The aircraft manufacturer sets the helicopter’s center of gravity limits. If operating outside of these limits, the helicopter may be uncontrollable in certain situations.
The effects of CG are a reason to perform a weight/balance calculation before each flight. When the CG is forward of limits, there may not be enough aft cyclic travel to stop the helicopter, in particular during the flare at the end of an autorotation. With a forward CG, the helicopter will hover nose low in no-wind conditions. This condition may not be as noticeable in windy conditions.
When the CG is aft of limits, excessive forward cyclic will be required for forward flight. Should a gust of wind occur, there may not be enough cyclic travel to adequately control the helicopter in forward flight.
Lateral CG limits are also established to ensure that cyclic travel is available to adequately control the aircraft during all phases of light. Smaller training helicopters may have certain limits, such as solo flight from the right seat, primarily due to the fuel tank locations. Depending where the fuel tanks are located, the CG will often move forward and to the opposite side as fuel is burned off during the flight, especially in small training helicopters.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 6-2
Principles of Helicopter Flight, 2nd Edition, pg. 270