Weight and Balance

the imaginary point at which all the weight is concentrated. To provide the necessary balance between longitudinal Datum stability and elevator control, the CG is usually located 100" slightly forward of the center of lift. This loading condition causes a nose-down tendency in flight, which is desirable during flight at a high AOA and slow speeds. 50 lb As mentioned earlier, a safe zone within which the balance point (CG) must fall is called the CG range. The extremities Fulcrum of the range are called the forward CG limits and aft CG Moment = 5,000 in-lb limits. These limits are usually specified in inches, along the Wt x Arm = Moment longitudinal axis of the airplane, measured from a reference (lb) x (in) = (in-lb) Note: The datum is assumed to be point called a datum reference. The datum is an arbitrary located at the fulcrum. 50 x 100 = 5,000 point, established by aircraft designers that may vary in location between different aircraft. [Figure 10-2] Figure 10-3. Determining moment. The distance from the datum to any component part or any in Figure 10-4), if a 100-pound weight is placed at a point object loaded on the aircraft is called the arm. When the (station) 25 inches from the datum, and another 50-pound object or component is located aft of the datum, it is measured weight is placed at a point (station) 50 inches from the datum, in positive inches; if located forward of the datum, it is the sum of the product of the two weights and their distances measured as negative inches or minus inches. The location total a moment of 5,000 in-lb, which will balance the board. of the object or part is often referred to as the station. If the weight of any object or component is multiplied by the Weight and Balance Restrictions distance from the datum (arm), the product is the moment. An aircraft’s weight and balance restrictions should be The moment is the measurement of the gravitational force closely followed. The loading conditions and empty weight that causes a tendency of the weight to rotate about a point of a particular aircraft may differ from that found in the or axis and is expressed in inch-pounds (in-lb). AFM/POH because modifications or equipment changes may have been made. Sample loading problems in the To illustrate, assume a weight of 50 pounds is placed on AFM/POH are intended for guidance only; therefore, each the board at a station or point 100 inches from the datum. aircraft must be treated separately. Although an aircraft is The downward force of the weight can be determined by certified for a specified maximum gross takeoff weight, it multiplying 50 pounds by 100 inches, which produces a may not safely take off at this weight under all conditions. moment of 5,000 in-lb. [Figure 10-3] Conditions that affect takeoff and climb performance, such as high elevations, high temperatures, and high humidity (high To establish a balance, a total of 5,000 in-lb must be applied density altitudes), may require a reduction in weight before to the other end of the board. Any combination of weight flight is attempted. Other factors to consider when computing and distance which, when multiplied, produces a 5,000 in- lb moment will balance the board. For example (illustrated Datum 50" 100" CG range 25" Fwd limit Aft limit Datum 100 50 lb 50 lb lb ( – ) ( + ) Arm Arm Fulcrum Moment = 700 in-lb 2,500 2,500 5,000 in-lb in-lb in-lb ( + ) Arm 70" Wt x Arm = Moment 100 x 25 = 2,500 10 lb (lb) x (in) = (in-lb) 50 x 50 = 2,500 Total = 5,000 Sta 0 Sta 70 Figure 10-2. Weight and balance. Figure 10-4. Establishing a balance. 10-6