jamb, and move the heli around so you can see the vertical line of the wall corner behind the helicopter.

Now try to align the main shaft with the vertical line. If the main shaft aligns perfectly with the line, then no adjustment is necessary. If the nose is light, then you will need to pull the battery pack forward. If the nose is heavy, then pull the battery pack backwards. Repeat until you find the correct battery position to balance the heli correctly.

If you use a tail servo mount on the ECO, the battery pack will need to be shifted forward quite a bit to adjust the CG properly. If you do this, you will need to put a rubber band over the two front side frame pegs to support the battery pack.

Note: Do not add weight to adjust the left/right CG of the helicopter. This can be adjusted using the transmitter left/right cyclic trim later.