Center of Gravity

ahxbell1.bmp (152418 bytes) The center of gravity (CG) is the point from which an airplane would hang if it were suspended by a cable. It's also the point at which the longitudinal, vertical, and lateral axes intersect. To ensure that an airplane is stable in flight and responds properly to control inputs, you must load your airplane carefully to keep the CG within its design range.

The CG Seesaw

An empty airplane is like a seesaw. It balances on its center of gravity. Each item added to the airplane shifts the CG slightly. Objects placed forward of the original CG tend to tip the airplane forward. Objects placed behind the CG tend to tip it backward. The amount of tipping force or "moment" depends on the weight of the object and its "arm"—the distance between the object and an arbitrary reference line called the datum. In many airplanes the datum is the firewall that separates the engine compartment from the cockpit.

Managing the CG

Pilots manage the CG by controlling how weight is distributed in the aircraft cabin. In most small airplanes, the fuel tanks and seats are located close to the optimum CG, so the CG doesn't move much as fuel, people, and luggage are added. Nevertheless, before every flight a pilot must ensure that the CG of the loaded airplane falls between the forward and aft limits specified by the manufacturer.

The CG and Stability

Keeping the CG within its design limits is critical because the position of the CG affects an airplane's stability, just as the position of a child on a seesaw changes the board's balance point.As the CG moves aft, an airplane becomes less stable in pitch. If the CG is too far aft, it may be impossible to lower the nose to recover from a stall.If the CG is too far forward, the airplane is "nose heavy," making it difficult or impossible to flare during the final phase of landing.