The center of gravity or CG of an aircraft is the point at which the entire weight of the aircraft is assumed to be concentrated and at which point the aircraft would balance if suspended there.
As a general rule of thumb, model aircraft are balanced at between 25 and 33 percent of the mean aerodynamic chord or MAC as measured from the leading edge of the wing on back. On models with a straight, untapered wing (referred to as constant chord), the MAC is itself constant across the width of the wing from root to tip and are measured close to the fuselage. Tapered or swept wings are measured at roughly half the width of the wing.
More specifically, if no factory documentation is available or if the wing is scratchbuilt, MAC may be determined by tracing the outline of the wing on a sheet of paper. The root and tip measurements should then be extended fore and aft with parallel lines and crossed with projection lines. In what is literally a case of "X marks the spot," a line drawn parallel to the root rib through where the projection lines cross represents the MAC; the center of gravity will be found at the aforementioned 25 to 33 percent of the MAC measured from the leading edge on back.
- Yarrish, Gerry. "Balancing Act: Solid Flight Performance Starts With Getting the CG Right." Model Airplane News, April 2007