W&B 3-6

Figure 3-9. Determining the CG with datum forward of an airplane with nosewheel landing gear. CG F × L W = D − = 128 − 114.8= ( ) 340 × 78 2,006( ) Figure 3-8. The datum is 100 inches forward of the wing root leading edge. D = 128.0 100.0 13.2 50 114.8 Datum L = 78.0 Figure 3-10. The datum is aft of the main wheels at the wing trailing edge. 153 D = 75 88.2 Datum L = 78.0 Figure 3-7. Determining the CG. CG Total moment Total weight = 65,756 2,006 = 32.8 inches behind the datum= Figure 3-6. The datum is located at the firewall. 46.032 32.8 Datum off, place chocks around the wheels to keep the airplane from rolling. Subtract the weight of the chocks, called tare weight, from the scale reading to determine the net weight at each weighing point. Multiply each net weight by its arm to determine its moment, and then determine the total weight and total moment. The CG is determined by dividing the total moment by the total weight. [Figure 3-7] The airplane illustrated in Figures 3-5 and 3-6 has a net weight of 2,006 pounds, and its CG is 32.8 inches behind the datum. EWCG Formulas A chart such as the one in Figure 3-5 helps the pilot visualize the weights, arms, and moments when solving an EWCG problem, but it is quicker to determine the EWCG by using formulas and an electronic calculator. The use of a calculator for solving these problems is described in Chapter 8, Use of Computers in Weight and Balance Computations. There are four possible conditions and their formulas that relate the location of CG to the datum. Notice that the formula for each condition first determines the moment of the nosewheel or tailwheel and then divides it by the total weight of the airplane. The arm thus determined is then added to or subtracted from the distance between the main wheels and the datum, distance D. Datum Forward of the Airplane—Nosewheel Landing Gear The datum of the airplane in Figure 3-8 is 100 inches forward of the leading edge of the wing root or 128 inches forward of the main-wheel weighing points. This is distance (D). The weight of the nosewheel (F) is 340 pounds, and the distance between main wheels and nosewheel (L) is 78 inches. The total weight of the airplane (W) is 2,006 pounds. Determine the CG by using the formula in Figure 3-9. The CG is 114.8 inches aft of the datum. This is 13.2 inches forward of the main-wheel weighing points, which proves the location of the datum has no effect on the location of the CG if all measurements are made from the same location. Datum Aft of the Main Wheels—Nosewheel Landing Gear The datum of some aircraft may be located aft of the main wheels. The airplane in this example is the same one just discussed, but the datum is at the intersection of the trailing edge of the wing with the fuselage. The distance (D) between the datum of the airplane in Figure 3-10 and the main-wheel