All mechanisms - not just RC Helicopters - have an optimum centre of gravity (C of G or CG). To understand the subject properly you have to go back to the days of high school physics; fulcrums and pivot points. Simply put, the C of G is the balance point, where the weight is equal on both sides.
In consistently weighted objects, the C of G is in the centre. However, RC helicopters are unequally balanced and therefore the C of G is towards the front. This was all worked out at the design stage, with the rotor mast being positioned for optimum balance – this is where the C of G will be.
A little leeway is allowed. For most electric helicopters the C of G is at the centre of the rotor mast. However, on some models – especially if they have been modified - it will be just in front of this point.
The C of G is practically the last thing to be checked before flying a kit built helicopter. However, it is a good thing to learn even if you only plan to fly RTF Electric Helicopters. If you upgrade, have a crash or even add a few ‘embellishments’ to your favourite bird, the C of G may alter. Occasionally, it can even happen in transit.
Checking is easy. First, make absolutely sure that every one of the rotor and RC components is correctly balanced. Also check that the battery and engine are firmly housed and correctly anchored. Now take off and hover. The tail to nose line should be parallel to the ground. If it isn’t, then the C of G needs adjustment.